Network Working Group B. Fraser Request for Comments: 2196 Editor FYI: 8 SEI/CMU Obsoletes: 1244 September 1997 Category: Informational
Site Security Handbook
Status of this Memo
This memo provides information for the Internet community. It does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
Abstract
This handbook is a guide to developing computer security policies and procedures for sites that have systems on the Internet. The purpose of this handbook is to provide practical guidance to administrators trying to secure their information and services. The subjects covered include policy content and formation, a broad range of technical system and network security topics, and security incident response.
Table of Contents
1. Introduction.................................................... 2 1.1 Purpose of this Work............................................ 3 1.2 Audience........................................................ 3 1.3 Definitions..................................................... 3 1.4 Related Work.................................................... 4 1.5 Basic Approach.................................................. 4 1.6 Risk Assessment................................................. 5 2. Security Policies............................................... 6 2.1 What is a Security Policy and Why Have One?..................... 6 2.2 What Makes a Good Security Policy?.............................. 9 2.3 Keeping the Policy Flexible..................................... 11 3. Architecture.................................................... 11 3.1 Objectives...................................................... 11 3.2 Network and Service Configuration............................... 14 3.3 Firewalls....................................................... 20 4. Security Services and Procedures................................ 24 4.1 Authentication.................................................. 24 4.2 Confidentiality................................................. 28 4.3 Integrity....................................................... 28
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4.4 Authorization................................................... 29 4.5 Access.......................................................... 30 4.6 Auditing........................................................ 34 4.7 Securing Backups................................................ 37 5. Security Incident Handling...................................... 37 5.1 Preparing and Planning for Incident Handling.................... 39 5.2 Notification and Points of Contact.............................. 42 5.3 Identifying an Incident......................................... 50 5.4 Handling an Incident............................................ 52 5.5 Aftermath of an Incident........................................ 58 5.6 Responsibilities................................................ 59 6. Ongoing Activities.............................................. 60 7. Tools and Locations............................................. 60 8. Mailing Lists and Other Resources............................... 62 9. References...................................................... 64
This document provides guidance to system and network administrators on how to address security issues within the Internet community. It builds on the foundation provided in RFC 1244 and is the collective work of a number of contributing authors. Those authors include: Jules P. Aronson (aronson@nlm.nih.gov), Nevil Brownlee (n.brownlee@auckland.ac.nz), Frank Byrum (byrum@norfolk.infi.net), Joao Nuno Ferreira (ferreira@rccn.net), Barbara Fraser (byf@cert.org), Steve Glass (glass@ftp.com), Erik Guttman (erik.guttman@eng.sun.com), Tom Killalea (tomk@nwnet.net), Klaus- Peter Kossakowski (kossakowski@cert.dfn.de), Lorna Leone (lorna@staff.singnet.com.sg), Edward.P.Lewis (Edward.P.Lewis.1@gsfc.nasa.gov), Gary Malkin (gmalkin@xylogics.com), Russ Mundy (mundy@tis.com), Philip J. Nesser (pjnesser@martigny.ai.mit.edu), and Michael S. Ramsey (msr@interpath.net).
In addition to the principle writers, a number of reviewers provided valuable comments. Those reviewers include: Eric Luiijf (luiijf@fel.tno.nl), Marijke Kaat (marijke.kaat@sec.nl), Ray Plzak (plzak@nic.mil) and Han Pronk (h.m.pronk@vka.nl).
A special thank you goes to Joyce Reynolds, ISI, and Paul Holbrook, CICnet, for their vision, leadership, and effort in the creation of the first version of this handbook. It is the working group's sincere hope that this version will be as helpful to the community as the earlier one was.
This handbook is a guide to setting computer security policies and procedures for sites that have systems on the Internet (however, the information provided should also be useful to sites not yet connected to the Internet). This guide lists issues and factors that a site must consider when setting their own policies. It makes a number of recommendations and provides discussions of relevant areas.
This guide is only a framework for setting security policies and procedures. In order to have an effective set of policies and procedures, a site will have to make many decisions, gain agreement, and then communicate and implement these policies.
The audience for this document are system and network administrators, and decision makers (typically "middle management") at sites. For brevity, we will use the term "administrator" throughout this document to refer to system and network administrators.
This document is not directed at programmers or those trying to create secure programs or systems. The focus of this document is on the policies and procedures that need to be in place to support the technical security features that a site may be implementing.
The primary audience for this work are sites that are members of the Internet community. However, this document should be useful to any site that allows communication with other sites. As a general guide to security policies, this document may also be useful to sites with isolated systems.
For the purposes of this guide, a "site" is any organization that owns computers or network-related resources. These resources may include host computers that users use, routers, terminal servers, PCs or other devices that have access to the Internet. A site may be an end user of Internet services or a service provider such as a mid- level network. However, most of the focus of this guide is on those end users of Internet services. We assume that the site has the ability to set policies and procedures for itself with the concurrence and support from those who actually own the resources. It will be assumed that sites that are parts of larger organizations will know when they need to consult, collaborate, or take recommendations from, the larger entity.
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The "Internet" is a collection of thousands of networks linked by a common set of technical protocols which make it possible for users of any one of the networks to communicate with, or use the services located on, any of the other networks (FYI4, RFC 1594).
The term "administrator" is used to cover all those people who are responsible for the day-to-day operation of system and network resources. This may be a number of individuals or an organization.
The term "security administrator" is used to cover all those people who are responsible for the security of information and information technology. At some sites this function may be combined with administrator (above); at others, this will be a separate position.
The term "decision maker" refers to those people at a site who set or approve policy. These are often (but not always) the people who own the resources.
The Site Security Handbook Working Group is working on a User's Guide to Internet Security. It will provide practical guidance to end users to help them protect their information and the resources they use.
This guide is written to provide basic guidance in developing a security plan for your site. One generally accepted approach to follow is suggested by Fites, et. al. [Fites 1989] and includes the following steps:
(1) Identify what you are trying to protect. (2) Determine what you are trying to protect it from. (3) Determine how likely the threats are. (4) Implement measures which will protect your assets in a cost- effective manner. (5) Review the process continuously and make improvements each time a weakness is found.
Most of this document is focused on item 4 above, but the other steps cannot be avoided if an effective plan is to be established at your site. One old truism in security is that the cost of protecting yourself against a threat should be less than the cost of recovering if the threat were to strike you. Cost in this context should be remembered to include losses expressed in real currency, reputation, trustworthiness, and other less obvious measures. Without reasonable knowledge of what you are protecting and what the likely threats are, following this rule could be difficult.
One of the most important reasons for creating a computer security policy is to ensure that efforts spent on security yield cost effective benefits. Although this may seem obvious, it is possible to be mislead about where the effort is needed. As an example, there is a great deal of publicity about intruders on computers systems; yet most surveys of computer security show that, for most organizations, the actual loss from "insiders" is much greater.
Risk analysis involves determining what you need to protect, what you need to protect it from, and how to protect it. It is the process of examining all of your risks, then ranking those risks by level of severity. This process involves making cost-effective decisions on what you want to protect. As mentioned above, you should probably not spend more to protect something than it is actually worth.
A full treatment of risk analysis is outside the scope of this document. [Fites 1989] and [Pfleeger 1989] provide introductions to this topic. However, there are two elements of a risk analysis that will be briefly covered in the next two sections:
(1) Identifying the assets (2) Identifying the threats
For each asset, the basic goals of security are availability, confidentiality, and integrity. Each threat should be examined with an eye to how the threat could affect these areas.
One step in a risk analysis is to identify all the things that need to be protected. Some things are obvious, like valuable proprietary information, intellectual property, and all the various pieces of hardware; but, some are overlooked, such as the people who actually use the systems. The essential point is to list all things that could be affected by a security problem.
One list of categories is suggested by Pfleeger [Pfleeger 1989]; this list is adapted from that source:
(1) Hardware: CPUs, boards, keyboards, terminals, workstations, personal computers, printers, disk drives, communication lines, terminal servers, routers.
Once the assets requiring protection are identified, it is necessary to identify threats to those assets. The threats can then be examined to determine what potential for loss exists. It helps to consider from what threats you are trying to protect your assets. The following are classic threats that should be considered. Depending on your site, there will be more specific threats that should be identified and addressed.
(1) Unauthorized access to resources and/or information (2) Unintented and/or unauthorized Disclosure of information (3) Denial of service
Throughout this document there will be many references to policies. Often these references will include recommendations for specific policies. Rather than repeat guidance in how to create and communicate such a policy, the reader should apply the advice presented in this chapter when developing any policy recommended later in this book.
The security-related decisions you make, or fail to make, as administrator largely determines how secure or insecure your network is, how much functionality your network offers, and how easy your network is to use. However, you cannot make good decisions about security without first determining what your security goals are. Until you determine what your security goals are, you cannot make effective use of any collection of security tools because you simply will not know what to check for and what restrictions to impose.
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For example, your goals will probably be very different from the goals of a product vendor. Vendors are trying to make configuration and operation of their products as simple as possible, which implies that the default configurations will often be as open (i.e., insecure) as possible. While this does make it easier to install new products, it also leaves access to those systems, and other systems through them, open to any user who wanders by.
Your goals will be largely determined by the following key tradeoffs:
(1) services offered versus security provided - Each service offered to users carries its own security risks. For some services the risk outweighs the benefit of the service and the administrator may choose to eliminate the service rather than try to secure it.
(2) ease of use versus security - The easiest system to use would allow access to any user and require no passwords; that is, there would be no security. Requiring passwords makes the system a little less convenient, but more secure. Requiring device-generated one-time passwords makes the system even more difficult to use, but much more secure.
(3) cost of security versus risk of loss - There are many different costs to security: monetary (i.e., the cost of purchasing security hardware and software like firewalls and one-time password generators), performance (i.e., encryption and decryption take time), and ease of use (as mentioned above). There are also many levels of risk: loss of privacy (i.e., the reading of information by unauthorized individuals), loss of data (i.e., the corruption or erasure of information), and the loss of service (e.g., the filling of data storage space, usage of computational resources, and denial of network access). Each type of cost must be weighed against each type of loss.
Your goals should be communicated to all users, operations staff, and managers through a set of security rules, called a "security policy." We are using this term, rather than the narrower "computer security policy" since the scope includes all types of information technology and the information stored and manipulated by the technology.
A security policy is a formal statement of the rules by which people who are given access to an organization's technology and information assets must abide.
The main purpose of a security policy is to inform users, staff and managers of their obligatory requirements for protecting technology and information assets. The policy should specify the mechanisms through which these requirements can be met. Another purpose is to provide a baseline from which to acquire, configure and audit computer systems and networks for compliance with the policy. Therefore an attempt to use a set of security tools in the absence of at least an implied security policy is meaningless.
An Appropriate Use Policy (AUP) may also be part of a security policy. It should spell out what users shall and shall not do on the various components of the system, including the type of traffic allowed on the networks. The AUP should be as explicit as possible to avoid ambiguity or misunderstanding. For example, an AUP might list any prohibited USENET newsgroups. (Note: Appropriate Use Policy is referred to as Acceptable Use Policy by some sites.)
In order for a security policy to be appropriate and effective, it needs to have the acceptance and support of all levels of employees within the organization. It is especially important that corporate management fully support the security policy process otherwise there is little chance that they will have the intended impact. The following is a list of individuals who should be involved in the creation and review of security policy documents:
(1) site security administrator (2) information technology technical staff (e.g., staff from computing center) (3) administrators of large user groups within the organization (e.g., business divisions, computer science department within a university, etc.) (4) security incident response team (5) representatives of the user groups affected by the security policy (6) responsible management (7) legal counsel (if appropriate)
The list above is representative of many organizations, but is not necessarily comprehensive. The idea is to bring in representation from key stakeholders, management who have budget and policy authority, technical staff who know what can and cannot be supported, and legal counsel who know the legal ramifications of various policy
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choices. In some organizations, it may be appropriate to include EDP audit personnel. Involving this group is important if resulting policy statements are to reach the broadest possible acceptance. It is also relevant to mention that the role of legal counsel will also vary from country to country.
The characteristics of a good security policy are:
(1) It must be implementable through system administration procedures, publishing of acceptable use guidelines, or other appropriate methods.
(2) It must be enforcible with security tools, where appropriate, and with sanctions, where actual prevention is not technically feasible.
(3) It must clearly define the areas of responsibility for the users, administrators, and management.
The components of a good security policy include:
(1) Computer Technology Purchasing Guidelines which specify required, or preferred, security features. These should supplement existing purchasing policies and guidelines.
(2) A Privacy Policy which defines reasonable expectations of privacy regarding such issues as monitoring of electronic mail, logging of keystrokes, and access to users' files.
(3) An Access Policy which defines access rights and privileges to protect assets from loss or disclosure by specifying acceptable use guidelines for users, operations staff, and management. It should provide guidelines for external connections, data communications, connecting devices to a network, and adding new software to systems. It should also specify any required notification messages (e.g., connect messages should provide warnings about authorized usage and line monitoring, and not simply say "Welcome").
(4) An Accountability Policy which defines the responsibilities of users, operations staff, and management. It should specify an audit capability, and provide incident handling guidelines (i.e., what to do and who to contact if a possible intrusion is detected).
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(5) An Authentication Policy which establishes trust through an effective password policy, and by setting guidelines for remote location authentication and the use of authentication devices (e.g., one-time passwords and the devices that generate them).
(6) An Availability statement which sets users' expectations for the availability of resources. It should address redundancy and recovery issues, as well as specify operating hours and maintenance down-time periods. It should also include contact information for reporting system and network failures.
(7) An Information Technology System & Network Maintenance Policy which describes how both internal and external maintenance people are allowed to handle and access technology. One important topic to be addressed here is whether remote maintenance is allowed and how such access is controlled. Another area for consideration here is outsourcing and how it is managed.
(8) A Violations Reporting Policy that indicates which types of violations (e.g., privacy and security, internal and external) must be reported and to whom the reports are made. A non- threatening atmosphere and the possibility of anonymous reporting will result in a greater probability that a violation will be reported if it is detected.
(9) Supporting Information which provides users, staff, and management with contact information for each type of policy violation; guidelines on how to handle outside queries about a security incident, or information which may be considered confidential or proprietary; and cross-references to security procedures and related information, such as company policies and governmental laws and regulations.
There may be regulatory requirements that affect some aspects of your security policy (e.g., line monitoring). The creators of the security policy should consider seeking legal assistance in the creation of the policy. At a minimum, the policy should be reviewed by legal counsel.
Once your security policy has been established it should be clearly communicated to users, staff, and management. Having all personnel sign a statement indicating that they have read, understood, and agreed to abide by the policy is an important part of the process. Finally, your policy should be reviewed on a regular basis to see if it is successfully supporting your security needs.
In order for a security policy to be viable for the long term, it requires a lot of flexibility based upon an architectural security concept. A security policy should be (largely) independent from specific hardware and software situations (as specific systems tend to be replaced or moved overnight). The mechanisms for updating the policy should be clearly spelled out. This includes the process, the people involved, and the people who must sign-off on the changes.
It is also important to recognize that there are exceptions to every rule. Whenever possible, the policy should spell out what exceptions to the general policy exist. For example, under what conditions is a system administrator allowed to go through a user's files. Also, there may be some cases when multiple users will have access to the same userid. For example, on systems with a "root" user, multiple system administrators may know the password and use the root account.
Another consideration is called the "Garbage Truck Syndrome." This refers to what would happen to a site if a key person was suddenly unavailable for his/her job function (e.g., was suddenly ill or left the company unexpectedly). While the greatest security resides in the minimum dissemination of information, the risk of losing critical information increases when that information is not shared. It is important to determine what the proper balance is for your site.
All sites should define a comprehensive security plan. This plan should be at a higher level than the specific policies discussed in chapter 2, and it should be crafted as a framework of broad guidelines into which specific policies will fit.
It is important to have this framework in place so that individual policies can be consistent with the overall site security architecture. For example, having a strong policy with regard to Internet access and having weak restrictions on modem usage is inconsistent with an overall philosophy of strong security restrictions on external access.
A security plan should define: the list of network services that will be provided; which areas of the organization will provide the services; who will have access to those services; how access will be provided; who will administer those services; etc.
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The plan should also address how incident will be handled. Chapter 5 provides an in-depth discussion of this topic, but it is important for each site to define classes of incidents and corresponding responses. For example, sites with firewalls should set a threshold on the number of attempts made to foil the firewall before triggering a response? Escallation levels should be defined for both attacks and responses. Sites without firewalls will have to determine if a single attempt to connect to a host constitutes an incident? What about a systematic scan of systems?
For sites connected to the Internet, the rampant media magnification of Internet related security incidents can overshadow a (potentially) more serious internal security problem. Likewise, companies who have never been connected to the Internet may have strong, well defined, internal policies but fail to adequately address an external connection policy.
There are many services which a site may wish to provide for its users, some of which may be external. There are a variety of security reasons to attempt to isolate services onto dedicated host computers. There are also performance reasons in most cases, but a detailed discussion is beyond to scope of this document.
The services which a site may provide will, in most cases, have different levels of access needs and models of trust. Services which are essential to the security or smooth operation of a site would be better off being placed on a dedicated machine with very limited access (see Section 3.1.3 "deny all" model), rather than on a machine that provides a service (or services) which has traditionally been less secure, or requires greater accessability by users who may accidentally suborn security.
It is also important to distinguish between hosts which operate within different models of trust (e.g., all the hosts inside of a firewall and any host on an exposed network).
Some of the services which should be examined for potential separation are outlined in section 3.2.3. It is important to remember that security is only as strong as the weakest link in the chain. Several of the most publicized penetrations in recent years have been through the exploitation of vulnerabilities in electronic mail systems. The intruders were not trying to steal electronic mail, but they used the vulnerability in that service to gain access to other systems.
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If possible, each service should be running on a different machine whose only duty is to provide a specific service. This helps to isolate intruders and limit potential harm.
There are two diametrically opposed underlying philosophies which can be adopted when defining a security plan. Both alternatives are legitimate models to adopt, and the choice between them will depend on the site and its needs for security.
The first option is to turn off all services and then selectively enable services on a case by case basis as they are needed. This can be done at the host or network level as appropriate. This model, which will here after be referred to as the "deny all" model, is generally more secure than the other model described in the next paragraph. More work is required to successfully implement a "deny all" configuration as well as a better understanding of services. Allowing only known services provides for a better analysis of a particular service/protocol and the design of a security mechanism suited to the security level of the site.
The other model, which will here after be referred to as the "allow all" model, is much easier to implement, but is generally less secure than the "deny all" model. Simply turn on all services, usually the default at the host level, and allow all protocols to travel across network boundaries, usually the default at the router level. As security holes become apparent, they are restricted or patched at either the host or network level.
Each of these models can be applied to different portions of the site, depending on functionality requirements, administrative control, site policy, etc. For example, the policy may be to use the "allow all" model when setting up workstations for general use, but adopt a "deny all" model when setting up information servers, like an email hub. Likewise, an "allow all" policy may be adopted for traffic between LAN's internal to the site, but a "deny all" policy can be adopted between the site and the Internet.
Be careful when mixing philosophies as in the examples above. Many sites adopt the theory of a hard "crunchy" shell and a soft "squishy" middle. They are willing to pay the cost of security for their external traffic and require strong security measures, but are unwilling or unable to provide similar protections internally. This works fine as long as the outer defenses are never breached and the internal users can be trusted. Once the outer shell (firewall) is breached, subverting the internal network is trivial.
There is a large variety of services which may be provided, both internally and on the Internet at large. Managing security is, in many ways, managing access to services internal to the site and managing how internal users access information at remote sites.
Services tend to rush like waves over the Internet. Over the years many sites have established anonymous FTP servers, gopher servers, wais servers, WWW servers, etc. as they became popular, but not particularly needed, at all sites. Evaluate all new services that are established with a skeptical attitude to determine if they are actually needed or just the current fad sweeping the Internet.
Bear in mind that security complexity can grow exponentially with the number of services provided. Filtering routers need to be modified to support the new protocols. Some protocols are inherently difficult to filter safely (e.g., RPC and UDP services), thus providing more openings to the internal network. Services provided on the same machine can interact in catastrophic ways. For example, allowing anonymous FTP on the same machine as the WWW server may allow an intruder to place a file in the anonymous FTP area and cause the HTTP server to execute it.
Many network administrators go to great lengths to protect the hosts on their networks. Few administrators make any effort to protect the networks themselves. There is some rationale to this. For example, it is far easier to protect a host than a network. Also, intruders are likely to be after data on the hosts; damaging the network would not serve their purposes. That said, there are still reasons to protect the networks. For example, an intruder might divert network traffic through an outside host in order to examine the data (i.e., to search for passwords). Also, infrastructure includes more than the networks and the routers which interconnect them. Infrastructure also includes network management (e.g., SNMP), services (e.g., DNS, NFS, NTP, WWW), and security (i.e., user authentication and access restrictions).
The infrastructure also needs protection against human error. When an administrator misconfigures a host, that host may offer degraded service. This only affects users who require that host and, unless
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that host is a primary server, the number of affected users will therefore be limited. However, if a router is misconfigured, all users who require the network will be affected. Obviously, this is a far larger number of users than those depending on any one host.
There are several problems to which networks are vulnerable. The classic problem is a "denial of service" attack. In this case, the network is brought to a state in which it can no longer carry legitimate users' data. There are two common ways this can be done: by attacking the routers and by flooding the network with extraneous traffic. Please note that the term "router" in this section is used as an example of a larger class of active network interconnection components that also includes components like firewalls, proxy- servers, etc.
An attack on the router is designed to cause it to stop forwarding packets, or to forward them improperly. The former case may be due to a misconfiguration, the injection of a spurious routing update, or a "flood attack" (i.e., the router is bombarded with unroutable packets, causing its performance to degrade). A flood attack on a network is similar to a flood attack on a router, except that the flood packets are usually broadcast. An ideal flood attack would be the injection of a single packet which exploits some known flaw in the network nodes and causes them to retransmit the packet, or generate error packets, each of which is picked up and repeated by another host. A well chosen attack packet can even generate an exponential explosion of transmissions.
Another classic problem is "spoofing." In this case, spurious routing updates are sent to one or more routers causing them to misroute packets. This differs from a denial of service attack only in the purpose behind the spurious route. In denial of service, the object is to make the router unusable; a state which will be quickly detected by network users. In spoofing, the spurious route will cause packets to be routed to a host from which an intruder may monitor the data in the packets. These packets are then re-routed to their correct destinations. However, the intruder may or may not have altered the contents of the packets.
The solution to most of these problems is to protect the routing update packets sent by the routing protocols in use (e.g., RIP-2, OSPF). There are three levels of protection: clear-text password, cryptographic checksum, and encryption. Passwords offer only minimal protection against intruders who do not have direct access to the physical networks. Passwords also offer some protection against misconfigured routers (i.e, routers which, out of the box, attempt to
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route packets). The advantage of passwords is that they have a very low overhead, in both bandwidth and CPU consumption. Checksums protect against the injection of spurious packets, even if the intruder has direct access to the physical network. Combined with a sequence number, or other unique identifier, a checksum can also protect again "replay" attacks, wherein an old (but valid at the time) routing update is retransmitted by either an intruder or a misbehaving router. The most security is provided by complete encryption of sequenced, or uniquely identified, routing updates. This prevents an intruder from determining the topology of the network. The disadvantage to encryption is the overhead involved in processing the updates.
RIP-2 (RFC 1723) and OSPF (RFC 1583) both support clear-text passwords in their base design specifications. In addition, there are extensions to each base protocol to support MD5 encryption.
Unfortunately, there is no adequate protection against a flooding attack, or a misbehaving host or router which is flooding the network. Fortunately, this type of attack is obvious when it occurs and can usually be terminated relatively simply.
There are many types of services and each has its own security requirements. These requirements will vary based on the intended use of the service. For example, a service which should only be usable within a site (e.g., NFS) may require different protection mechanisms than a service provided for external use. It may be sufficient to protect the internal server from external access. However, a WWW server, which provides a home page intended for viewing by users anywhere on the Internet, requires built-in protection. That is, the service/protocol/server must provide whatever security may be required to prevent unauthorized access and modification of the Web database.
Internal services (i.e., services meant to be used only by users within a site) and external services (i.e., services deliberately made available to users outside a site) will, in general, have protection requirements which differ as previously described. It is therefore wise to isolate the internal services to one set of server host computers and the external services to another set of server host computers. That is, internal and external servers should not be co-located on the same host computer. In fact, many sites go so far
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as to have one set of subnets (or even different networks) which are accessible from the outside and another set which may be accessed only within the site. Of course, there is usually a firewall which connects these partitions. Great care must be taken to ensure that such a firewall is operating properly.
There is increasing interest in using intranets to connect different parts of a organization (e.g., divisions of a company). While this document generally differentiates between external and internal (public and private), sites using intranets should be aware that they will need to consider three separations and take appropriate actions when designing and offering services. A service offered to an intranet would be neither public, nor as completely private as a service to a single organizational subunit. Therefore, the service would need its own supporting system, separated from both external and internal services and networks.
One form of external service deserves some special consideration, and that is anonymous, or guest, access. This may be either anonymous FTP or guest (unauthenticated) login. It is extremely important to ensure that anonymous FTP servers and guest login userids are carefully isolated from any hosts and file systems from which outside users should be kept. Another area to which special attention must be paid concerns anonymous, writable access. A site may be legally responsible for the content of publicly available information, so careful monitoring of the information deposited by anonymous users is advised.
Now we shall consider some of the most popular services: name service, password/key service, authentication/proxy service, electronic mail, WWW, file transfer, and NFS. Since these are the most frequently used services, they are the most obvious points of attack. Also, a successful attack on one of these services can produce disaster all out of proportion to the innocence of the basic service.
The Internet uses the Domain Name System (DNS) to perform address resolution for host and network names. The Network Information Service (NIS) and NIS+ are not used on the global Internet, but are subject to the same risks as a DNS server. Name-to-address resolution is critical to the secure operation of any network. An attacker who can successfully control or impersonate a DNS server can re-route traffic to subvert security protections. For example, routine traffic can be diverted to a compromised system to be monitored; or, users can be tricked into providing authentication secrets. An organization should create well known, protected sites
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to act as secondary name servers and protect their DNS masters from denial of service attacks using filtering routers.
Traditionally, DNS has had no security capabilities. In particular, the information returned from a query could not be checked for modification or verified that it had come from the name server in question. Work has been done to incorporate digital signatures into the protocol which, when deployed, will allow the integrity of the information to be cryptographically verified (see RFC 2065).
Password and key servers generally protect their vital information (i.e., the passwords and keys) with encryption algorithms. However, even a one-way encrypted password can be determined by a dictionary attack (wherein common words are encrypted to see if they match the stored encryption). It is therefore necessary to ensure that these servers are not accessable by hosts which do not plan to use them for the service, and even those hosts should only be able to access the service (i.e., general services, such as Telnet and FTP, should not be allowed by anyone other than administrators).
A proxy server provides a number of security enhancements. It allows sites to concentrate services through a specific host to allow monitoring, hiding of internal structure, etc. This funnelling of services creates an attractive target for a potential intruder. The type of protection required for a proxy server depends greatly on the proxy protocol in use and the services being proxied. The general rule of limiting access only to those hosts which need the services, and limiting access by those hosts to only those services, is a good starting point.
Electronic mail (email) systems have long been a source for intruder break-ins because email protocols are among the oldest and most widely deployed services. Also, by it's very nature, an email server requires access to the outside world; most email servers accept input from any source. An email server generally consists of two parts: a receiving/sending agent and a processing agent. Since email is delivered to all users, and is usually private, the processing agent typically requires system (root) privileges to deliver the mail. Most email implementations perform both portions of the service, which means the receiving agent also has system privileges. This opens several security holes which this document will not describe. There are some implementations available which allow a separation of
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the two agents. Such implementations are generally considered more secure, but still require careful installation to avoid creating a security problem.
The Web is growing in popularity exponentially because of its ease of use and the powerful ability to concentrate information services. Most WWW servers accept some type of direction and action from the persons accessing their services. The most common example is taking a request from a remote user and passing the provided information to a program running on the server to process the request. Some of these programs are not written with security in mind and can create security holes. If a Web server is available to the Internet community, it is especially important that confidential information not be co-located on the same host as that server. In fact, it is recommended that the server have a dedicated host which is not "trusted" by other internal hosts.
Many sites may want to co-locate FTP service with their WWW service. But this should only occur for anon-ftp servers that only provide information (ftp-get). Anon-ftp puts, in combination with WWW, might be dangerous (e.g., they could result in modifications to the information your site is publishing to the web) and in themselves make the security considerations for each service different.
FTP and TFTP both allow users to receive and send electronic files in a point-to-point manner. However, FTP requires authentication while TFTP requires none. For this reason, TFTP should be avoided as much as possible.
Improperly configured FTP servers can allow intruders to copy, replace and delete files at will, anywhere on a host, so it is very important to configure this service correctly. Access to encrypted passwords and proprietary data, and the introduction of Trojan horses are just a few of the potential security holes that can occur when the service is configured incorrectly. FTP servers should reside on their own host. Some sites choose to co-locate FTP with a Web server, since the two protocols share common security considerations However, the the practice isn't recommended, especially when the FTP service allows the deposit of files (see section on WWW above). As mentioned in the opening paragraphs of section 3.2.3, services offered internally to your site should not be co-located with services offered externally. Each should have its own host.
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TFTP does not support the same range of functions as FTP, and has no security whatsoever. This service should only be considered for internal use, and then it should be configured in a restricted way so that the server only has access to a set of predetermined files (instead of every world-readable file on the system). Probably the most common usage of TFTP is for downloading router configuration files to a router. TFTP should reside on its own host, and should not be installed on hosts supporting external FTP or Web access.
The Network File Service allows hosts to share common disks. NFS is frequently used by diskless hosts who depend on a disk server for all of their storage needs. Unfortunately, NFS has no built-in security. It is therefore necessary that the NFS server be accessable only by those hosts which are using it for service. This is achieved by specifying which hosts the file system is being exported to and in what manner (e.g., read-only, read-write, etc.). Filesystems should not be exported to any hosts outside the local network since this will require that the NFS service be accessible externally. Ideally, external access to NFS service should be stopped by a firewall.
It is amazing how often a site will overlook the most obvious weakness in its security by leaving the security server itself open to attack. Based on considerations previously discussed, it should be clear that: the security server should not be accessible from off-site; should offer minimum access, except for the authentication function, to users on-site; and should not be co-located with any other servers. Further, all access to the node, including access to the service itself, should be logged to provide a "paper trail" in the event of a security breach.
One of the most widely deployed and publicized security measures in use on the Internet is a "firewall." Firewalls have been given the reputation of a general panacea for many, if not all, of the Internet security issues. They are not. Firewalls are just another tool in the quest for system security. They provide a certain level of protection and are, in general, a way of implementing security policy at the network level. The level of security that a firewall provides can vary as much as the level of security on a particular machine. There are the traditional trade-offs between security, ease of use, cost, complexity, etc.
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A firewall is any one of several mechanisms used to control and watch access to and from a network for the purpose of protecting it. A firewall acts as a gateway through which all traffic to and from the protected network and/or systems passes. Firewalls help to place limitations on the amount and type of communication that takes place between the protected network and the another network (e.g., the Internet, or another piece of the site's network).
A firewall is generally a way to build a wall between one part of a network, a company's internal network, for example, and another part, the global Internet, for example. The unique feature about this wall is that there needs to be ways for some traffic with particular characteristics to pass through carefully monitored doors ("gateways"). The difficult part is establishing the criteria by which the packets are allowed or denied access through the doors. Books written on firewalls use different terminology to describe the various forms of firewalls. This can be confusing to system administrators who are not familiar with firewalls. The thing to note here is that there is no fixed terminology for the description of firewalls.
Firewalls are not always, or even typically, a single machine. Rather, firewalls are often a combination of routers, network segments, and host computers. Therefore, for the purposes of this discussion, the term "firewall" can consist of more than one physical device. Firewalls are typically built using two different components, filtering routers and proxy servers.
Filtering routers are the easiest component to conceptualize in a firewall. A router moves data back and forth between two (or more) different networks. A "normal" router takes a packet from network A and "routes" it to its destination on network B. A filtering router does the same thing but decides not only how to route the packet, but whether it should route the packet. This is done by installing a series of filters by which the router decides what to do with any given packet of data.
A discussion concerning capabilities of a particular brand of router, running a particular software version is outside the scope of this document. However, when evaluating a router to be used for filtering packets, the following criteria can be important when implementing a filtering policy: source and destination IP address, source and destination TCP port numbers, state of the TCP "ack" bit, UDP source and destination port numbers, and direction of packet flow (i.e.. A- >B or B->A). Other information necessary to construct a secure filtering scheme are whether the router reorders filter instructions (designed to optimize filters, this can sometimes change the meaning and cause unintended access), and whether it is possible to apply
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filters for inbound and outbound packets on each interface (if the router filters only outbound packets then the router is "outside" of its filters and may be more vulnerable to attack). In addition to the router being vulnerable, this distinction between applying filters on inbound or outbound packets is especially relevant for routers with more than 2 interfaces. Other important issues are the ability to create filters based on IP header options and the fragment state of a packet. Building a good filter can be very difficult and requires a good understanding of the type of services (protocols) that will be filtered.
For better security, the filters usually restrict access between the two connected nets to just one host, the bastion host. It is only possible to access the other network via this bastion host. As only this host, rather than a few hundred hosts, can get attacked, it is easier to maintain a certain level of security because only this host has to be protected very carefully. To make resources available to legitimate users across this firewall, services have to be forwarded by the bastion host. Some servers have forwarding built in (like DNS-servers or SMTP-servers), for other services (e.g., Telnet, FTP, etc.), proxy servers can be used to allow access to the resources across the firewall in a secure way.
A proxy server is way to concentrate application services through a single machine. There is typically a single machine (the bastion host) that acts as a proxy server for a variety of protocols (Telnet, SMTP, FTP, HTTP, etc.) but there can be individual host computers for each service. Instead of connecting directly to an external server, the client connects to the proxy server which in turn initiates a connection to the requested external server. Depending on the type of proxy server used, it is possible to configure internal clients to perform this redirection automatically, without knowledge to the user, others might require that the user connect directly to the proxy server and then initiate the connection through a specified format.
There are significant security benefits which can be derived from using proxy servers. It is possible to add access control lists to protocols, requiring users or systems to provide some level of authentication before access is granted. Smarter proxy servers, sometimes called Application Layer Gateways (ALGs), can be written which understand specific protocols and can be configured to block only subsections of the protocol. For example, an ALG for FTP can tell the difference between the "put" command and the "get" command; an organization may wish to allow users to "get" files from the Internet, but not be able to "put" internal files on a remote server. By contrast, a filtering router could either block all FTP access, or none, but not a subset.
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Proxy servers can also be configured to encrypt data streams based on a variety of parameters. An organization might use this feature to allow encrypted connections between two locations whose sole access points are on the Internet.
Firewalls are typically thought of as a way to keep intruders out, but they are also often used as a way to let legitimate users into a site. There are many examples where a valid user might need to regularly access the "home" site while on travel to trade shows and conferences, etc. Access to the Internet is often available but may be through an untrusted machine or network. A correctly configured proxy server can allow the correct users into the site while still denying access to other users.
The current best effort in firewall techniques is found using a combination of a pair of screening routers with one or more proxy servers on a network between the two routers. This setup allows the external router to block off any attempts to use the underlying IP layer to break security (IP spoofing, source routing, packet fragments), while allowing the proxy server to handle potential security holes in the higher layer protocols. The internal router's purpose is to block all traffic except to the proxy server. If this setup is rigidly implemented, a high level of security can be achieved.
Most firewalls provide logging which can be tuned to make security administration of the network more convenient. Logging may be centralized and the system may be configured to send out alerts for abnormal conditions. It is important to regularly monitor these logs for any signs of intrusions or break-in attempts. Since some intruders will attempt to cover their tracks by editing logs, it is desirable to protect these logs. A variety of methods is available, including: write once, read many (WORM) drives; papers logs; and centralized logging via the "syslog" utility. Another technique is to use a "fake" serial printer, but have the serial port connected to an isolated machine or PC which keeps the logs.
Firewalls are available in a wide range of quality and strengths. Commercial packages start at approximately $10,000US and go up to over $250,000US. "Home grown" firewalls can be built for smaller amounts of capital. It should be remembered that the correct setup of a firewall (commercial or homegrown) requires a significant amount of skill and knowledge of TCP/IP. Both types require regular maintenance, installation of software patches and updates, and regular monitoring. When budgeting for a firewall, these additional costs should be considered in addition to the cost of the physical elements of the firewall.
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As an aside, building a "home grown" firewall requires a significant amount of skill and knowledge of TCP/IP. It should not be trivially attempted because a perceived sense of security is worse in the long run than knowing that there is no security. As with all security measures, it is important to decide on the threat, the value of the assets to be protected, and the costs to implement security.
A final note about firewalls. They can be a great aid when implementing security for a site and they protect against a large variety of attacks. But it is important to keep in mind that they are only one part of the solution. They cannot protect your site against all types of attack.
This chapter guides the reader through a number of topics that should be addressed when securing a site. Each section touches on a security service or capability that may be required to protect the information and systems at a site. The topics are presented at a fairly high-level to introduce the reader to the concepts.
Throughout the chapter, you will find significant mention of cryptography. It is outside the scope of this document to delve into details concerning cryptography, but the interested reader can obtain more information from books and articles listed in the reference section of this document.
For many years, the prescribed method for authenticating users has been through the use of standard, reusable passwords. Originally, these passwords were used by users at terminals to authenticate themselves to a central computer. At the time, there were no networks (internally or externally), so the risk of disclosure of the clear text password was minimal. Today, systems are connected together through local networks, and these local networks are further connected together and to the Internet. Users are logging in from all over the globe; their reusable passwords are often transmitted across those same networks in clear text, ripe for anyone in-between to capture. And indeed, the CERT* Coordination Center and other response teams are seeing a tremendous number of incidents involving packet sniffers which are capturing the clear text passwords.
With the advent of newer technologies like one-time passwords (e.g., S/Key), PGP, and token-based authentication devices, people are using password-like strings as secret tokens and pins. If these secret tokens and pins are not properly selected and protected, the authentication will be easily subverted.
As mentioned above, given today's networked environments, it is recommended that sites concerned about the security and integrity of their systems and networks consider moving away from standard, reusable passwords. There have been many incidents involving Trojan network programs (e.g., telnet and rlogin) and network packet sniffing programs. These programs capture clear text hostname/account name/password triplets. Intruders can use the captured information for subsequent access to those hosts and accounts. This is possible because 1) the password is used over and over (hence the term "reusable"), and 2) the password passes across the network in clear text.
Several authentication techniques have been developed that address this problem. Among these techniques are challenge-response technologies that provide passwords that are only used once (commonly called one-time passwords). There are a number of products available that sites should consider using. The decision to use a product is the responsibility of each organization, and each organization should perform its own evaluation and selection.
Kerberos is a distributed network security system which provides for authentication across unsecured networks. If requested by the application, integrity and encryption can also be provided. Kerberos was originally developed at the Massachusetts Institute of Technology (MIT) in the mid 1980s. There are two major releases of Kerberos, version 4 and 5, which are for practical purposes, incompatible.
Kerberos relies on a symmetric key database using a key distribution center (KDC) which is known as the Kerberos server. A user or service (known as "principals") are granted electronic "tickets" after properly communicating with the KDC. These tickets are used for authentication between principals. All tickets include a time stamp which limits the time period for which the ticket is valid. Therefore, Kerberos clients and server must have a secure time source, and be able to keep time accurately.
The practical side of Kerberos is its integration with the application level. Typical applications like FTP, telnet, POP, and NFS have been integrated with the Kerberos system. There are a variety of implementations which have varying levels of integration. Please see the Kerberos FAQ available at http://www.ov.com/misc/krb- faq.html for the latest information.
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4.1.3 Choosing and Protecting Secret Tokens and PINs
When selecting secret tokens, take care to choose them carefully. Like the selection of passwords, they should be robust against brute force efforts to guess them. That is, they should not be single words in any language, any common, industry, or cultural acronyms, etc. Ideally, they will be longer rather than shorter and consist of pass phrases that combine upper and lower case character, digits, and other characters.
Once chosen, the protection of these secret tokens is very important. Some are used as pins to hardware devices (like token cards) and these should not be written down or placed in the same location as the device with which they are associated. Others, such as a secret Pretty Good Privacy (PGP) key, should be protected from unauthorized access.
One final word on this subject. When using cryptography products, like PGP, take care to determine the proper key length and ensure that your users are trained to do likewise. As technology advances, the minimum safe key length continues to grow. Make sure your site keeps up with the latest knowledge on the technology so that you can ensure that any cryptography in use is providing the protection you believe it is.
While the need to eliminate the use of standard, reusable passwords cannot be overstated, it is recognized that some organizations may still be using them. While it's recommended that these organizations transition to the use of better technology, in the mean time, we have the following advice to help with the selection and maintenance of traditional passwords. But remember, none of these measures provides protection against disclosure due to sniffer programs.
(1) The importance of robust passwords - In many (if not most) cases of system penetration, the intruder needs to gain access to an account on the system. One way that goal is typically accomplished is through guessing the password of a legitimate user. This is often accomplished by running an automated password cracking program, which utilizes a very large dictionary, against the system's password file. The only way to guard against passwords being disclosed in this manner is through the careful selection of passwords which cannot be easily guessed (i.e., combinations of numbers, letters, and punctuation characters). Passwords should also be as long as the system supports and users can tolerate.
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(2) Changing default passwords - Many operating systems and application programs are installed with default accounts and passwords. These must be changed immediately to something that cannot be guessed or cracked.
(3) Restricting access to the password file - In particular, a site wants to protect the encrypted password portion of the file so that would-be intruders don't have them available for cracking. One effective technique is to use shadow passwords where the password field of the standard file contains a dummy or false password. The file containing the legitimate passwords are protected elsewhere on the system.
(4) Password aging - When and how to expire passwords is still a subject of controversy among the security community. It is generally accepted that a password should not be maintained once an account is no longer in use, but it is hotly debated whether a user should be forced to change a good password that's in active use. The arguments for changing passwords relate to the prevention of the continued use of penetrated accounts. However, the opposition claims that frequent password changes lead to users writing down their passwords in visible areas (such as pasting them to a terminal), or to users selecting very simple passwords that are easy to guess. It should also be stated that an intruder will probably use a captured or guessed password sooner rather than later, in which case password aging provides little if any protection.
While there is no definitive answer to this dilemma, a password policy should directly address the issue and provide guidelines for how often a user should change the password. Certainly, an annual change in their password is usually not difficult for most users, and you should consider requiring it. It is recommended that passwords be changed at least whenever a privileged account is compromised, there is a critical change in personnel (especially if it is an administrator!), or when an account has been compromised. In addition, if a privileged account password is compromised, all passwords on the system should be changed.
(5) Password/account blocking - Some sites find it useful to disable accounts after a predefined number of failed attempts to authenticate. If your site decides to employ this mechanism, it is recommended that the mechanism not "advertise" itself. After
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disabling, even if the correct password is presented, the message displayed should remain that of a failed login attempt. Implementing this mechanism will require that legitimate users contact their system administrator to request that their account be reactivated.
(6) A word about the finger daemon - By default, the finger daemon displays considerable system and user information. For example, it can display a list of all users currently using a system, or all the contents of a specific user's .plan file. This information can be used by would-be intruders to identify usernames and guess their passwords. It is recommended that sites consider modifying finger to restrict the information displayed.
There will be information assets that your site will want to protect from disclosure to unauthorized entities. Operating systems often have built-in file protection mechanisms that allow an administrator to control who on the system can access, or "see," the contents of a given file. A stronger way to provide confidentiality is through encryption. Encryption is accomplished by scrambling data so that it is very difficult and time consuming for anyone other than the authorized recipients or owners to obtain the plain text. Authorized recipients and the owner of the information will possess the corresponding decryption keys that allow them to easily unscramble the text to a readable (clear text) form. We recommend that sites use encryption to provide confidentiality and protect valuable information.
The use of encryption is sometimes controlled by governmental and site regulations, so we encourage administrators to become informed of laws or policies that regulate its use before employing it. It is outside the scope of this document to discuss the various algorithms and programs available for this purpose, but we do caution against the casual use of the UNIX crypt program as it has been found to be easily broken. We also encourage everyone to take time to understand the strength of the encryption in any given algorithm/product before using it. Most well-known products are well-documented in the literature, so this should be a fairly easy task.
As an administrator, you will want to make sure that information (e.g., operating system files, company data, etc.) has not been altered in an unauthorized fashion. This means you will want to provide some assurance as to the integrity of the information on your
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systems. One way to provide this is to produce a checksum of the unaltered file, store that checksum offline, and periodically (or when desired) check to make sure the checksum of the online file hasn't changed (which would indicate the data has been modified).
Some operating systems come with checksumming programs, such as the UNIX sum program. However, these may not provide the protection you actually need. Files can be modified in such a way as to preserve the result of the UNIX sum program! Therefore, we suggest that you use a cryptographically strong program, such as the message digesting program MD5 [ref], to produce the checksums you will be using to assure integrity.
There are other applications where integrity will need to be assured, such as when transmitting an email message between two parties. There are products available that can provide this capability. Once you identify that this is a capability you need, you can go about identifying technologies that will provide it.
Authorization refers to the process of granting privileges to processes and, ultimately, users. This differs from authentication in that authentication is the process used to identify a user. Once identified (reliably), the privileges, rights, property, and permissible actions of the user are determined by authorization.
Explicitly listing the authorized activities of each user (and user process) with respect to all resources (objects) is impossible in a reasonable system. In a real system certain techniques are used to simplify the process of granting and checking authorization(s).
One approach, popularized in UNIX systems, is to assign to each object three classes of user: owner, group and world. The owner is either the creator of the object or the user assigned as owner by the super-user. The owner permissions (read, write and execute) apply only to the owner. A group is a collection of users which share access rights to an object. The group permissions (read, write and execute) apply to all users in the group (except the owner). The world refers to everybody else with access to the system. The world permissions (read, write and execute) apply to all users (except the owner and members of the group).
Another approach is to attach to an object a list which explicitly contains the identity of all permitted users (or groups). This is an Access Control List (ACL). The advantage of ACLs are that they are
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easily maintained (one central list per object) and it's very easy to visually check who has access to what. The disadvantages are the extra resources required to store such lists, as well as the vast number of such lists required for large systems.
Restrict physical access to hosts, allowing access only to those people who are supposed to use the hosts. Hosts include "trusted" terminals (i.e., terminals which allow unauthenticated use such as system consoles, operator terminals and terminals dedicated to special tasks), and individual microcomputers and workstations, especially those connected to your network. Make sure people's work areas mesh well with access restrictions; otherwise they will find ways to circumvent your physical security (e.g., jamming doors open).
Keep original and backup copies of data and programs safe. Apart from keeping them in good condition for backup purposes, they must be protected from theft. It is important to keep backups in a separate location from the originals, not only for damage considerations, but also to guard against thefts.
Portable hosts are a particular risk. Make sure it won't cause problems if one of your staff's portable computer is stolen. Consider developing guidelines for the kinds of data that should be allowed to reside on the disks of portable computers as well as how the data should be protected (e.g., encryption) when it is on a portable computer.
Other areas where physical access should be restricted is the wiring closets and important network elements like file servers, name server hosts, and routers.
By "walk-up" connections, we mean network connection points located to provide a convenient way for users to connect a portable host to your network.
Consider whether you need to provide this service, bearing in mind that it allows any user to attach an unauthorized host to your network. This increases the risk of attacks via techniques such as
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IP address spoofing, packet sniffing, etc. Users and site management must appreciate the risks involved. If you decide to provide walk-up connections, plan the service carefully and define precisely where you will provide it so that you can ensure the necessary physical access security.
A walk-up host should be authenticated before its user is permitted to access resources on your network. As an alternative, it may be possible to control physical access. For example, if the service is to be used by students, you might only provide walk-up connection sockets in student laboratories.
If you are providing walk-up access for visitors to connect back to their home networks (e.g., to read e-mail, etc.) in your facility, consider using a separate subnet that has no connectivity to the internal network.
Keep an eye on any area that contains unmonitored access to the network, such as vacant offices. It may be sensible to disconnect such areas at the wiring closet, and consider using secure hubs and monitoring attempts to connect unauthorized hosts.
Technologies considered here include X.25, ISDN, SMDS, DDS and Frame Relay. All are provided via physical links which go through telephone exchanges, providing the potential for them to be diverted. Crackers are certainly interested in telephone switches as well as in data networks!
With switched technologies, use Permanent Virtual Circuits or Closed User Groups whenever this is possible. Technologies which provide authentication and/or encryption (such as IPv6) are evolving rapidly; consider using them on links where security is important.
Although they provide convenient access to a site for its users, they can also provide an effective detour around the site's firewalls. For this reason it is essential to maintain proper control of modems.
Don't allow users to install a modem line without proper authorization. This includes temporary installations (e.g., plugging a modem into a facsimile or telephone line overnight).
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Maintain a register of all your modem lines and keep your register up to date. Conduct regular (ideally automated) site checks for unauthorized modems.
A username and password check should be completed before a user can access anything on your network. Normal password security considerations are particularly important (see section 4.1.1).
Remember that telephone lines can be tapped, and that it is quite easy to intercept messages to cellular phones. Modern high-speed modems use more sophisticated modulation techniques, which makes them somewhat more difficult to monitor, but it is prudent to assume that hackers know how to eavesdrop on your lines. For this reason, you should use one-time passwords if at all possible.
It is helpful to have a single dial-in point (e.g., a single large modem pool) so that all users are authenticated in the same way.
Users will occasionally mis-type a password. Set a short delay - say two seconds - after the first and second failed logins, and force a disconnect after the third. This will slow down automated password attacks. Don't tell the user whether the username, the password, or both, were incorrect.
Some dial-in servers offer call-back facilities (i.e., the user dials in and is authenticated, then the system disconnects the call and calls back on a specified number). Call-back is useful since if someone were to guess a username and password, they are disconnected, and the system then calls back the actual user whose password was cracked; random calls from a server are suspicious, at best. This does mean users may only log in from one location (where the server is configured to dial them back), and of course there may be phone charges associated with there call-back location.
This feature should be used with caution; it can easily be bypassed. At a minimum, make sure that the return call is never made from the same modem as the incoming one. Overall, although call-back can improve modem security, you should not depend on it alone.
All logins, whether successful or unsuccessful should be logged. However, do not keep correct passwords in the log. Rather, log them simply as a successful login attempt. Since most bad passwords are
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mistyped by authorized users, they only vary by a single character from the actual password. Therefore if you can't keep such a log secure, don't log it at all.
If Calling Line Identification is available, take advantage of it by recording the calling number for each login attempt. Be sensitive to the privacy issues raised by Calling Line Identification. Also be aware that Calling Line Identification is not to be trusted (since intruders have been known to break into phone switches and forward phone numbers or make other changes); use the data for informational purposes only, not for authentication.
Many sites use a system default contained in a message of the day file for their opening banner. Unfortunately, this often includes the type of host hardware or operating system present on the host. This can provide valuable information to a would-be intruder. Instead, each site should create its own specific login banner, taking care to only include necessary information.
Display a short banner, but don't offer an "inviting" name (e.g., University of XYZ, Student Records System). Instead, give your site name, a short warning that sessions may be monitored, and a username/password prompt. Verify possible legal issues related to the text you put into the banner.
For high-security applications, consider using a "blind" password (i.e., give no response to an incoming call until the user has typed in a password). This effectively simulates a dead modem.
Dial-out users should also be authenticated, particularly since your site will have to pay their telephone charges.
Never allow dial-out from an unauthenticated dial-in call, and consider whether you will allow it from an authenticated one. The goal here is to prevent callers using your modem pool as part of a chain of logins. This can be hard to detect, particularly if a hacker sets up a path through several hosts on your site.
At a minimum, don't allow the same modems and phone lines to be used for both dial-in and dial-out. This can be implemented easily if you run separate dial-in and dial-out modem pools.
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4.5.4.7 Make Your Modem Programming as "Bullet-proof" as Possible
Be sure modems can't be reprogrammed while they're in service. At a minimum, make sure that three plus signs won't put your dial-in modems into command mode!
Program your modems to reset to your standard configuration at the start of each new call. Failing this, make them reset at the end of each call. This precaution will protect you against accidental reprogramming of your modems. Resetting at both the end and the beginning of each call will assure an even higher level of confidence that a new caller will not inherit a previous caller's session.
Check that your modems terminate calls cleanly. When a user logs out from an access server, verify that the server hangs up the phone line properly. It is equally important that the server forces logouts from whatever sessions were active if the user hangs up unexpectedly.
This section covers the procedures for collecting data generated by network activity, which may be useful in analyzing the security of a network and responding to security incidents.
Audit data should include any attempt to achieve a different security level by any person, process, or other entity in the network. This includes login and logout, super user access (or the non-UNIX equivalent), ticket generation (for Kerberos, for example), and any other change of access or status. It is especially important to note "anonymous" or "guest" access to public servers.
The actual data to collect will differ for different sites and for different types of access changes within a site. In general, the information you want to collect includes: username and hostname, for login and logout; previous and new access rights, for a change of access rights; and a timestamp. Of course, there is much more information which might be gathered, depending on what the system makes available and how much space is available to store that information.
One very important note: do not gather passwords. This creates an enormous potential security breach if the audit records should be improperly accessed. Do not gather incorrect passwords either, as they often differ from valid passwords by only a single character or transposition.
The collection process should be enacted by the host or resource being accessed. Depending on the importance of the data and the need to have it local in instances in which services are being denied, data could be kept local to the resource until needed or be transmitted to storage after each event.
There are basically three ways to store audit records: in a read/write file on a host, on a write-once/read-many device (e.g., a CD-ROM or a specially configured tape drive), or on a write-only device (e.g., a line printer). Each method has advantages and disadvantages.
File system logging is the least resource intensive of the three methods and the easiest to configure. It allows instant access to the records for analysis, which may be important if an attack is in progress. File system logging is also the least reliable method. If the logging host has been compromised, the file system is usually the first thing to go; an intruder could easily cover up traces of the intrusion.
Collecting audit data on a write-once device is slightly more effort to configure than a simple file, but it has the significant advantage of greatly increased security because an intruder could not alter the data showing that an intrusion has occurred. The disadvantage of this method is the need to maintain a supply of storage media and the cost of that media. Also, the data may not be instantly available.
Line printer logging is useful in system where permanent and immediate logs are required. A real time system is an example of this, where the exact point of a failure or attack must be recorded. A laser printer, or other device which buffers data (e.g., a print server), may suffer from lost data if buffers contain the needed data at a critical instant. The disadvantage of, literally, "paper trails" is the need to keep the printer fed and the need to scan records by hand. There is also the issue of where to store the, potentially, enormous volume of paper which may be generated.
For each of the logging methods described, there is also the issue of securing the path between the device generating the log and actual logging device (i.e., the file server, tape/CD-ROM drive, printer). If that path is compromised, logging can be stopped or spoofed or both. In an ideal world, the logging device would be directly
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attached by a single, simple, point-to-point cable. Since that is usually impractical, the path should pass through the minimum number of networks and routers. Even if logs can be blocked, spoofing can be prevented with cryptographic checksums (it probably isn't necessary to encrypt the logs because they should not contain sensitive information in the first place).
Collecting audit data may result in a rapid accumulation of bytes so storage availability for this information must be considered in advance. There are a few ways to reduce the required storage space. First, data can be compressed, using one of many methods. Or, the required space can be minimized by keeping data for a shorter period of time with only summaries of that data kept in long-term archives. One major drawback to the latter method involves incident response. Often, an incident has been ongoing for some period of time when a site notices it and begins to investigate. At that point in time, it's very helpful to have detailed audit logs available. If these are just summaries, there may not be sufficient detail to fully handle the incident.
Audit data should be some of the most carefully secured data at the site and in the backups. If an intruder were to gain access to audit logs, the systems themselves, in addition to the data, would be at risk.
Audit data may also become key to the investigation, apprehension, and prosecution of the perpetrator of an incident. For this reason, it is advisable to seek the advice of legal council when deciding how audit data should be treated. This should happen before an incident occurs.
If a data handling plan is not adequately defined prior to an incident, it may mean that there is no recourse in the aftermath of an event, and it may create liability resulting from improper treatment of the data.
Due to the content of audit data, there are a number of legal questions that arise which might need to be addressed by your legal counsel. If you collect and save audit data, you need to be prepared for consequences resulting both from its existence and its content.
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One area concerns the privacy of individuals. In certain instances, audit data may contain personal information. Searching through the data, even for a routine check of the system's security, could represent an invasion of privacy.
A second area of concern involves knowledge of intrusive behavior originating from your site. If an organization keeps audit data, is it responsible for examining it to search for incidents? If a host in one organization is used as a launching point for an attack against another organization, can the second organization use the audit data of the first organization to prove negligence on the part of that organization?
The above examples are meant to be comprehensive, but should motivate your organization to consider the legal issues involved with audit data.
The procedure of creating backups is a classic part of operating a computer system. Within the context of this document, backups are addressed as part of the overall security plan of a site. There are several aspects to backups that are important within this context:
(1) Make sure your site is creating backups (2) Make sure your site is using offsite storage for backups. The storage site should be carefully selected for both its security and its availability. (3) Consider encrypting your backups to provide additional protection of the information once it is off-site. However, be aware that you will need a good key management scheme so that you'll be able to recover data at any point in the future. Also, make sure you will have access to the necessary decryption programs at such time in the future as you need to perform the decryption. (4) Don't always assume that your backups are good. There have been many instances of computer security incidents that have gone on for long periods of time before a site has noticed the incident. In such cases, backups of the affected systems are also tainted. (5) Periodically verify the correctness and completeness of your backups.
This chapter of the document will supply guidance to be used before, during, and after a computer security incident occurs on a host, network, site, or multi-site environment. The operative philosophy in the event of a breach of computer security is to react according
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to a plan. This is true whether the breach is the result of an external intruder attack, unintentional damage, a student testing some new program to exploit a software vulnerability, or a disgruntled employee. Each of the possible types of events, such as those just listed, should be addressed in advance by adequate contingency plans.
Traditional computer security, while quite important in the overall site security plan, usually pays little attention to how to actually handle an attack once one occurs. The result is that when an attack is in progress, many decisions are made in haste and can be damaging to tracking down the source of the incident, collecting evidence to be used in prosecution efforts, preparing for the recovery of the system, and protecting the valuable data contained on the system.
One of the most important, but often overlooked, benefits for efficient incident handling is an economic one. Having both technical and managerial personnel respond to an incident requires considerable resources. If trained to handle incidents efficiently, less staff time is required when one occurs.
Due to the world-wide network most incidents are not restricted to a single site. Operating systems vulnerabilities apply (in some cases) to several millions of systems, and many vulnerabilities are exploited within the network itself. Therefore, it is vital that all sites with involved parties be informed as soon as possible.
Another benefit is related to public relations. News about computer security incidents tends to be damaging to an organization's stature among current or potential clients. Efficient incident handling minimizes the potential for negative exposure.
A final benefit of efficient incident handling is related to legal issues. It is possible that in the near future organizations may be held responsible because one of their nodes was used to launch a network attack. In a similar vein, people who develop patches or workarounds may be sued if the patches or workarounds are ineffective, resulting in compromise of the systems, or, if the patches or workarounds themselves damage systems. Knowing about operating system vulnerabilities and patterns of attacks, and then taking appropriate measures to counter these potential threats, is critical to circumventing possible legal problems.
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The sections in this chapter provide an outline and starting point for creating your site's policy for handling security incidents. The sections are:
(1) Preparing and planning (what are the goals and objectives in handling an incident). (2) Notification (who should be contacted in the case of an incident). - Local managers and personnel - Law enforcement and investigative agencies - Computer security incidents handling teams - Affected and involved sites - Internal communications - Public relations and press releases (3) Identifying an incident (is it an incident and how serious is it). (4) Handling (what should be done when an incident occurs). - Notification (who should be notified about the incident) - Protecting evidence and activity logs (what records should be kept from before, during, and after the incident) - Containment (how can the damage be limited) - Eradication (how to eliminate the reasons for the incident) - Recovery (how to reestablish service and systems) - Follow Up (what actions should be taken after the incident) (5) Aftermath (what are the implications of past incidents). (6) Administrative response to incidents.
The remainder of this chapter will detail the issues involved in each of the important topics listed above, and provide some guidance as to what should be included in a site policy for handling incidents.
Part of handling an incident is being prepared to respond to an incident before the incident occurs in the first place. This includes establishing a suitable level of protections as explained in the preceding chapters. Doing this should help your site prevent incidents as well as limit potential damage resulting from them when they do occur. Protection also includes preparing incident handling guidelines as part of a contingency plan for your organization or site. Having written plans eliminates much of the ambiguity which occurs during an incident, and will lead to a more appropriate and thorough set of responses. It is vitally important to test the proposed plan before an incident occurs through "dry runs". A team might even consider hiring a tiger team to act in parallel with the dry run. (Note: a tiger team is a team of specialists that try to penetrate the security of a system.)
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Learning to respond efficiently to an incident is important for a number of reasons:
(1) Protecting the assets which could be compromised (2) Protecting resources which could be utilized more profitably if an incident did not require their services (3) Complying with (government or other) regulations (4) Preventing the use of your systems in attacks against other systems (which could cause you to incur legal liability) (5) Minimizing the potential for negative exposure
As in any set of pre-planned procedures, attention must be paid to a set of goals for handling an incident. These goals will be prioritized differently depending on the site. A specific set of objectives can be identified for dealing with incidents:
(1) Figure out how it happened. (2) Find out how to avoid further exploitation of the same vulnerability. (3) Avoid escalation and further incidents. (4) Assess the impact and damage of the incident. (5) Recover from the incident. (6) Update policies and procedures as needed. (7) Find out who did it (if appropriate and possible).
Due to the nature of the incident, there might be a conflict between analyzing the original source of a problem and restoring systems and services. Overall goals (like assuring the integrity of critical systems) might be the reason for not analyzing an incident. Of course, this is an important management decision; but all involved parties must be aware that without analysis the same incident may happen again.
It is also important to prioritize the actions to be taken during an incident well in advance of the time an incident occurs. Sometimes an incident may be so complex that it is impossible to do everything at once to respond to it; priorities are essential. Although priorities will vary from institution to institution, the following suggested priorities may serve as a starting point for defining your organization's response:
(1) Priority one -- protect human life and people's safety; human life always has precedence over all other considerations.
(2) Priority two -- protect classified and/or sensitive data. Prevent exploitation of classified and/or sensitive systems, networks or sites. Inform affected
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classified and/or sensitive systems, networks or sites about already occurred penetrations. (Be aware of regulations by your site or by government)
(3) Priority three -- protect other data, including proprietary, scientific, managerial and other data, because loss of data is costly in terms of resources. Prevent exploitations of other systems, networks or sites and inform already affected systems, networks or sites about successful penetrations.
(4) Priority four -- prevent damage to systems (e.g., loss or alteration of system files, damage to disk drives, etc.). Damage to systems can result in costly down time and recovery.
(5) Priority five -- minimize disruption of computing resources (including processes). It is better in many cases to shut a system down or disconnect from a network than to risk damage to data or systems. Sites will have to evaluate the trade-offs between shutting down and disconnecting, and staying up. There may be service agreements in place that may require keeping systems up even in light of further damage occurring. However, the damage and scope of an incident may be so extensive that service agreements may have to be over-ridden.
An important implication for defining priorities is that once human life and national security considerations have been addressed, it is generally more important to save data than system software and hardware. Although it is undesirable to have any damage or loss during an incident, systems can be replaced. However, the loss or compromise of data (especially classified or proprietary data) is usually not an acceptable outcome under any circumstances.
Another important concern is the effect on others, beyond the systems and networks where the incident occurs. Within the limits imposed by government regulations it is always important to inform affected parties as soon as possible. Due to the legal implications of this topic, it should be included in the planned procedures to avoid further delays and uncertainties for the administrators.
Any plan for responding to security incidents should be guided by local policies and regulations. Government and private sites that deal with classified material have specific rules that they must follow.
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The policies chosen by your site on how it reacts to incidents will shape your response. For example, it may make little sense to create mechanisms to monitor and trace intruders if your site does not plan to take action against the intruders if they are caught. Other organizations may have policies that affect your plans. Telephone companies often release information about telephone traces only to law enforcement agencies.
Handling incidents can be tedious and require any number of routine tasks that could be handled by support personnel. To free the technical staff it may be helpful to identify support staff who will help with tasks like: photocopying, fax'ing, etc.
It is important to establish contacts with various personnel before a real incident occurs. Many times, incidents are not real emergencies. Indeed, often you will be able to handle the activities internally. However, there will also be many times when others outside your immediate department will need to be included in the incident handling. These additional contacts include local managers and system administrators, administrative contacts for other sites on the Internet, and various investigative organizations. Getting to know these contacts before incidents occurs will help to make your incident handling process more efficient.
For each type of communication contact, specific "Points of Contact" (POC) should be defined. These may be technical or administrative in nature and may include legal or investigative agencies as well as service providers and vendors. When establishing these contact, it is important to decide how much information will be shared with each class of contact. It is especially important to define, ahead of time, what information will be shared with the users at a site, with the public (including the press), and with other sites.
Settling these issues are especially important for the local person responsible for handling the incident, since that is the person responsible for the actual notification of others. A list of contacts in each of these categories is an important time saver for this person during an incident. It can be quite difficult to find an appropriate person during an incident when many urgent events are ongoing. It is strongly recommended that all relevant telephone numbers (also electronic mail addresses and fax numbers) be included in the site security policy. The names and contact information of all individuals who will be directly involved in the handling of an incident should be placed at the top of this list.
When an incident is under way, a major issue is deciding who is in charge of coordinating the activity of the multitude of players. A major mistake that can be made is to have a number of people who are each working independently, but are not working together. This will only add to the confusion of the event and will probably lead to wasted or ineffective effort.
The single POC may or may not be the person responsible for handling the incident. There are two distinct roles to fill when deciding who shall be the POC and who will be the person in charge of the incident. The person in charge of the incident will make decisions as to the interpretation of policy applied to the event. In contrast, the POC must coordinate the effort of all the parties involved with handling the event.
The POC must be a person with the technical expertise to successfully coordinate the efforts of the system managers and users involved in monitoring and reacting to the attack. Care should be taken when identifying who this person will be. It should not necessarily be the same person who has administrative responsibility for the compromised systems since often such administrators have knowledge only sufficient for the day to day use of the computers, and lack in depth technical expertise.
Another important function of the POC is to maintain contact with law enforcement and other external agencies to assure that multi-agency involvement occurs. The level of involvement will be determined by management decisions as well as legal constraints.
A single POC should also be the single person in charge of collecting evidence, since as a rule of thumb, the more people that touch a potential piece of evidence, the greater the possibility that it will be inadmissible in court. To ensure that evidence will be acceptable to the legal community, collecting evidence should be done following predefined procedures in accordance with local laws and legal regulations.
One of the most critical tasks for the POC is the coordination of all relevant processes. Responsibilities may be distributed over the whole site, involving multiple independent departments or groups. This will require a well coordinated effort in order to achieve overall success. The situation becomes even more complex if multiple sites are involved. When this happens, rarely will a single POC at one site be able to adequately coordinate the handling of the entire incident. Instead, appropriate incident response teams should be involved.
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The incident handling process should provide some escalation mechanisms. In order to define such a mechanism, sites will need to create an internal classification scheme for incidents. Associated with each level of incident will be the appropriate POC and procedures. As an incident is escalated, there may be a change in the POC which will need to be communicated to all others involved in handling the incident. When a change in the POC occurs, old POC should brief the new POC in all background information.
Lastly, users must know how to report suspected incidents. Sites should establish reporting procedures that will work both during and outside normal working hours. Help desks are often used to receive these reports during normal working hours, while beepers and telephones can be used for out of hours reporting.
In the event of an incident that has legal consequences, it is important to establish contact with investigative agencies (e.g, the FBI and Secret Service in the U.S.) as soon as possible. Local law enforcement, local security offices, and campus police departments should also be informed as appropriate. This section describes many of the issues that will be confronted, but it is acknowledged that each organization will have its own local and governmental laws and regulations that will impact how they interact with law enforcement and investigative agencies. The most important point to make is that each site needs to work through these issues.
A primary reason for determining these point of contact well in advance of an incident is that once a major attack is in progress, there is little time to call these agencies to determine exactly who the correct point of contact is. Another reason is that it is important to cooperate with these agencies in a manner that will foster a good working relationship, and that will be in accordance with the working procedures of these agencies. Knowing the working procedures in advance, and the expectations of your point of contact is a big step in this direction. For example, it is important to gather evidence that will be admissible in any subsequent legal proceedings, and this will require prior knowledge of how to gather such evidence. A final reason for establishing contacts as soon as possible is that it is impossible to know the particular agency that will assume jurisdiction in any given incident. Making contacts and finding the proper channels early on will make responding to an incident go considerably more smoothly.
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If your organization or site has a legal counsel, you need to notify this office soon after you learn that an incident is in progress. At a minimum, your legal counsel needs to be involved to protect the legal and financial interests of your site or organization. There are many legal and practical issues, a few of which are:
(1) Whether your site or organization is willing to risk negative publicity or exposure to cooperate with legal prosecution efforts.
(2) Downstream liability--if you leave a compromised system as is so it can be monitored and another computer is damaged because the attack originated from your system, your site or organization may be liable for damages incurred.
(3) Distribution of information--if your site or organization distributes information about an attack in which another site or organization may be involved or the vulnerability in a product that may affect ability to market that product, your site or organization may again be liable for any damages (including damage of reputation).
(4) Liabilities due to monitoring--your site or organization may be sued if users at your site or elsewhere discover that your site is monitoring account activity without informing users.
Unfortunately, there are no clear precedents yet on the liabilities or responsibilities of organizations involved in a security incident or who might be involved in supporting an investigative effort. Investigators will often encourage organizations to help trace and monitor intruders. Indeed, most investigators cannot pursue computer intrusions without extensive support from the organizations involved. However, investigators cannot provide protection from liability claims, and these kinds of efforts may drag out for months and may take a lot of effort.
On the other hand, an organization's legal council may advise extreme caution and suggest that tracing activities be halted and an intruder shut out of the system. This, in itself, may not provide protection from liability, and may prevent investigators from identifying the perpetrator.
The balance between supporting investigative activity and limiting liability is tricky. You'll need to consider the advice of your legal counsel and the damage the intruder is causing (if any) when making your decision about what to do during any particular incident.
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Your legal counsel should also be involved in any decision to contact investigative agencies when an incident occurs at your site. The decision to coordinate efforts with investigative agencies is most properly that of your site or organization. Involving your legal counsel will also foster the multi-level coordination between your site and the particular investigative agency involved, which in turn results in an efficient division of labor. Another result is that you are likely to obtain guidance that will help you avoid future legal mistakes.
Finally, your legal counsel should evaluate your site's written procedures for responding to incidents. It is essential to obtain a "clean bill of health" from a legal perspective before you actually carry out these procedures.
It is vital, when dealing with investigative agencies, to verify that the person who calls asking for information is a legitimate representative from the agency in question. Unfortunately, many well intentioned people have unknowingly leaked sensitive details about incidents, allowed unauthorized people into their systems, etc., because a caller has masqueraded as a representative of a government agency. (Note: this word of caution actually applies to all external contacts.)
A similar consideration is using a secure means of communication. Because many network attackers can easily re-route electronic mail, avoid using electronic mail to communicate with other agencies (as well as others dealing with the incident at hand). Non-secured phone lines (the phones normally used in the business world) are also frequent targets for tapping by network intruders, so be careful!
There is no one established set of rules for responding to an incident when the local government becomes involved. Normally (in the U.S.), except by legal order, no agency can force you to monitor, to disconnect from the network, to avoid telephone contact with the suspected attackers, etc. Each organization will have a set of local and national laws and regulations that must be adhered to when handling incidents. It is recommended that each site be familiar with those laws and regulations, and identify and get know the contacts for agencies with jurisdiction well in advance of handling an incident.
There are currently a number of of Computer Security Incident Response teams (CSIRTs) such as the CERT Coordination Center, the German DFN-CERT, and other teams around the globe. Teams exist for many major government agencies and large corporations. If such a
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team is available, notifying it should be of primary consideration during the early stages of an incident. These teams are responsible for coordinating computer security incidents over a range of sites and larger entities. Even if the incident is believed to be contained within a single site, it is possible that the information available through a response team could help in fully resolving the incident.
If it is determined that the breach occurred due to a flaw in the system's hardware or software, the vendor (or supplier) and a Computer Security Incident Handling team should be notified as soon as possible. This is especially important because many other systems are vulnerable, and these vendor and response team organizations can help disseminate help to other affected sites.
In setting up a site policy for incident handling, it may be desirable to create a subgroup, much like those teams that already exist, that will be responsible for handling computer security incidents for the site (or organization). If such a team is created, it is essential that communication lines be opened between this team and other teams. Once an incident is under way, it is difficult to open a trusted dialogue between other teams if none has existed before.
If an incident has an impact on other sites, it is good practice to inform them. It may be obvious from the beginning that the incident is not limited to the local site, or it may emerge only after further analysis.
Each site may choose to contact other sites directly or they can pass the information to an appropriate incident response team. It is often very difficult to find the responsible POC at remote sites and the incident response team will be able to facilitate contact by making use of already established channels.
The legal and liability issues arising from a security incident will differ from site to site. It is important to define a policy for the sharing and logging of information about other sites before an incident occurs.
Information about specific people is especially sensitive, and may be subject to privacy laws. To avoid problems in this area, irrelevant information should be deleted and a statement of how to handle the remaining information should be included. A clear statement of how this information is to be used is essential. No one who informs a site of a security incident wants to read about it in the public
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press. Incident response teams are valuable in this respect. When they pass information to responsible POCs, they are able to protect the anonymity of the original source. But, be aware that, in many cases, the analysis of logs and information at other sites will reveal addresses of your site.
All the problems discussed above should be not taken as reasons not to involve other sites. In fact, the experiences of existing teams reveal that most sites informed about security problems are not even aware that their site had been compromised. Without timely information, other sites are often unable to take action against intruders.
It is crucial during a major incident to communicate why certain actions are being taken, and how the users (or departments) are expected to behave. In particular, it should be made very clear to users what they are allowed to say (and not say) to the outside world (including other departments). For example, it wouldn't be good for an organization if users replied to customers with something like, "I'm sorry the systems are down, we've had an intruder and we are trying to clean things up." It would be much better if they were instructed to respond with a prepared statement like, "I'm sorry our systems are unavailable, they are being maintained for better service in the future."
Communications with customers and contract partners should be handled in a sensible, but sensitive way. One can prepare for the main issues by preparing a checklist. When an incident occurs, the checklist can be used with the addition of a sentence or two for the specific circumstances of the incident.
Public relations departments can be very helpful during incidents. They should be involved in all planning and can provide well constructed responses for use when contact with outside departments and organizations is necessary.
There has been a tremendous growth in the amount of media coverage dedicated to computer security incidents in the United States. Such press coverage is bound to extend to other countries as the Internet continues to grow and expand internationally. Readers from countries where such media attention has not yet occurred, can learn from the experiences in the U.S. and should be forwarned and prepared.
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One of the most important issues to consider is when, who, and how much to release to the general public through the press. There are many issues to consider when deciding this particular issue. First and foremost, if a public relations office exists for the site, it is important to use this office as liaison to the press. The public relations office is trained in the type and wording of information released, and will help to assure that the image of the site is protected during and after the incident (if possible). A public relations office has the advantage that you can communicate candidly with them, and provide a buffer between the constant press attention and the need of the POC to maintain control over the incident.
If a public relations office is not available, the information released to the press must be carefully considered. If the information is sensitive, it may be advantageous to provide only minimal or overview information to the press. It is quite possible that any information provided to the press will be quickly reviewed by the perpetrator of the incident. Also note that misleading the press can often backfire and cause more damage than releasing sensitive information.
While it is difficult to determine in advance what level of detail to provide to the press, some guidelines to keep in mind are:
(1) Keep the technical level of detail low. Detailed information about the incident may provide enough information for others to launch similar attacks on other sites, or even damage the site's ability to prosecute the guilty party once the event is over.
(2) Keep the speculation out of press statements. Speculation of who is causing the incident or the motives are very likely to be in error and may cause an inflamed view of the incident.
(3) Work with law enforcement professionals to assure that evidence is protected. If prosecution is involved, assure that the evidence collected is not divulged to the press.
(4) Try not to be forced into a press interview before you are prepared. The popular press is famous for the "2 am" interview, where the hope is to catch the interviewee off guard and obtain information otherwise not available.
(5) Do not allow the press attention to detract from the handling of the event. Always remember that the successful closure of an incident is of primary importance.
This stage involves determining if a problem really exists. Of course many if not most signs often associated with virus infection, system intrusions, malicious users, etc., are simply anomalies such as hardware failures or suspicious system/user behavior. To assist in identifying whether there really is an incident, it is usually helpful to obtain and use any detection software which may be available. Audit information is also extremely useful, especially in determining whether there is a network attack. It is extremely important to obtain a system snapshot as soon as one suspects that something is wrong. Many incidents cause a dynamic chain of events to occur, and an initial system snapshot may be the most valuable tool for identifying the problem and any source of attack. Finally, it is important to start a log book. Recording system events, telephone conversations, time stamps, etc., can lead to a more rapid and systematic identification of the problem, and is the basis for subsequent stages of incident handling.
There are certain indications or "symptoms" of an incident that deserve special attention:
(1) System crashes. (2) New user accounts (the account RUMPLESTILTSKIN has been unexpectedly created), or high activity on a previously low usage account. (3) New files (usually with novel or strange file names, such as data.xx or k or .xx ). (4) Accounting discrepancies (in a UNIX system you might notice the shrinking of an accounting file called /usr/admin/lastlog, something that should make you very suspicious that there may be an intruder). (5) Changes in file lengths or dates (a user should be suspicious if .EXE files in an MS DOS computer have unexplainedly grown by over 1800 bytes). (6) Attempts to write to system (a system manager notices that a privileged user in a VMS system is attempting to alter RIGHTSLIST.DAT). (7) Data modification or deletion (files start to disappear). (8) Denial of service (a system manager and all other users become locked out of a UNIX system, now in single user mode). (9) Unexplained, poor system performance (10) Anomalies ("GOTCHA" is displayed on the console or there are frequent unexplained "beeps"). (11) Suspicious probes (there are numerous unsuccessful login attempts from another node).
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(12) Suspicious browsing (someone becomes a root user on a UNIX system and accesses file after file on many user accounts.) (13) Inability of a user to log in due to modifications of his/her account.
By no means is this list comprehensive; we have just listed a number of common indicators. It is best to collaborate with other technical and computer security personnel to make a decision as a group about whether an incident is occurring.
Along with the identification of the incident is the evaluation of the scope and impact of the problem. It is important to correctly identify the boundaries of the incident in order to effectively deal with it and prioritize responses.
In order to identify the scope and impact a set of criteria should be defined which is appropriate to the site and to the type of connections available. Some of the issues include:
(1) Is this a multi-site incident? (2) Are many computers at your site affected by this incident? (3) Is sensitive information involved? (4) What is the entry point of the incident (network, phone line, local terminal, etc.)? (5) Is the press involved? (6) What is the potential damage of the incident? (7) What is the estimated time to close out the incident? (8) What resources could be required to handle the incident? (9) Is law enforcement involved?
The analysis of the damage and extent of the incident can be quite time consuming, but should lead to some insight into the nature of the incident, and aid investigation and prosecution. As soon as the breach has occurred, the entire system and all of its components should be considered suspect. System software is the most probable target. Preparation is key to be able to detect all changes for a possibly tainted system. This includes checksumming all media from the vendor using a algorithm which is resistant to tampering. (See sections 4.3)
Assuming original vendor distribution media are available, an analysis of all system files should commence, and any irregularities should be noted and referred to all parties involved in handling the incident. It can be very difficult, in some cases, to decide which
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backup media are showing a correct system status. Consider, for example, that the incident may have continued for months or years before discovery, and the suspect may be an employee of the site, or otherwise have intimate knowledge or access to the systems. In all cases, the pre-incident preparation will determine what recovery is possible.
If the system supports centralized logging (most do), go back over the logs and look for abnormalities. If process accounting and connect time accounting is enabled, look for patterns of system usage. To a lesser extent, disk usage may shed light on the incident. Accounting can provide much helpful information in an analysis of an incident and subsequent prosecution. Your ability to address all aspects of a specific incident strongly depends on the success of this analysis.
Certain steps are necessary to take during the handling of an incident. In all security related activities, the most important point to be made is that all sites should have policies in place. Without defined policies and goals, activities undertaken will remain without focus. The goals should be defined by management and legal counsel in advance.
One of the most fundamental objectives is to restore control of the affected systems and to limit the impact and damage. In the worst case scenario, shutting down the system, or disconnecting the system from the network, may the only practical solution.
As the activities involved are complex, try to get as much help as necessary. While trying to solve the problem alone, real damage might occur due to delays or missing information. Most administrators take the discovery of an intruder as a personal challenge. By proceeding this way, other objectives as outlined in the local policies may not always be considered. Trying to catch intruders may be a very low priority, compared to system integrity, for example. Monitoring a hacker's activity is useful, but it might not be considered worth the risk to allow the continued access.
5.4.1 Types of Notification and Exchange of Information
When you have confirmed that an incident is occurring, the appropriate personnel must be notified. How this notification is achieved is very important to keeping the event under control both from a technical and emotional standpoint. The circumstances should be described in as much detail as possible, in order to aid prompt acknowledgment and understanding of the problem. Great care should
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be taken when determining to which groups detailed technical information is given during the notification. For example, it is helpful to pass this kind of information to an incident handling team as they can assist you by providing helpful hints for eradicating the vulnerabilities involved in an incident. On the other hand, putting the critical knowledge into the public domain (e.g., via USENET newsgroups or mailing lists) may potentially put a large number of systems at risk of intrusion. It is invalid to assume that all administrators reading a particular newsgroup have access to operating system source code, or can even understand an advisory well enough to take adequate steps.
First of all, any notification to either local or off-site personnel must be explicit. This requires that any statement (be it an electronic mail message, phone call, fax, beeper, or semaphone) providing information about the incident be clear, concise, and fully qualified. When you are notifying others that will help you handle an event, a "smoke screen" will only divide the effort and create confusion. If a division of labor is suggested, it is helpful to provide information to each participant about what is being accomplished in other efforts. This will not only reduce duplication of effort, but allow people working on parts of the problem to know where to obtain information relevant to their part of the incident.
Another important consideration when communicating about the incident is to be factual. Attempting to hide aspects of the incident by providing false or incomplete information may not only prevent a successful resolution to the incident, but may even worsen the situation.
The choice of language used when notifying people about the incident can have a profound effect on the way that information is received. When you use emotional or inflammatory terms, you raise the potential for damage and negative outcomes of the incident. It is important to remain calm both in written and spoken communications.
Another consideration is that not all people speak the same language. Due to this fact, misunderstandings and delay may arise, especially if it is a multi-national incident. Other international concerns include differing legal implications of a security incident and cultural differences. However, cultural differences do not only exist between countries. They even exist within countries, between different social or user groups. For example, an administrator of a university system might be very relaxed about attempts to connect to the system via telnet, but the administrator of a military system is likely to consider the same action as a possible attack.
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Another issue associated with the choice of language is the notification of non-technical or off-site personnel. It is important to accurately describe the incident without generating undue alarm or confusion. While it is more difficult to describe the incident to a non-technical audience, it is often more important. A non-technical description may be required for upper-level management, the press, or law enforcement liaisons. The importance of these communications cannot be underestimated and may make the difference between resolving the incident properly and escalating to some higher level of damage.
If an incident response team becomes involved, it might be necessary to fill out a template for the information exchange. Although this may seem to be an additional burden and adds a certain delay, it helps the team to act on this minimum set of information. The response team may be able to respond to aspects of the incident of which the local administrator is unaware. If information is given out to someone else, the following minimum information should be provided:
(1) timezone of logs, ... in GMT or local time (2) information about the remote system, including host names, IP addresses and (perhaps) user IDs (3) all log entries relevant for the remote site (4) type of incident (what happened, why should you care)
If local information (i.e., local user IDs) is included in the log entries, it will be necessary to sanitize the entries beforehand to avoid privacy issues. In general, all information which might assist a remote site in resolving an incident should be given out, unless local policies prohibit this.
When you respond to an incident, document all details related to the incident. This will provide valuable information to yourself and others as you try to unravel the course of events. Documenting all details will ultimately save you time. If you don't document every relevant phone call, for example, you are likely to forget a significant portion of information you obtain, requiring you to contact the source of information again. At the same time, recording details will provide evidence for prosecution efforts, providing the case moves in that direction. Documenting an incident will also help you perform a final assessment of damage (something your management, as well as law enforcement officers, will want to know), and will provide the basis for later phases of the handling process: eradication, recovery, and follow-up "lessons learned."
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During the initial stages of an incident, it is often infeasible to determine whether prosecution is viable, so you should document as if you are gathering evidence for a court case. At a minimum, you should record:
(1) all system events (audit records) (2) all actions you take (time tagged) (3) all external conversations (including the person with whom you talked, the date and time, and the content of the conversation)
The most straightforward way to maintain documentation is keeping a log book. This allows you to go to a centralized, chronological source of information when you need it, instead of requiring you to page through individual sheets of paper. Much of this information is potential evidence in a court of law. Thus, when a legal follow-up is a possibility, one should follow the prepared procedures and avoid jeopardizing the legal follow-up by improper handling of possible evidence. If appropriate, the following steps may be taken.
(1) Regularly (e.g., every day) turn in photocopied, signed copies of your logbook (as well as media you use to record system events) to a document custodian. (2) The custodian should store these copied pages in a secure place (e.g., a safe). (3) When you submit information for storage, you should receive a signed, dated receipt from the document custodian.
Failure to observe these procedures can result in invalidation of any evidence you obtain in a court of law.
The purpose of containment is to limit the extent of an attack. An essential part of containment is decision making (e.g., determining whether to shut a system down, disconnect from a network, monitor system or network activity, set traps, disable functions such as remote file transfer, etc.).
Sometimes this decision is trivial; shut the system down if the information is classified, sensitive, or proprietary. Bear in mind that removing all access while an incident is in progress obviously notifies all users, including the alleged problem users, that the administrators are aware of a problem; this may have a deleterious
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effect on an investigation. In some cases, it is prudent to remove all access or functionality as soon as possible, then restore normal operation in limited stages. In other cases, it is worthwhile to risk some damage to the system if keeping the system up might enable you to identify an intruder.
This stage should involve carrying out predetermined procedures. Your organization or site should, for example, define acceptable risks in dealing with an incident, and should prescribe specific actions and strategies accordingly. This is especially important when a quick decision is necessary and it is not possible to first contact all involved parties to discuss the decision. In the absence of predefined procedures, the person in charge of the incident will often not have the power to make difficult management decisions (like to lose the results of a costly experiment by shutting down a system). A final activity that should occur during this stage of incident handling is the notification of appropriate authorities.
Once the incident has been contained, it is time to eradicate the cause. But before eradicating the cause, great care should be taken to collect all necessary information about the compromised system(s) and the cause of the incident as they will likely be lost when cleaning up the system.
Software may be available to help you in the eradication process, such as anti-virus software. If any bogus files have been created, archive them before deleting them. In the case of virus infections, it is important to clean and reformat any media containing infected files. Finally, ensure that all backups are clean. Many systems infected with viruses become periodically re-infected simply because people do not systematically eradicate the virus from backups. After eradication, a new backup should be taken.
Removing all vulnerabilities once an incident has occurred is difficult. The key to removing vulnerabilities is knowledge and understanding of the breach.
It may be necessary to go back to the original distribution media and re-customize the system. To facilitate this worst case scenario, a record of the original system setup and each customization change should be maintained. In the case of a network-based attack, it is important to install patches for each operating system vulnerability which was exploited.
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As discussed in section 5.4.2, a security log can be most valuable during this phase of removing vulnerabilities. The logs showing how the incident was discovered and contained can be used later to help determine how extensive the damage was from a given incident. The steps taken can be used in the future to make sure the problem does not resurface. Ideally, one should automate and regularly apply the same test as was used to detect the security incident.
If a particular vulnerability is isolated as having been exploited, the next step is to find a mechanism to protect your system. The security mailing lists and bulletins would be a good place to search for this information, and you can get advice from incident response teams.
Once the cause of an incident has been eradicated, the recovery phase defines the next stage of action. The goal of recovery is to return the system to normal. In general, bringing up services in the order of demand to allow a minimum of user inconvenience is the best practice. Understand that the proper recovery procedures for the system are extremely important and should be specific to the site.
Once you believe that a system has been restored to a "safe" state, it is still possible that holes, and even traps, could be lurking in the system. One of the most important stages of responding to incidents is also the most often omitted, the follow-up stage. In the follow-up stage, the system should be monitored for items that may have been missed during the cleanup stage. It would be prudent to utilize some of the tools mentioned in chapter 7 as a start. Remember, these tools don't replace continual system monitoring and good systems administration practices.
The most important element of the follow-up stage is performing a postmortem analysis. Exactly what happened, and at what times? How well did the staff involved with the incident perform? What kind of information did the staff need quickly, and how could they have gotten that information as soon as possible? What would the staff do differently next time?
After an incident, it is prudent to write a report describing the exact sequence of events: the method of discovery, correction procedure, monitoring procedure, and a summary of lesson learned. This will aid in the clear understanding of the problem. Creating a formal chronology of events (including time stamps) is also important for legal reasons.
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A follow-up report is valuable for many reasons. It provides a reference to be used in case of other similar incidents. It is also important to, as quickly as possible obtain a monetary estimate of the amount of damage the incident caused. This estimate should include costs associated with any loss of software and files (especially the value of proprietary data that may have been disclosed), hardware damage, and manpower costs to restore altered files, reconfigure affected systems, and so forth. This estimate may become the basis for subsequent prosecution activity. The report can also help justify an organization's computer security effort to management.
In the wake of an incident, several actions should take place. These actions can be summarized as follows:
(1) An inventory should be taken of the systems' assets, (i.e., a careful examination should determine how the system was affected by the incident).
(2) The lessons learned as a result of the incident should be included in revised security plan to prevent the incident from re-occurring.
(3) A new risk analysis should be developed in light of the incident.
(4) An investigation and prosecution of the individuals who caused the incident should commence, if it is deemed desirable.
If an incident is based on poor policy, and unless the policy is changed, then one is doomed to repeat the past. Once a site has recovered from and incident, site policy and procedures should be reviewed to encompass changes to prevent similar incidents. Even without an incident, it would be prudent to review policies and procedures on a regular basis. Reviews are imperative due to today's changing computing environments.
The whole purpose of this post mortem process is to improve all security measures to protect the site against future attacks. As a result of an incident, a site or organization should gain practical knowledge from the experience. A concrete goal of the post mortem is to develop new proactive methods. Another important facet of the aftermath may be end user and administrator education to prevent a reoccurrence of the security problem.
It is one thing to protect one's own network, but quite another to assume that one should protect other networks. During the handling of an incident, certain system vulnerabilities of one's own systems and the systems of others become apparent. It is quite easy and may even be tempting to pursue the intruders in order to track them. Keep in mind that at a certain point it is possible to "cross the line," and, with the best of intentions, become no better than the intruder.
The best rule when it comes to propriety is to not use any facility of remote sites which is not public. This clearly excludes any entry onto a system (such as a remote shell or login session) which is not expressly permitted. This may be very tempting; after a breach of security is detected, a system administrator may have the means to "follow it up," to ascertain what damage is being done to the remote site. Don't do it! Instead, attempt to reach the appropriate point of contact for the affected site.
During a security incident there are two choices one can make. First, a site can choose to watch the intruder in the hopes of catching him; or, the site can go about cleaning up after the incident and shut the intruder out of the systems. This is a decision that must be made very thoughtfully, as there may be legal liabilities if you choose to leave your site open, knowing that an intruder is using your site as a launching pad to reach out to other sites. Being a good Internet citizen means that you should try to alert other sites that may have been impacted by the intruder. These affected sites may be readily apparent after a thorough review of your log files.
When a security incident involves a user, the site's security policy should describe what action is to be taken. The transgression should be taken seriously, but it is very important to be sure of the role the user played. Was the user naive? Could there be a mistake in attributing the security breach to the user? Applying administrative action that assumes the user intentionally caused the incident may
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not be appropriate for a user who simply made a mistake. It may be appropriate to include sanctions more suitable for such a situation in your policies (e.g., education or reprimand of a user) in addition to more stern measures for intentional acts of intrusion and system misuse.
At this point in time, your site has hopefully developed a complete security policy and has developed procedures to assist in the configuration and management of your technology in support of those policies. How nice it would be if you could sit back and relax at this point and know that you were finished with the job of security. Unfortunately, that isn't possible. Your systems and networks are not a static environment, so you will need to review policies and procedures on a regular basis. There are a number of steps you can take to help you keep up with the changes around you so that you can initiate corresponding actions to address those changes. The following is a starter set and you may add others as appropriate for your site.
(1) Subscribe to advisories that are issued by various security incident response teams, like those of the CERT Coordination Center, and update your systems against those threats that apply to your site's technology.
(2) Monitor security patches that are produced by the vendors of your equipment, and obtain and install all that apply.
(3) Actively watch the configurations of your systems to identify any changes that may have occurred, and investigate all anomalies.
(4) Review all security policies and procedures annually (at a minimum).
(5) Read relevant mailing lists and USENET newsgroups to keep up to date with the latest information being shared by fellow administrators.
(6) Regularly check for compliance with policies and procedures. This audit should be performed by someone other than the people who define or implement the policies and procedures.
This chapter provides a brief list of publicly available security technology which can be downloaded from the Internet. Many of the items described below will undoubtedly be surpassed or made obsolete before this document is published.
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Some of the tools listed are applications such as end user programs (clients) and their supporting system infrastructure (servers). Others are tools that a general user will never see or need to use, but may be used by applications, or by administrators to troubleshoot security problems or to guard against intruders.
A sad fact is that there are very few security conscious applications currently available. Primarily, this is caused by the need for a security infrastructure which must first be put into place for most applications to operate securely. There is considerable effort currently taking place to build this infrastructure so that applications can take advantage of secure communications.
Most of the tools and applications described below can be found in one of the following archive sites:
It is important to note that many sites, including CERT and COAST are mirrored throughout the Internet. Be careful to use a "well known" mirror site to retrieve software, and to use verification tools (md5 checksums, etc.) to validate that software. A clever cracker might advertise security software that has intentionally been designed to provide access to data or systems.
Tools
COPS DES Drawbridge identd (not really a security tool) ISS Kerberos logdaemon lsof MD5 PEM PGP rpcbind/portmapper replacement SATAN sfingerd S/KEY smrsh
It would be impossible to list all of the mail-lists and other resources dealing with site security. However, these are some "jump- points" from which the reader can begin. All of these references are for the "INTERNET" constituency. More specific (vendor and geographical) resources can be found through these references.
Mailing Lists
(1) CERT(TM) Advisory Send mail to: cert-advisory-request@cert.org Message Body: subscribe cert <FIRST NAME> <LAST NAME>
A CERT advisory provides information on how to obtain a patch or details of a workaround for a known computer security problem. The CERT Coordination Center works with vendors to produce a workaround or a patch for a problem, and does not publish vulnerability information until a workaround or a patch is available. A CERT advisory may also be a warning to our constituency about ongoing attacks (e.g., "CA-91:18.Active.Internet.tftp.Attacks").
CERT advisories are also published on the USENET newsgroup: comp.security.announce
CERT advisory archives are available via anonymous FTP from info.cert.org in the /pub/cert_advisories directory.
(2) VIRUS-L List Send mail to: listserv%lehiibm1.bitnet@mitvma.mit.edu Message Body: subscribe virus-L FIRSTNAME LASTNAME
VIRUS-L is a moderated mailing list with a focus on computer virus issues. For more information, including a copy of the posting guidelines, see the file "virus-l.README", available by anonymous FTP from cs.ucr.edu.
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(3) Internet Firewalls Send mail to: majordomo@greatcircle.com Message Body: subscribe firewalls user@host
The Firewalls mailing list is a discussion forum for firewall administrators and implementors.
USENET newsgroups
(1) comp.security.announce The comp.security.announce newsgroup is moderated and is used solely for the distribution of CERT advisories.
(2) comp.security.misc The comp.security.misc is a forum for the discussion of computer security, especially as it relates to the UNIX(r) Operating System.
(3) alt.security The alt.security newsgroup is also a forum for the discussion of computer security, as well as other issues such as car locks and alarm systems.
(4) comp.virus The comp.virus newsgroup is a moderated newsgroup with a focus on computer virus issues. For more information, including a copy of the posting guidelines, see the file "virus-l.README", available via anonymous FTP on info.cert.org in the /pub/virus-l directory.
(5) comp.risks The comp.risks newsgroup is a moderated forum on the risks to the public in computers and related systems.
Computer Security Resource Clearinghouse. The main focus is on crisis response information; information on computer security-related threats, vulnerabilities, and solutions. At the same time, the Clearinghouse strives to be a general index to computer security information on a broad variety of subjects, including general risks, privacy, legal issues, viruses, assurance, policy, and training.
This Reference Index contains a list of links to information sources on Network and Computer Security. There is no implied fitness to the Tools, Techniques and Documents contained within this archive. Many if not all of these items work well, but we do not guarantee that this will be so. This information is for the education and legitimate use of computer security techniques only.
This page features general information about computer security. Information is organized by source and each section is organized by topic. Recent modifications are noted in What's New page.
This archive at the National Institute of Standards and Technology's Computer Security Resource Clearinghouse page contains a number of announcements, programs, and documents related to computer security.
* CERT and Tripwire are registered in the U.S. Patent and Trademark Office
The following references may not be available in all countries.
[Appelman, et. al., 1995] Appelman, Heller, Ehrman, White, and McAuliffe, "The Law and The Internet", USENIX 1995 Technical Conference on UNIX and Advanced Computing, New Orleans, LA, January 16-20, 1995.
[ABA, 1989] American Bar Association, Section of Science and Technology, "Guide to the Prosecution of Telecommunication Fraud by the Use of Computer Crime Statutes", American Bar Association, 1989.
[Aucoin, 1989] R. Aucoin, "Computer Viruses: Checklist for Recovery", Computers in Libraries, Vol. 9, No. 2, Pg. 4, February 1989.
[Barrett, 1996] D. Barrett, "Bandits on the Information Superhighway", O'Reilly & Associates, Sebastopol, CA, 1996.
[Bates, 1992] R. Bates, "Disaster Recovery Planning: Networks, Telecommunications and Data Communications", McGraw-Hill, 1992.
[Bellovin, 1989] S. Bellovin, "Security Problems in the TCP/IP Protocol Suite", Computer Communication Review, Vol 19, 2, pp. 32-48, April 1989.
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[Bellovin, 1990] S. Bellovin, and M. Merritt, "Limitations of the Kerberos Authentication System", Computer Communications Review, October 1990.
[Bellovin, 1992] S. Bellovin, "There Be Dragon", USENIX: Proceedings of the Third Usenix Security Symposium, Baltimore, MD. September, 1992.
[Bender, 1894] D. Bender, "Computer Law: Evidence and Procedure", M. Bender, New York, NY, 1978-present.
[Brand, 1990] R. Brand, "Coping with the Threat of Computer Security Incidents: A Primer from Prevention through Recovery", R. Brand, 8 June 1990.
[Brock, 1989] J. Brock, "November 1988 Internet Computer Virus and the Vulnerability of National Telecommunications Networks to Computer Viruses", GAO/T-IMTEC-89-10, Washington, DC, 20 July 1989.
[BS 7799] British Standard, BS Tech Cttee BSFD/12, Info. Sec. Mgmt, "BS 7799 : 1995 Code of Practice for Information Security Management", British Standards Institution, London, 54, Effective 15 February 1995.
[Caelli, 1988] W. Caelli, Editor, "Computer Security in the Age of Information", Proceedings of the Fifth IFIP International Conference on Computer Security, IFIP/Sec '88.
[Cavazos and Morin, 1995] E. Cavazos and G. Morin, "Cyber-Space and The Law", MIT Press, Cambridge, MA, 1995.
[CCH, 1989] Commerce Clearing House, "Guide to Computer Law", (Topical Law Reports), Chicago, IL., 1989.
[Chapman, 1992] B. Chapman, "Network(In) Security Through IP Packet Filtering", USENIX: Proceedings of the Third UNIX Security Symposium, Baltimore, MD, September 1992.
[Chapman and Zwicky, 1995] B. Chapman and E. Zwicky, "Building Internet Firewalls", O'Reilly and Associates, Sebastopol, CA, 1995.
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[Cheswick, 1990] B. Cheswick, "The Design of a Secure Internet Gateway", Proceedings of the Summer Usenix Conference, Anaheim, CA, June 1990.
[Cheswick1] W. Cheswick, "An Evening with Berferd In Which a Cracker is Lured, Endured, and Studied", AT&T Bell Laboratories.
[Cheswick and Bellovin, 1994] W. Cheswick and S. Bellovin, "Firewalls and Internet Security: Repelling the Wily Hacker", Addison-Wesley, Reading, MA, 1994.
[Conly, 1989] C. Conly, "Organizing for Computer Crime Investigation and Prosecution", U.S. Dept. of Justice, Office of Justice Programs, Under Contract Number OJP-86-C-002, National Institute of Justice, Washington, DC, July 1989.
[Cooper, 1989] J. Cooper, "Computer and Communications Security: Strategies for the 1990s", McGraw-Hill, 1989.
[CPSR, 1989] Computer Professionals for Social Responsibility, "CPSR Statement on the Computer Virus", CPSR, Communications of the ACM, Vol. 32, No. 6, Pg. 699, June 1989.
[CSC-STD-002-85, 1985] Department of Defense, "Password Management Guideline", CSC-STD-002-85, 12 April 1985, 31 pages.
[Curry, 1990] D. Curry, "Improving the Security of Your UNIX System", SRI International Report ITSTD-721-FR-90-21, April 1990.
[Curry, 1992] D. Curry, "UNIX System Security: A Guide for Users and Systems Administrators", Addision-Wesley, Reading, MA, 1992.
[DDN88] Defense Data Network, "BSD 4.2 and 4.3 Software Problem Resolution", DDN MGT Bulletin #43, DDN Network Information Center, 3 November 1988.
[DDN89] DCA DDN Defense Communications System, "DDN Security Bulletin 03", DDN Security Coordination Center, 17 October 1989.
[Denning, 1990] P. Denning, Editor, "Computers Under Attack: Intruders, Worms, and Viruses", ACM Press, 1990.
[Eichin and Rochlis, 1989] M. Eichin, and J. Rochlis, "With Microscope and Tweezers: An Analysis of the Internet Virus of November 1988", Massachusetts Institute of Technology, February 1989.
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[Eisenberg, et. al., 89] T. Eisenberg, D. Gries, J. Hartmanis, D. Holcomb, M. Lynn, and T. Santoro, "The Computer Worm", Cornell University, 6 February 1989.
[Ermann, Willians, and Gutierrez, 1990] D. Ermann, M. Williams, and C. Gutierrez, Editors, "Computers, Ethics, and Society", Oxford University Press, NY, 1990. (376 pages, includes bibliographical references).
[Farmer and Spafford, 1990] D. Farmer and E. Spafford, "The COPS Security Checker System", Proceedings of the Summer 1990 USENIX Conference, Anaheim, CA, Pgs. 165-170, June 1990.
[Fenwick, 1985] W. Fenwick, Chair, "Computer Litigation, 1985: Trial Tactics and Techniques", Litigation Course Handbook Series No. 280, Prepared for distribution at the Computer Litigation, 1985: Trial Tactics and Techniques Program, February-March 1985.
[Fites 1989] M. Fites, P. Kratz, and A. Brebner, "Control and Security of Computer Information Systems", Computer Science Press, 1989.
[Fites, Johnson, and Kratz, 1992] Fites, Johnson, and Kratz, "The Computer Virus Crisis", Van Hostrand Reinhold, 2nd edition, 1992.
[Forester and Morrison, 1990] T. Forester, and P. Morrison, "Computer Ethics: Tales and Ethical Dilemmas in Computing", MIT Press, Cambridge, MA, 1990.
[Foster and Morrision, 1990] T. Forester, and P. Morrison, "Computer Ethics: Tales and Ethical Dilemmas in Computing", MIT Press, Cambridge, MA, 1990. (192 pages including index.)
[GAO/IMTEX-89-57, 1989] U.S. General Accounting Office, "Computer Security - Virus Highlights Need for Improved Internet Management", United States General Accounting Office, Washington, DC, 1989.
[Garfinkel and Spafford, 1991] S. Garfinkel, and E. Spafford, "Practical Unix Security", O'Reilly & Associates, ISBN 0-937175-72-2, May 1991.
[Garfinkel, 1995] S. Garfinkel, "PGP:Pretty Good Privacy", O'Reilly & Associates, Sebastopol, CA, 1996.
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[Garfinkel and Spafford, 1996] S. Garfinkel and E. Spafford, "Practical UNIX and Internet Security", O'Reilly & Associates, Sebastopol, CA, 1996.
[Gemignani, 1989] M. Gemignani, "Viruses and Criminal Law", Communications of the ACM, Vol. 32, No. 6, Pgs. 669-671, June 1989.
[Goodell, 1996] J. Goodell, "The Cyberthief and the Samurai: The True Story of Kevin Mitnick-And The Man Who Hunted Him Down", Dell Publishing, 1996.
[Gould, 1989] C. Gould, Editor, "The Information Web: Ethical and Social Implications of Computer Networking", Westview Press, Boulder, CO, 1989.
[Greenia, 1989] M. Greenia, "Computer Security Information Sourcebook", Lexikon Services, Sacramento, CA, 1989.
[Hafner and Markoff, 1991] K. Hafner and J. Markoff, "Cyberpunk: Outlaws and Hackers on the Computer Frontier", Touchstone, Simon & Schuster, 1991.
[Hess, Safford, and Pooch] D. Hess, D. Safford, and U. Pooch, "A Unix Network Protocol Security Study: Network Information Service", Texas A&M University.
[Hoffman, 1990] L. Hoffman, "Rogue Programs: Viruses, Worms, and Trojan Horses", Van Nostrand Reinhold, NY, 1990. (384 pages, includes bibliographical references and index.)
[Howard, 1995] G. Howard, "Introduction to Internet Security: From Basics to Beyond", Prima Publishing, Rocklin, CA, 1995.
[Huband and Shelton, 1986] F. Huband, and R. Shelton, Editors, "Protection of Computer Systems and Software: New Approaches for Combating Theft of Software and Unauthorized Intrusion", Papers presented at a workshop sponsored by the National Science Foundation, 1986.
[Hughes, 1995] L. Hughes Jr., "Actually Useful Internet Security Techniques", New Riders Publishing, Indianapolis, IN, 1995.
[IAB-RFC1087, 1989] Internet Activities Board, "Ethics and the Internet", RFC 1087, IAB, January 1989. Also appears in the Communications of the ACM, Vol. 32, No. 6, Pg. 710, June 1989.
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[Icove, Seger, and VonStorch, 1995] D. Icove, K. Seger, and W. VonStorch, "Computer Crime: A Crimefighter's Handbook", O'Reilly & Associates, Sebastopol, CA, 1995.
[Johnson and Podesta] D. Johnson, and J. Podesta, "Formulating A Company Policy on Access to and Use and Disclosure of Electronic Mail on Company Computer Systems".
[Kane, 1994] P. Kane, "PC Security and Virus Protection Handbook: The Ongoing War Against Information Sabotage", M&T Books, 1994.
[Kaufman, Perlman, and Speciner, 1995] C. Kaufman, R. Perlman, and M. Speciner, "Network Security: PRIVATE Communication in a PUBLIC World", Prentice Hall, Englewood Cliffs, NJ, 1995.
[Kent, 1990] S. Kent, "E-Mail Privacy for the Internet: New Software and Strict Registration Procedures will be Implemented this Year", Business Communications Review, Vol. 20, No. 1, Pg. 55, 1 January 1990.
[Levy, 1984] S. Levy, "Hacker: Heroes of the Computer Revolution", Delta, 1984.
[Lewis, 1996] S. Lewis, "Disaster Recovery Yellow Pages", The Systems Audit Group, 1996.
[Littleman, 1996] J. Littleman, "The Fugitive Game: Online with Kevin Mitnick", Little, Brown, Boston, MA., 1996.
[Lu and Sundareshan, 1989] W. Lu and M. Sundareshan, "Secure Communication in Internet Environments: A Hierarchical Key Management Scheme for End-to-End Encryption", IEEE Transactions on Communications, Vol. 37, No. 10, Pg. 1014, 1 October 1989.
[Lu and Sundareshan, 1990] W. Lu and M. Sundareshan, "A Model for Multilevel Security in Computer Networks", IEEE Transactions on Software Engineering, Vol. 16, No. 6, Page 647, 1 June 1990.
[Martin and Schinzinger, 1989] M. Martin, and R. Schinzinger, "Ethics in Engineering", McGraw Hill, 2nd Edition, 1989.
[Merkle] R. Merkle, "A Fast Software One Way Hash Function", Journal of Cryptology, Vol. 3, No. 1.
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[McEwen, 1989] J. McEwen, "Dedicated Computer Crime Units", Report Contributors: D. Fester and H. Nugent, Prepared for the National Institute of Justice, U.S. Department of Justice, by Institute for Law and Justice, Inc., under contract number OJP-85-C-006, Washington, DC, 1989.
[MIT, 1989] Massachusetts Institute of Technology, "Teaching Students About Responsible Use of Computers", MIT, 1985-1986. Also reprinted in the Communications of the ACM, Vol. 32, No. 6, Pg. 704, Athena Project, MIT, June 1989.
[Mogel, 1989] Mogul, J., "Simple and Flexible Datagram Access Controls for UNIX-based Gateways", Digital Western Research Laboratory Research Report 89/4, March 1989.
[Muffett, 1992] A. Muffett, "Crack Version 4.1: A Sensible Password Checker for Unix"
[NCSC-89-660-P, 1990] National Computer Security Center, "Guidelines for Formal Verification Systems", Shipping list no.: 89-660-P, The Center, Fort George G. Meade, MD, 1 April 1990.
[NCSC-89-254-P, 1988] National Computer Security Center, "Glossary of Computer Security Terms", Shipping list no.: 89-254-P, The Center, Fort George G. Meade, MD, 21 October 1988.
[NCSC-C1-001-89, 1989] Tinto, M., "Computer Viruses: Prevention, Detection, and Treatment", National Computer Security Center C1 Technical Report C1-001-89, June 1989.
[NCSC Conference, 1989] National Computer Security Conference, "12th National Computer Security Conference: Baltimore Convention Center, Baltimore, MD, 10-13 October, 1989: Information Systems Security, Solutions for Today - Concepts for Tomorrow", National Institute of Standards and National Computer Security Center, 1989.
[NCSC-CSC-STD-003-85, 1985] National Computer Security Center, "Guidance for Applying the Department of Defense Trusted Computer System Evaluation Criteria in Specific Environments", CSC-STD-003-85, NCSC, 25 June 1985.
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[NCSC-STD-004-85, 1985] National Computer Security Center, "Technical Rationale Behind CSC-STD-003-85: Computer Security Requirements", CSC-STD-004-85, NCSC, 25 June 1985.
[NCSC-STD-005-85, 1985] National Computer Security Center, "Magnetic Remanence Security Guideline", CSC-STD-005-85, NCSC, 15 November 1985.
[NCSC-TCSEC, 1985] National Computer Security Center, "Trusted Computer System Evaluation Criteria", DoD 5200.28-STD, CSC-STD-001- 83, NCSC, December 1985.
[NCSC-TG-003, 1987] NCSC, "A Guide to Understanding DISCRETIONARY ACCESS CONTROL in Trusted Systems", NCSC-TG-003, Version-1, 30 September 1987, 29 pages.
[NCSC-TG-001, 1988] NCSC, "A Guide to Understanding AUDIT in Trusted Systems", NCSC-TG-001, Version-2, 1 June 1988, 25 pages.
[NCSC-TG-004, 1988] National Computer Security Center, "Glossary of Computer Security Terms", NCSC-TG-004, NCSC, 21 October 1988.
[NCSC-TG-005, 1987] National Computer Security Center, "Trusted Network Interpretation", NCSC-TG-005, NCSC, 31 July 1987.
[NCSC-TG-006, 1988] NCSC, "A Guide to Understanding CONFIGURATION MANAGEMENT in Trusted Systems", NCSC-TG-006, Version-1, 28 March 1988, 31 pages.
[NCSC-TRUSIX, 1990] National Computer Security Center, "Trusted UNIX Working Group (TRUSIX) rationale for selecting access control list features for the UNIX system", Shipping list no.: 90-076-P, The Center, Fort George G. Meade, MD, 1990.
[NRC, 1991] National Research Council, "Computers at Risk: Safe Computing in the Information Age", National Academy Press, 1991.
[Nemeth, et. al, 1995] E. Nemeth, G. Snyder, S. Seebass, and T. Hein, "UNIX Systems Administration Handbook", Prentice Hall PTR, Englewood Cliffs, NJ, 2nd ed. 1995.
[NIST, 1989] National Institute of Standards and Technology, "Computer Viruses and Related Threats: A Management Guide", NIST Special Publication 500-166, August 1989.
[NSA] National Security Agency, "Information Systems Security Products and Services Catalog", NSA, Quarterly Publication.
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[NSF, 1988] National Science Foundation, "NSF Poses Code of Networking Ethics", Communications of the ACM, Vol. 32, No. 6, Pg. 688, June 1989. Also appears in the minutes of the regular meeting of the Division Advisory Panel for Networking and Communications Research and Infrastructure, Dave Farber, Chair, November 29-30, 1988.
[NTISSAM, 1987] NTISS, "Advisory Memorandum on Office Automation Security Guideline", NTISSAM COMPUSEC/1-87, 16 January 1987, 58 pages.
[OTA-CIT-310, 1987] United States Congress, Office of Technology Assessment, "Defending Secrets, Sharing Data: New Locks and Keys for Electronic Information", OTA-CIT-310, October 1987.
[OTA-TCT-606] Congress of the United States, Office of Technology Assessment, "Information Security and Privacy in Network Environments", OTA-TCT-606, September 1994.
[Palmer and Potter, 1989] I. Palmer, and G. Potter, "Computer Security Risk Management", Van Nostrand Reinhold, NY, 1989.
[Parker, 1989] D. Parker, "Computer Crime: Criminal Justice Resource Manual", U.S. Dept. of Justice, National Institute of Justice, Office of Justice Programs, Under Contract Number OJP-86-C-002, Washington, D.C., August 1989.
[Parker, Swope, and Baker, 1990] D. Parker, S. Swope, and B. Baker, "Ethical Conflicts: Information and Computer Science, Technology and Business", QED Information Sciences, Inc., Wellesley, MA. (245 pages).
[Pfleeger, 1989] C. Pfleeger, "Security in Computing", Prentice-Hall, Englewood Cliffs, NJ, 1989.
[Quarterman, 1990] J. Quarterman, J., "The Matrix: Computer Networks and Conferencing Systems Worldwide", Digital Press, Bedford, MA, 1990.
[Ranum1, 1992] M. Ranum, "An Internet Firewall", Proceedings of World Conference on Systems Management and Security, 1992.
[Ranum2, 1992] M. Ranum, "A Network Firewall", Digital Equipment Corporation Washington Open Systems Resource Center, June 12, 1992.
[Ranum, 1993] M. Ranum, "Thinking About Firewalls", 1993.
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[Ranum and Avolio, 1994] M. Ranum and F. Avolio, "A Toolkit and Methods for Internet Firewalls", Trustest Information Systems, 1994.
[Reinhardt, 1992] R. Reinhardt, "An Architectural Overview of UNIX Network Security"
[Reinhardt, 1993] R. Reinhardt, "An Architectural Overview of UNIX Network Security", ARINC Research Corporation, February 18, 1993.
[Reynolds-RFC1135, 1989] The Helminthiasis of the Internet, RFC 1135, USC/Information Sciences Institute, Marina del Rey, CA, December 1989.
[Russell and Gangemi, 1991] D. Russell and G. Gangemi, "Computer Security Basics" O'Reilly & Associates, Sebastopol, CA, 1991.
[Schneier 1996] B. Schneier, "Applied Cryptography: Protocols, Algorithms, and Source Code in C", John Wiley & Sons, New York, second edition, 1996.
[Seeley, 1989] D. Seeley, "A Tour of the Worm", Proceedings of 1989 Winter USENIX Conference, Usenix Association, San Diego, CA, February 1989.
[Shaw, 1986] E. Shaw Jr., "Computer Fraud and Abuse Act of 1986", Congressional Record (3 June 1986), Washington, D.C., 3 June 1986.
[Shimomura, 1996] T. Shimomura with J. Markoff, "Takedown:The Pursuit and Capture of Kevin Mitnick, America's Most Wanted Computer Outlaw- by the Man Who Did It", Hyperion, 1996.
[Shirey, 1990] R. Shirey, "Defense Data Network Security Architecture", Computer Communication Review, Vol. 20, No. 2, Page 66, 1 April 1990.
[Slatalla and Quittner, 1995] M. Slatalla and J. Quittner, "Masters of Deception: The Gang that Ruled Cyberspace", Harper Collins Publishers, 1995.
[Smith, 1989] M. Smith, "Commonsense Computer Security: Your Practical Guide to Preventing Accidental and Deliberate Electronic Data Loss", McGraw-Hill, New York, NY, 1989.
[Smith, 1995] D. Smith, "Forming an Incident Response Team", Sixth Annual Computer Security Incident Handling Workshop, Boston, MA, July 25-29, 1995.
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[Spafford, 1988] E. Spafford, "The Internet Worm Program: An Analysis", Computer Communication Review, Vol. 19, No. 1, ACM SIGCOM, January 1989. Also issued as Purdue CS Technical Report CSD-TR-823, 28 November 1988.
[Spafford, 1989] G. Spafford, "An Analysis of the Internet Worm", Proceedings of the European Software Engineering Conference 1989, Warwick England, September 1989. Proceedings published by Springer- Verlag as: Lecture Notes in Computer Science #387. Also issued as Purdue Technical Report #CSD-TR-933.
[Spafford, Keaphy, and Ferbrache, 1989] E. Spafford, K. Heaphy, and D. Ferbrache, "Computer Viruses: Dealing with Electronic Vandalism and Programmed Threats", ADAPSO, 1989. (109 pages.)
[Stallings1, 1995] W. Stallings, "Internet Security Handbook", IDG Books, Foster City CA, 1995.
[Stallings2, 1995] W. Stallings, "Network and InterNetwork Security", Prentice Hall, , 1995.
[Stallings3, 1995] W. Stallings, "Protect Your Privacy: A Guide for PGP Users" PTR Prentice Hall, 1995.
[Stoll, 1988] C. Stoll, "Stalking the Wily Hacker", Communications of the ACM, Vol. 31, No. 5, Pgs. 484-497, ACM, New York, NY, May 1988.
[Stoll, 1989] C. Stoll, "The Cuckoo's Egg", ISBN 00385-24946-2, Doubleday, 1989.
[Treese and Wolman, 1993] G. Treese and A. Wolman, "X Through the Firewall, and Other Applications Relays", Digital Equipment Corporation, Cambridge Research Laboratory, CRL 93/10, May 3, 1993.
[Trible, 1986] P. Trible, "The Computer Fraud and Abuse Act of 1986", U.S. Senate Committee on the Judiciary, 1986.
[Venema] W. Venema, "TCP WRAPPER: Network monitoring, access control, and booby traps", Mathematics and Computing Science, Eindhoven University of Technology, The Netherlands.
[USENIX, 1993] USENIX, "USENIX Symposium Proceedings: UNIX Security IV", Santa Clara, CA, October 4-6, 1993.
[USENIX, 1995] USENIX, "The Fifth USENIX UNIX Security Symposium", Salt Lake City, UT, June 5-7, 1995.
[Wood, et.al., 1987] C. Wood, W. Banks, S. Guarro, A. Garcia, V. Hampel, and H. Sartorio, "Computer Security: A Comprehensive Controls Checklist", John Wiley and Sons, Interscience Publication, 1987.
[Wrobel, 1993] L. Wrobel, "Writing Disaster Recovery Plans for Telecommunications Networks and LANS", Artech House, 1993.
[Vallabhaneni, 1989] S. Vallabhaneni, "Auditing Computer Security: A Manual with Case Studies", Wiley, New York, NY, 1989.
Security Considerations
This entire document discusses security issues.
Editor Information
Barbara Y. Fraser Software Engineering Institute Carnegie Mellon University 5000 Forbes Avenue Pittsburgh, PA 15213