Internet Engineering Task Force (IETF)                      J. Livingood
Request for Comments: 6561                                       N. Mody
Category: Informational                                     M. O'Reirdan
ISSN: 2070-1721                                                  Comcast
                                                              March 2012

Recommendations for the Remediation of Bots in ISP Networks

Abstract

   This document contains recommendations on how Internet Service
   Providers can use various remediation techniques to manage the
   effects of malicious bot infestations on computers used by their
   subscribers.  Internet users with infected computers are exposed to
   risks such as loss of personal data and increased susceptibility to
   online fraud.  Such computers can also become inadvertent
   participants in or components of an online crime network, spam
   network, and/or phishing network as well as be used as a part of a
   distributed denial-of-service attack.  Mitigating the effects of and
   remediating the installations of malicious bots will make it more
   difficult for botnets to operate and could reduce the level of online
   crime on the Internet in general and/or on a particular Internet
   Service Provider's network.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6561.

Copyright Notice

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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   1. Introduction ....................................................3
      1.1. Key Terminology ............................................3
           1.1.1. Malicious Bots, or Bots .............................3
           1.1.2. Bot Networks, or Botnets ............................4
           1.1.3. Host ................................................5
           1.1.4. Malware .............................................5
           1.1.5. Fast Flux ...........................................5
   2. Problem Statement ...............................................6
   3. Important Notice of Limitations and Scope .......................7
   4. Detection of Bots ...............................................8
   5. Notification to Internet Users .................................12
      5.1. Email Notification ........................................13
      5.2. Telephone Call Notification ...............................13
      5.3. Postal Mail Notification ..................................14
      5.4. Walled Garden Notification ................................14
      5.5. Instant Message Notification ..............................16
      5.6. Short Message Service (SMS) Notification ..................16
      5.7. Web Browser Notification ..................................17
      5.8. Considerations for Notification to Public Network
           Locations .................................................18
      5.9. Considerations for Notification to Network
           Locations Using a Shared IP Address .......................18
      5.10. Notification and End User Expertise ......................19
   6. Remediation of Hosts Infected with a Bot .......................19
      6.1. Guided Remediation Process ................................21
      6.2. Professionally Assisted Remediation Process ...............22
   7. Failure or Refusal to Remediate ................................23
   8. Sharing of Data from the User to the ISP .......................23
   9. Security Considerations ........................................23
   10. Privacy Considerations ........................................24
   11. Acknowledgements ..............................................24
   12. Informative References ........................................26
   Appendix A.  Examples of Third-Party Malware Lists ................28

1. Introduction

   This document contains recommendations on how Internet Service
   Providers can use various remediation techniques to manage the
   effects of malicious bot infestations on computers used by their
   subscribers.  Internet users with infected computers are exposed to
   risks such as loss of personal data and increased susceptibility to
   online fraud.  Such computers can also become inadvertent
   participants in or components of an online crime network, spam
   network, and/or phishing network as well as be used as a part of a
   distributed denial-of-service attack.  Mitigating the effects of and
   remediating the installations of malicious bots will make it more
   difficult for botnets to operate and could reduce the level of online
   crime on the Internet in general and/or on a particular Internet
   Service Provider's network.

1.1. Key Terminology

This section defines the key terms used in this document.

1.1.1. Malicious Bots, or Bots

   A malicious or potentially malicious bot (derived from the word
   "robot", hereafter simply referred to as a "bot") refers to a program
   that is installed on a system in order to enable that system to
   automatically (or semi-automatically) perform a task or set of tasks
   typically under the command and control of a remote administrator, or
   "bot master".  Bots are also known as "zombies".  Such bots may have
   been installed surreptitiously, without the user's full understanding
   of what the bot will do once installed, unknowingly as part of
   another software installation, under false pretenses, and/or in a
   variety of other possible ways.

   It is important to note that there are "good" bots.  Such good bots
   are often found interacting with a computing resource in environments
   such as gaming and Internet Relay Chat (IRC) [RFC1459], where a
   continual, interactive presence can be a requirement for
   participating in the games.  Since such good bots are performing
   useful, lawful, and non-disruptive functions, there is no reason for
   a provider to monitor for their presence and/or alert users to their
   presence.

   While there may be good, or harmless bots, for the purposes of this
   document, all mention of bots shall assume that the bots involved are
   malicious or potentially malicious in nature.  Such malicious bots
   shall generally be assumed to have been deployed without the
   permission or conscious understanding of a particular Internet user.
   Thus, without a user's knowledge, bots may transform the user's

   computing device into a platform from which malicious activities can
   be conducted.  In addition, included explicitly in this category are
   potentially malicious bots, which may initially appear neutral but
   may simply be waiting for remote instructions to transform and/or
   otherwise begin engaging in malicious behavior.  In general,
   installation of a malicious bot without user knowledge and consent is
   considered in most regions to be unlawful, and the activities of
   malicious bots typically involve unlawful or other maliciously
   disruptive activities.

1.1.2. Bot Networks, or Botnets

   A "bot network", or "botnet", is defined as a concerted network of
   bots capable of acting on instructions generated remotely.  The
   malicious activities are either focused on the information on the
   local machine or acting to provide services for remote machines.
   Bots are highly customizable so they can be programmed to do many
   things.  The major malicious activities include but are not limited
   to identity theft, spam, spim (spam over Instant Messaging (IM)),
   spit (spam over Internet telephony), email address harvesting,
   distributed denial-of-service (DDoS) attacks, key-logging, fraudulent
   DNS pharming (redirection), hosting proxy services, fast flux (see
   Section 1.1.5) hosting, hosting of illegal content, use in man-in-
   the-middle attacks, and click fraud.

   Infection vectors (infection pathways) include un-patched operating
   systems, software vulnerabilities (which include so-called zero-day
   vulnerabilities where no patch yet exists), weak/non-existent
   passwords, malicious web sites, un-patched browsers, malware,
   vulnerable helper applications, inherently insecure protocols,
   protocols implemented without security features switched on, and
   social engineering techniques to gain access to the user's computer.
   The detection and destruction of bots is an ongoing issue and also a
   constant battle between the Internet security community and network
   security engineers on the one hand and bot developers on the other.

   Initially, some bots used IRC to communicate but were easy to shut
   down if the command and control server was identified and
   deactivated.  Newer command and control methods have evolved, such
   that those currently employed by bot masters make them much more
   resistant to deactivation.  With the introduction of peer-to-peer
   (P2P) architectures and associated protocols, the use of HTTP and
   other resilient communication protocols, and the widespread adoption
   of encryption, bots are considerably more difficult to identify and
   isolate from typical network usage.  As a result, increased reliance
   is being placed on anomaly detection and behavioral analysis, both
   locally and remotely, to identify bots.

1.1.3. Host

   As used in the context of this document, the host or computer of an
   end user is intended to refer to a computing device that connects to
   the Internet.  This encompasses devices used by Internet users such
   as personal computers (including laptops, desktops, and netbooks),
   mobile phones, smart phones, home gateway devices, and other end user
   computing devices that are connected or can connect to the public
   Internet and/or private IP networks.

   Increasingly, other household systems and devices contain embedded
   hosts that are connected to or can connect to the public Internet
   and/or private IP networks.  However, these devices may not be under
   interactive control of the Internet user, such as may be the case
   with various smart home and smart grid devices.

1.1.4. Malware

   Malware is short for "malicious software".  In this case, malicious
   bots are considered a subset of malware.  Other forms of malware
   could include viruses and other similar types of software.  Internet
   users can sometimes cause their hosts to be infected with malware,
   which may include a bot or cause a bot to install itself, via
   inadvertently accessing a specific web site, downloading a file, or
   other activities.

   In other cases, Internet-connected hosts may become infected with
   malware through externally initiated malicious activities such as the
   exploitation of vulnerabilities or the brute force guessing of access
   credentials.

1.1.5. Fast Flux

   Domain Name System (DNS) fast fluxing occurs when a domain is bound
   in DNS using A records to multiple IP addresses, each of which has a
   very short Time-to-Live (TTL) value associated with it.  This means
   that the domain resolves to varying IP addresses over a short period
   of time.

   DNS fast flux is typically used in conjunction with proxies that are
   normally run on compromised user hosts.  These proxies route the web
   requests to the real host, which serves the data being sought.  The
   effect of this is to make the detection of the real host much more
   difficult and to ensure that the backend or hidden site remains up
   for as long as possible.

2. Problem Statement

   Hosts used by Internet users, which in this case are customers of an
   Internet Service Provider (ISP), can be infected with malware that
   may contain and/or install one or more bots on a host.  They can
   present a major problem for an ISP for a number of reasons (not to
   mention, of course, the problems created for users).  First, these
   bots can be used to send spam, in some cases very large volumes of
   spam [Spamalytics].  This spam can result in extra cost for the ISPs
   in terms of wasted network, server, and/or personnel resources, among
   many other potential costs and side effects.  Such spam can also
   negatively affect the reputation of the ISP, their customers, and the
   email reputation of the IP address space used by the ISP (often
   referred to simply as "IP reputation").  A further potential
   complication is that IP space compromised by bad reputation may
   continue to carry this bad reputation even when used for entirely
   innocent purposes following reassignment of that IP space.

   In addition, these bots can act as platforms for directing,
   participating in, or otherwise conducting attacks on critical
   Internet infrastructure [Threat-Report].  Bots are frequently used as
   part of coordinated DDoS attacks for criminal, political, or other
   motivations [Gh0st][Dragon][DDoS].  For example, bots have been used
   to attack Internet resources and infrastructure ranging from web
   sites to email servers and DNS servers, as well as the critical
   Internet infrastructure of entire countries [Estonia][Combat-Zone].
   Motivations for such coordinated DDoS attacks can range from criminal
   extortion attempts through to online protesting and nationalistic
   fervor [Whiz-Kid].  DDoS attacks may also be motivated by simple
   personal vendettas or by persons simply seeking a cheap thrill at the
   expense of others.

   There is good evidence to suggest that bots are being used in the
   corporate environment for purposes of corporate espionage including
   the exfiltration of corporate financial data and intellectual
   property.  This also extends to the possibility of bots being used
   for state-sponsored purposes such as espionage.

   While any computing device can be infected with bots, the majority of
   bot infections affect the personal computers used by Internet end
   users.  As a result of the role of ISPs in providing IP connectivity,
   among many other services, to Internet users, these ISPs are in a
   unique position to be able to attempt to detect and observe botnets
   operating in their networks.  Furthermore, ISPs may also be in a
   unique position to be able to notify their customers of actual,
   potential, or likely infection by bots or other infection.

   From the perspective of end users, being notified that they may have
   an infected computer on their network is important information.  Once
   they know this, they can take steps to remove the bots, resolve any
   problems that may stem from the bot infection, and protect themselves
   against future threats.  It is important to notify users that they
   may be infected with a bot because bots can consume vast amounts of
   local computing and network resources, enable theft of personal
   information (including personal financial information), enable the
   host to be used for criminal activities (that may result in the
   Internet user being legally culpable), and destroy or leave the host
   in an unrecoverable state via "kill switch" bot technologies.

   As a result, the intent of this document is to provide guidance to
   ISPs and other organizations for the remediation of hosts infected
   with bots, so as to reduce the size of botnets and minimize the
   potential harm that bots can inflict upon Internet infrastructure in
   general as well as on individual Internet users.  Efforts by ISPs and
   other organizations can, over time, reduce the pool of hosts infected
   with bots on the Internet, which in turn could result in smaller
   botnets with less capability for disruption.

   The potential mitigation of bots is accomplished through a process of
   detection, notification to Internet users, and remediation of bot
   infections with a variety of tools, as described later in this
   document.

3. Important Notice of Limitations and Scope

   The techniques described in this document in no way guarantee the
   remediation of all bots.  Bot removal is potentially a task requiring
   specialized knowledge, skills, and tools; it may be beyond the
   ability of average users.  Attempts at bot removal may frequently be
   unsuccessful, or only partially successful, leaving the user's system
   in an unstable and unsatisfactory state or even in a state where it
   is still infected.  Attempts at bot removal can result in side
   effects ranging from a loss of data to partial or complete loss of
   system usability.

   In general, the only way a user can be sure they have removed some of
   today's increasingly sophisticated malware is by "nuking-and-paving"
   the system: reformatting the drive, reinstalling the operating system
   and applications (including all patches) from scratch, and then
   restoring user files from a known clean backup.  However, the
   introduction of persistent memory-based malware may mean that, in
   some cases, this may not be enough and may prove to be more than any
   end user can be reasonably expected to resolve [BIOS].  Experienced
   users would have to re-flash or re-image persistent memory sections
   or components of their hosts in order to remove persistent memory-

   based malware.  However, in some cases, not even nuking-and-paving
   the system will solve the problem, which calls for hard drive
   replacement and/or complete replacement of the host.

   Devices with embedded operating systems, such as video gaming
   consoles and smart home appliances, will most likely be beyond a
   user's capability to remediate by themselves and could therefore
   require the aid of vendor-specific advice, updates, and tools.
   However, in some cases, such devices will have a function or switch
   to enable the user to reset that device to a factory default
   configuration, which may sometimes enable the user to remediate the
   infection.  Care should be taken when imparting remediation advice to
   Internet users given the increasingly wide array of computing devices
   that can be, or could be, infected by bots in the future.

   This document is not intended to address the issues relating to the
   prevention of bots on an end user device.  This is out of the scope
   of this document.

4. Detection of Bots

   An ISP must first identify that an Internet user is infected or
   likely to have been infected with a bot (a user is assumed to be
   their customer or otherwise connected to the ISP's network).  The ISP
   should attempt to detect the presence of bots using methods,
   processes, and tools that maintain the privacy of the personally
   identifiable information (PII) of their customers.  The ISP should
   not block legitimate traffic in the course of bot detection and
   should instead employ detection methods, tools, and processes that
   seek to be non-disruptive and transparent to Internet users and end
   user applications.

   Detection methods, tools, and processes may include analysis of
   specific network and/or application traffic flows (such as traffic to
   an email server), analysis of aggregate network and/or application
   traffic data, data feeds received from other ISPs and organizations
   (such as lists of the ISP's IP addresses that have been reported to
   have sent spam), feedback from the ISP's customers or other Internet
   users, as well as a wide variety of other possibilities.  In
   practice, it has proven effective to confirm a bot infection through
   the use of a combination of multiple bot detection data points.  This
   can help to corroborate information of varying dependability or
   consistency, as well as to avoid or minimize the possibility of false
   positive identification of hosts.  Detection should also, where
   possible and feasible, attempt to classify the specific bot infection
   type in order to confirm that it is malicious in nature, estimate the
   variety and severity of threats it may pose (such as spam bot, key-
   logging bot, file distribution bot, etc.), and determine potential

   methods for eventual remediation.  However, given the dynamic nature
   of botnet management and the criminal incentives to seek quick
   financial rewards, botnets frequently update or change their core
   capabilities.  As a consequence, botnets that are initially detected
   and classified by the ISP as made up of one particular type of bot
   need to be continuously monitored and tracked in order to correctly
   identify the threat the botnet poses at any particular point in time.

   Detection is also time sensitive.  If complex analysis is required
   and multiple confirmations are needed to verify a bot is indeed
   present, then it is possible that the bot may cause some damage (to
   either the infected host or a remotely targeted system) before it can
   be stopped.  This means that an ISP needs to balance the desire or
   need to definitively classify and/or confirm the presence of a bot,
   which may take an extended period of time, with the ability to
   predict the likelihood of a bot in a very short period of time.  Such
   determinations must have a relatively low false positive rate in
   order to maintain the trust of users.  This "definitive-versus-
   likely" challenge is difficult and, when in doubt, ISPs should err on
   the side of caution by communicating that a bot infection has taken
   place.  This also means that Internet users may benefit from the
   installation of client-based security software on their host.  This
   can enable rapid heuristically based detection of bot activity, such
   as the detection of a bot as it starts to communicate with other
   botnets and execute commands.  Any bot detection system should also
   be capable of adapting, either via manual intervention or
   automatically, in order to cope with a rapidly evolving threat.

   As noted above, detection methods, tools, and processes should ensure
   that privacy of customers' personally identifiable information (PII)
   is maintained.  This protection afforded to PII should also extend to
   third parties processing data on behalf of ISPs.  While bot detection
   methods, tools, and processes are similar to spam and virus defenses
   deployed by the ISP for the benefit of their customers (and may be
   directly related to those defenses), attempts to detect bots should
   take into account the need of an ISP to take care to ensure any PII
   collected or incidentally detected is properly protected.  This is
   important because just as spam defenses may involve scanning the
   content of email messages, which may contain PII, then so too may bot
   defenses similarly come into incidental contact with PII.  The
   definition of PII varies from one jurisdiction to the next so proper
   care should be taken to ensure that any actions taken comply with
   legislation and good practice in the jurisdiction in which the PII is
   gathered.  Finally, depending upon the geographic region within which
   an ISP operates, certain methods relating to bot detection may need
   to be included in relevant terms of service documents or other
   documents that are available to the customers of a particular ISP.

   There are several bot detection methods, tools, and processes that an
   ISP may choose to utilize, as noted in the list below.  It is
   important to note that the technical solutions available are
   relatively immature and are likely to change over time, evolving
   rapidly in the coming years.  While these items are described in
   relation to ISPs, they may also be applicable to organizations
   operating other networks, such as campus networks and enterprise
   networks.

   a.  Where it is not legally proscribed and an accepted industry
       practice in a particular market region, an ISP may in some manner
       "scan" its IP space in order to detect un-patched or otherwise
       vulnerable hosts or to detect the signs of infection.  This may
       provide the ISP with the opportunity to easily identify Internet
       users who appear already to be infected or are at great risk of
       being infected with a bot.  ISPs should note that some types of
       port scanning may leave network services in a hung state or
       render them unusable due to common frailties and that many modern
       firewall and host-based intrusion detection implementations may
       alert the Internet user to the scan.  As a result, the scan may
       be interpreted as a malicious attack against the host.
       Vulnerability scanning has a higher probability of leaving
       accessible network services and applications in a damaged state
       and will often result in a higher probability of detection by the
       Internet user and subsequent interpretation as a targeted attack.
       Depending upon the vulnerability for which an ISP may be
       scanning, some automated methods of vulnerability checking may
       result in data being altered or created afresh on the Internet
       user's host, which can be a problem in many legal environments.
       It should also be noted that due to the prevalence of Network
       Address Translation devices, Port Address Translation devices,
       and/or firewall devices in user networks, network-based
       vulnerability scanning may be of limited value.  Thus, while we
       note that this is one technique that may be utilized, it is
       unlikely to be particularly effective and has problematic side
       effects, which leads the authors to recommend against the use of
       this particular method.

   b.  An ISP may also communicate and share selected data, via feedback
       loops or other mechanisms, with various third parties.  Feedback
       loops are consistently formatted feeds of real-time (or nearly
       real-time) abuse reports offered by threat data clearinghouses,
       security alert organizations, other ISPs, and other
       organizations.  The formats for feedback loops include those
       defined in both the Abuse Reporting Format (ARF) [RFC5965] and
       the Incident Object Description Exchange Format (IODEF)
       [RFC5070].  The data may include, but is not limited to, IP
       addresses of hosts that appear to be either definitely or

       probably infected, IP addresses, domain names or fully qualified
       domain names (FQDNs) known to host malware and/or be involved in
       the command and control of botnets, recently tested or discovered
       techniques for detecting or remediating bot infections, new
       threat vectors, and other relevant information.  A few good
       examples of data sharing are noted in Appendix A.

   c.  An ISP may use Netflow [RFC3954] or other similar passive network
       monitoring to identify network anomalies that may be indicative
       of botnet attacks or bot communications.  For example, an ISP may
       be able to identify compromised hosts by identifying traffic
       destined to IP addresses associated with the command and control
       of botnets or destined to the combination of an IP address and
       control port associated with a command and control network
       (sometimes command and control traffic comes from a host that has
       legitimate traffic).  In addition, bots may be identified when a
       remote host is under a DDoS attack, because hosts participating
       in the attack will likely be infected by a bot.  This can often
       be observed at network borders although ISPs should beware of
       source IP address spoofing techniques that may be employed to
       avoid or confuse detection.

   d.  An ISP may use DNS-based techniques to perform detection.  For
       example, a given classified bot may be known to query a specific
       list of domain names at specific times or on specific dates (in
       the example of the so-called "Conficker" bot (see [Conficker]),
       often by matching DNS queries to a well-known list of domains
       associated with malware.  In many cases, such lists are
       distributed by or shared using third parties, such as threat data
       clearinghouses.

   e.  Because hosts infected by bots are frequently used to send spam
       or participate in DDoS attacks, the ISP servicing those hosts
       will normally receive complaints about the malicious network
       traffic.  Those complaints may be sent to role accounts specified
       in RFC 2142 [RFC2142], such as abuse@, or to other relevant
       addresses such as to abuse or security addresses specified by the
       site as part of its WHOIS (or other) contact data.

   f.  ISPs may also discover likely bot-infected hosts located on other
       networks.  Thus, when legally permissible in a particular market
       region, it may be worthwhile for ISPs to share information
       relating to those compromised hosts with the relevant remote
       network operator, security researchers, and blocklist operators.

   g.  ISPs may operate or subscribe to services that provide
       "sinkholing" or "honeynet" capabilities.  This may enable the ISP
       to obtain near-real-time lists of bot-infected hosts as they
       attempt to join a larger botnet or propagate to other hosts on a
       network.

   h.  ISP industry associations should examine the possibility of
       collating statistics from ISP members in order to provide good
       statistics about bot infections based on real ISP data.

   i.  An Intrusion Detection System (IDS) can be a useful tool to
       actually help identify the malware.  An IDS tool such as Snort
       (open source IDS platform; see [Snort]) can be placed in a walled
       garden and used to analyze end user traffic to confirm malware
       type.  This will help with remediation of the infected device.

5. Notification to Internet Users

   Once an ISP has detected a bot, or the strong likelihood of a bot,
   steps should be undertaken to inform the Internet user that they may
   have a bot-related problem.  An ISP should decide the most
   appropriate method or methods for providing notification to one or
   more of their customers or Internet users, depending upon a range of
   factors including the technical capabilities of the ISP, the
   technical attributes of its network, financial considerations,
   available server resources, available organizational resources, the
   number of likely infected hosts detected at any given time, and the
   severity of any possible threats.  Such notification methods may
   include one or more of the methods described in the following
   subsections, as well as other possible methods not described below.

   It is important to note that none of these methods are guaranteed to
   be one hundred percent successful and that each has its own set of
   limitations.  In addition, in some cases, an ISP may determine that a
   combination of two or more methods is most appropriate and effective
   and reduces the chance that malware may block a notification.  As
   such, the authors recommend the use of multiple notification methods.
   Finally, notification is also considered time sensitive; if the user
   does not receive or view the notification in a timely fashion, then a
   particular bot could launch an attack, exploit the user, or cause
   other harm.  If possible, an ISP should establish a preferred means
   of communication when the subscriber first signs up for service.  As
   a part of the notification process, ISPs should maintain a record of
   the allocation of IP addresses to subscribers for a period long
   enough to allow any commonly used bot detection technology to be able
   to accurately link an infected IP address to a subscriber.  This

   record should only be maintained for a period of time that is
   necessary to support bot detection, but no longer, in order to
   protect the privacy of the individual subscriber.

   One important factor to bear in mind is that notification to end
   users needs to be resistant to potential spoofing.  This should be
   done to protect, as reasonably as possible, against the potential of
   legitimate notifications being spoofed and/or used by parties with
   intent to perform additional malicious attacks against victims of
   malware or even to deliver additional malware.

   It should be possible for the end user to indicate the preferred
   means of notification on an opt-in basis for that notification
   method.  It is recommended that the end user should not be allowed to
   opt out of notification entirely.

   When users are notified, an ISP should endeavor to give as much
   information as possible to the end user regarding which bot detection
   methods are employed at the ISP, consonant with not providing
   information to those creating or deploying the bots so that they
   would be able to avoid detection.

5.1. Email Notification

   This is a common form of notification used by ISPs.  One drawback of
   using email is that it is not guaranteed to be viewed within a
   reasonable time frame, if at all.  The user may be using a different
   primary email address than the one they provided to the ISP.  In
   addition, some ISPs do not provide an email account at all as part of
   a bundle of Internet services and/or do not have a need for or method
   by which to request or retain the primary email addresses of Internet
   users of their networks.  Another possibility is that the user, their
   email client, and/or their email servers could determine or classify
   such a notification as spam, which could delete the message or
   otherwise file it in an email folder that the user may not check on a
   regular and/or timely basis.  Bot masters have also been known to
   impersonate the ISP or trusted sender and send fraudulent emails to
   the users.  This technique of social engineering often leads to new
   bot infestations.  Finally, if the user's email credentials are
   compromised, then a hacker and/or a bot could simply access the
   user's email account and delete the email before it is read by the
   user.

5.2. Telephone Call Notification

   A telephone call may be an effective means of communication in
   particularly high-risk situations.  However, telephone calls may not
   be feasible due to the cost of making a large number of calls, as

   measured in either time, money, organizational resources, server
   resources, or some other means.  In addition, there is no guarantee
   that the user will answer their phone.  To the extent that the
   telephone number called by the ISP can be answered by the infected
   computing device, the bot on that host may be able to disconnect,
   divert, or otherwise interfere with an incoming call.  Users may also
   interpret such a telephone notification as a telemarketing call and
   therefore not welcome it or not accept the call at all.  Finally,
   even if a representative of the ISP is able to connect with and speak
   to a user, that user is very likely to lack the necessary technical
   expertise to understand or be able to effectively deal with the
   threat.

5.3. Postal Mail Notification

   This form of notification is probably the least popular and effective
   means of communication, due to preparation time, delivery time, the
   cost of printing and paper, and the cost of postage.

5.4. Walled Garden Notification

   Placing a user in a walled garden is another approach that ISPs may
   take to notify users.  A "walled garden" refers to an environment
   that controls the information and services that a subscriber is
   allowed to utilize and what network access permissions are granted.
   A walled garden implementation can range from strict to leaky.  In a
   strict walled garden environment, access to most Internet resources
   is typically limited by the ISP.  In contrast, a leaky walled garden
   environment permits access to all Internet resources, except those
   deemed malicious, and ensures access to those that can be used to
   notify users of infections.

   Walled gardens are effective because it is possible to notify the
   user and simultaneously block all communication between the bot and
   the command and control channel.  While in many cases the user is
   almost guaranteed to view the notification message and take any
   appropriate remediation actions, this approach can pose other
   challenges.  For example, it is not always the case that a user is
   actively utilizing a host that implements a web browser, has a web
   browser actively running on it, or operates another application that
   uses ports that are redirected to the walled garden.  In one example,
   a user could be playing a game online, via the use of a dedicated,
   Internet-connected game console.  In another example, the user may
   not be using a host with a web browser when they are placed in the
   walled garden and may instead be in the course of a telephone
   conversation or may be expecting to receive a call using a Voice over
   IP (VoIP) device of some type.  As a result, the ISP may feel the
   need to maintain a potentially lengthy white list of domains that are

   not subject to the typical restrictions of a walled garden, which
   could well prove to be an onerous task from an operational
   perspective.

   For these reasons, the implementation of a leaky walled garden makes
   more sense, but a leaky walled garden has a different set of
   drawbacks.  The ISP has to assume that the user will eventually use a
   web browser to acknowledge the notification; otherwise, the user will
   remain in the walled garden and not know it.  If the intent of the
   leaky walled garden is solely to notify the user about the bot
   infection, then the leaky walled garden is not ideal because
   notification is time sensitive, and the user may not receive the
   notification until the user invokes a request for the targeted
   service and/or resource.  This means the bot can potentially do more
   damage.  Additionally, the ISP has to identify which services and/or
   resources to restrict for the purposes of notification.  This does
   not have to be resource specific and can be time based and/or policy
   based.  An example of how notification could be made on a timed basis
   could involve notification for all HTTP requests every 10 minutes, or
   show the notification for one in five HTTP requests.

   The ISP has several options to determine when to let the user out of
   the walled garden.  One approach may be to let the user determine
   when to exit.  This option is suggested when the primary purpose of
   the walled garden is to notify users and provide information on
   remediation only, particularly since notification is not a guarantee
   of successful remediation.  It could also be the case that, for
   whatever reason, the user makes the judgment that they cannot then
   take the time to remediate their host and that other online
   activities that they would like to resume are more important.  Exit
   from the walled garden may also involve a process to verify that it
   is indeed the user who is requesting exit from the walled garden and
   not the bot.

   Once the user acknowledges the notification, they may decide either
   to remediate and exit the walled garden or to exit the walled garden
   without remediating the issue.  Another approach may be to enforce a
   stricter policy and require the user to clean the host prior to
   permitting the user to exit the walled garden, though this may not be
   technically feasible depending upon the type of bot, obfuscation
   techniques employed by a bot, and/or a range of other factors.  Thus,
   the ISP may also need to support tools to scan the infected host (in
   the style of a virus scan, rather than a port scan) and determine
   whether it is still infected or rely on user judgment that the bot
   has been disabled or removed.  One challenge with this approach is
   that the user might have multiple hosts sharing a single IP address,
   such as via a common home gateway device that performs Network

   Address Translation (NAT).  In such a case, the ISP may need to
   determine from user feedback, or other means, that all affected hosts
   have been remediated, which may or may not be technically feasible.

   Finally, when a walled garden is used, a list of well-known addresses
   for both operating system vendors and security vendors should be
   created and maintained in a white list that permits access to these
   sites.  This can be important for allowing access from the walled
   garden by end users in search of operating system and application
   patches.  It is recommended that walled gardens be seriously
   considered as a method of notification as they are easy to implement
   and proven to be effective as a means of getting end user attention.

5.5. Instant Message Notification

   IM provides the ISP with a simple means to communicate with the user.
   There are several advantages to using IM that make it an attractive
   option.  If the ISP provides IM service and the user subscribes to
   it, then the user can be notified easily.  IM-based notification can
   be a cost-effective means to communicate with users automatically
   from an IM alert system or by a manual process, involving the ISP's
   support staff.  Ideally, the ISP should allow the user to register
   their IM identity in an ISP account management system and grant
   permission to be contacted via this means.  If the IM service
   provider supports off-line messaging, then the user can be notified
   regardless of whether they are currently logged into the IM system.

   There are several drawbacks with this communications method.  There
   is a high probability that a subscriber may interpret the
   communication to be spim and thus ignore it.  Also, not every user
   uses IM and/or the user may not provide their IM identity to the ISP
   so some alternative means have to be used.  Even in those cases where
   a user does have an IM address, they may not be signed onto that IM
   system when the notification is attempted.  There may be a privacy
   concern on the part of users when such an IM notification must be
   transmitted over a third-party network and/or IM service.  As such,
   should this method be used, the notification should be discreet and
   not include any PII in the notification itself.

5.6. Short Message Service (SMS) Notification

   SMS allows the ISP to send a brief description of the problem to
   notify the user of the issue, typically to a mobile device such as a
   mobile phone or smart phone.  Ideally, the ISP should allow the user
   to register their mobile number and/or SMS address in an ISP account
   management system and grant permission to be contacted via this
   means.  The primary advantage of SMS is that users are familiar with

   receiving text messages and are likely to read them.  However, users
   may not act on the notification immediately if they are not in front
   of their host at the time of the SMS notification.

   One disadvantage is that ISPs may have to follow up with an alternate
   means of notification if not all of the necessary information may be
   conveyed in one message, given constraints on the number of
   characters in an individual message (typically 140 characters).
   Another disadvantage with SMS is the cost associated with it.  The
   ISP has to either build its own SMS gateway to interface with the
   various wireless network service providers or use a third-party SMS
   clearinghouse (relay) to notify users.  In both cases, an ISP may
   incur fees related to SMS notifications, depending upon the method
   used to send the notifications.  An additional downside is that SMS
   messages sent to a user may result in a charge to the user by their
   wireless provider, depending upon the plan to which they subscribe
   and the country in which the user resides.  Another minor
   disadvantage is that it is possible to notify the wrong user if the
   intended user changes their mobile number but forgets to update it
   with the ISP.

   There are several other drawbacks with this communications method.
   There is a high probability that subscriber may interpret the
   communication to be spam and thus ignore it.  Also, not every user
   uses SMS, and/or the user may not provide their SMS address or mobile
   number to the ISP.  Even in those cases where a user does have an SMS
   address or mobile number, their device may not be powered on or
   otherwise available on a wireless network when the notification is
   attempted.  There may also be a privacy concern on the part of users
   when such an SMS notification must be transmitted over a third-party
   network and/or SMS clearinghouse.  As such, should this method be
   used, the notification should be discreet and not include any PII in
   the notification itself.

5.7. Web Browser Notification

   Near-real-time notification to the user's web browser is another
   technique that may be utilized for notifying the user [RFC6108],
   though how such a system might operate is outside the scope of this
   document.  Such a notification could have a comparative advantage
   over a walled garden notification, in that it does not restrict
   traffic to a specified list of destinations in the same way that a
   walled garden would, by definition.  However, as with a walled garden
   notification, there is no guarantee that a user is making use of a
   web browser at any given time, though such a system could certainly
   provide a notification when such a browser is eventually used.
   Compared to a walled garden, a web browser notification is probably

   preferred from the perspective of Internet users, as it does not have
   the risk of disrupting non-web sessions, such as online games, VoIP
   calls, etc. (as noted in Section 5.4).

   There are alternative methods of web browser notification offered
   commercially by a number of vendors.  Many of the techniques used are
   proprietary, and it is not within the scope of this document to
   describe how they are implemented.  These techniques have been
   successfully implemented at several ISPs.

   It should be noted that web notification is only intended to notify
   devices running a web browser.

5.8. Considerations for Notification to Public Network Locations

   Delivering a notification to a location that provides a shared public
   network, such as a train station, public square, coffee shop, or
   similar location may be of low value since the users connecting to
   such networks are typically highly transient and generally not known
   to site or network administrators.  For example, a system may detect
   that a host on such a network has a bot, but by the time a
   notification is generated, that user has departed from the network
   and moved elsewhere.

5.9. Considerations for Notification to Network Locations Using a
Shared IP Address

   Delivering a notification to a location that accesses the Internet
   routed through one or more shared public IP addresses may be of low
   value since it may be quite difficult to differentiate between users
   when providing a notification.  For example, on a business network of
   500 users, all sharing one public IP address, it may be sub-optimal
   to provide a notification to all 500 users if you only need one
   specific user to be notified and take action.  As a result, such
   networks may find value in establishing a localized bot detection and
   notification system, just as they are likely to also establish other
   localized systems for security, file sharing, email, and so on.

   However, should an ISP implement some form of notification to such
   networks, it may be better to simply send notifications to a
   designated network administrator at the site.  In such a case, the
   local network administrator may like to receive additional
   information in such a notification, such as a date and timestamp, the
   source port of the infected system, and malicious sites and ports
   that may have been visited.

5.10. Notification and End User Expertise

   The ultimate effectiveness of any of the aforementioned forms of
   notification is heavily dependent upon both the expertise of the end
   user and the wording of any such notification.  For example, while a
   user may receive and acknowledge a notification, that user may lack
   the necessary technical expertise to understand or be able to deal
   effectively with the threat.  As a result, it is important that such
   notifications use clear and easily understood language, so that the
   majority of users (who are non-technical) may understand the
   notification.  In addition, a notification should provide easily
   understood guidance on how to remediate a threat as described in
   Section 6, potentially with one path for technical users to take and
   another for non-technical users.

6. Remediation of Hosts Infected with a Bot

   This section covers the different options available to remediate a
   host, which means to remove, disable, or otherwise render a bot
   harmless.  Prior to this step, an ISP has detected the bot, notified
   the user that one of their hosts is infected with a bot, and now may
   provide some recommended means to clean the host.  The generally
   recommended approach is to provide the necessary tools and education
   to the user so that they may perform bot remediation themselves,
   particularly given the risks and difficulties inherent in attempting
   to remove a bot.

   For example, this may include the creation of a special web site with
   security-oriented content that is dedicated for this purpose.  This
   should be a well-publicized security web site to which a user with a
   bot infection can be directed to for remediation.  This security web
   site should clearly explain why the user was notified and may include
   an explanation of what bots are and the threats that they pose.
   There should be a clear explanation of the steps that the user should
   take in order to attempt to clean their host and information on how
   users can keep the host free of future infections.  The security web
   site should also have a guided process that takes non-technical users
   through the remediation process, on an easily understood, step-by-
   step basis.

   In terms of the text used to explain what bots are and the threats
   that they pose, something simple such as this may suffice:

      What is a bot?  A bot is a piece of software, generally installed
      on your machine without your knowledge, which either sends spam or
      tries to steal your personal information.  They can be very
      difficult to spot, though you may have noticed that your computer
      is running much more slowly than usual or you may notice regular

      disk activity even when you are not doing anything.  Ignoring this
      problem is risky to you and your personal information.  Thus, bots
      need to be removed to protect your data and your personal
      information.

   Many bots are designed to work in a very stealthy manner, and as
   such, there may be a need to make sure that the Internet user
   understands the magnitude of the threat faced despite the stealthy
   nature of the bot.

   It is also important to note that it may not be immediately apparent
   to the Internet user precisely which devices have been infected with
   a particular bot.  This may be due to the user's home network
   configuration, which may encompass several hosts, where a home
   gateway that performs Network Address Translation (NAT) to share a
   single public IP address has been used.  Therefore, any of these
   devices can be infected with a bot.  The consequence of this for an
   ISP is that remediation advice may not ultimately be immediately
   actionable by the Internet user, as that user may need to perform
   additional investigation within their own home network.

   An added complication is that the user may have a bot infection on a
   device such as a video console, multimedia system, appliance, or
   other end user computing device that does not have a typical desktop
   computing interface.  As a result, diligence needs to be taken by the
   ISP where possible such that it can identify and communicate the
   specific nature of the device that has been infected with a bot and
   provide further appropriate remediation advice.  If the ISP cannot
   pin down the device or identify its type, then it should make it
   clear to the user that any initial advice given is generic and
   further advice can be given (or is available) once the type of
   infected device is known.

   There are a number of forums that exist online to provide security-
   related support to end users.  These forums are staffed by volunteers
   and often are focused around the use of a common tool set to help end
   users to remediate hosts infected with malware.  It may be
   advantageous to ISPs to foster a relationship with one or more
   forums, perhaps by offering free hosting or other forms of
   sponsorship.

   It is also important to keep in mind that not all users will be
   technically adept, as noted in Section 5.10.  As a result, it may be
   more effective to provide a range of suggestion options for
   remediation.  This may include, for example, a very detailed "do it
   yourself" approach for experts, a simpler guided process for the
   average user, and even assisted remediation as described in
   Section 6.2.

6.1. Guided Remediation Process

   Minimally, the Guided Remediation Process should include the
   following goals, with options and/or recommendations for achieving
   them:

   1.  Back up personal files.  For example:

          Before you start, make sure to back up all of your important
          data.  (You should do this on a regular basis anyway.)  You
          can back up your files manually or using a system backup
          software utility, which may be part of your Operating System
          (OS).  You can back up your files to a USB Thumb Drive (aka
          USB Key), a writable CD/DVD-ROM, an external hard drive, a
          network file server, or an Internet-based backup service.

       It may be advisable to suggest that the user backup is performed
       onto separate backup media or devices if they suspect bot
       infection.

   2.  Download OS patches and Anti-Virus (A/V) software updates.  For
       example, links could be provided to Microsoft Windows updates,
       Apple Mac OS updates, or other major operating systems that are
       relevant to users and their devices.

   3.  Configure the host to automatically install updates for the OS,
       A/V, and other common web browsers such as Microsoft Internet
       Explorer, Mozilla Firefox, Apple Safari, Opera, and Google
       Chrome.

   4.  Get professional assistance if they are unable to remove the bots
       themselves.  If purchasing professional assistance, then the user
       should be encouraged to predetermine how much they are willing to
       pay for that help.  For example, if the host that is being
       remediated is old and can easily be replaced with a new, faster,
       larger, and more reliable system for a certain cost, then it
       makes no sense to spend more than that cost to fix the old host.
       On the other hand, if the customer has a brand-new host, it might
       make perfect sense to spend the money to attempt to remediate it.

   5.  To continue, regardless of whether the user or a knowledgeable
       technical assistant is working on remediating the host, the first
       task should be to determine which of multiple potentially
       infected machines may be the one that needs attention (in the
       common case of multiple hosts in a home network).  Sometimes, as
       in cases where there is only a single directly attached host, or
       the user has been noticing problems with one of their hosts, this
       can be easy.  Other times, it may be more difficult, especially

       if there are no clues as to which host is infected.  If the user
       is behind a home gateway/router, then the first task may be to
       ascertain which of the machines is infected.  In some cases, the
       user may have to check all machines to identify the infected one.

   6.  ISPs may also look at offering a CD/DVD with remediation
       processes and software in the event that a host is so badly
       infected as to be unable to communicate over the Internet.

   7.  User surveys to solicit feedback on whether the notification and
       remediation process is effective and what recommended changes
       could be made in order to improve the ease, understandability,
       and effectiveness the remediation process.

   8.  If the user is interested in reporting the host's bot infection
       to an applicable law enforcement authority, then the host
       effectively becomes a cyber "crime scene", and the infection
       should not be mitigated unless or until law enforcement has
       collected the necessary evidence.  For individuals in this
       situation, the ISP may wish to provide links to local, state,
       federal, or other relevant computer crime offices.  (Note: Some
       "minor" incidents, even if highly traumatic to the user, may not
       be sufficiently serious for law enforcement to commit some of
       their limited resources to an investigation.)  In addition,
       individual regions may have other, specialized computer crime
       organizations to which these incidents can be reported.  For
       example, in the United States, that organization is the Internet
       Crime Complaint Center, at http://www.ic3.gov.

9. Users may also be interested in links to security expert forums,
where other users can assist them.

6.2. Professionally Assisted Remediation Process

   It should be acknowledged that, based on the current state of
   remediation tools and the technical abilities of end users, that many
   users may be unable to remediate on their own.  As a result, it is
   recommended that users have the option for professional assistance.
   This may entail online or telephone assistance for remediation, as
   well as working face to face with a professional who has training and
   expertise in the removal of malware.  It should be made clear at the
   time of offering this service that this service is intended for those
   that do not have the skills or confidence to attempt remediation and
   is not intended as an up-sell by the ISP.

7. Failure or Refusal to Remediate

   ISP systems should track the bot infection history of hosts in order
   to detect when users consistently fail to remediate or refuse to take
   any steps to remediate.  In such cases, ISPs may need to consider
   taking additional steps to protect their network, other users and
   hosts on that network, and other networks.  Such steps may include a
   progression of actions up to and including account termination.
   Refusal to remediate can be viewed as a business issue, and as such,
   no technical recommendation is possible.

8. Sharing of Data from the User to the ISP

   As an additional consideration, it may be useful to create a process
   by which users could choose, at their option and with their express
   consent, to share data regarding their bot infections with their ISP
   and/or another authorized third party.  Such third parties may
   include governmental entities that aggregate threat data, such as the
   Internet Crime Complaint Center referred to earlier in this document,
   academic institutions, and/or security researchers.  While in many
   cases the information shared with the user's ISP or designated third
   parties will only be used for aggregated statistical analysis, it is
   also possible that certain research needs may be best met with more
   detailed data.  Thus, any such data sharing from a user to the ISP or
   authorized third party may contain some type of personally
   identifiable information, either by design or inadvertently.  As a
   result, any such data sharing should be enabled on an opt-in basis,
   where users review and approve of the data being shared and the
   parties with which it is to be shared, unless the ISP is already
   required to share such data in order to comply with local laws and
   applicable regulations.

9.  Security Considerations

   This document describes in detail the numerous security risks and
   concerns relating to botnets.  As such, it has been appropriate to
   include specific information about security in each section above.
   This document describes the security risks related to malicious bot
   infections themselves, such as enabling identity theft, theft of
   authentication credentials, and the use of a host to unwittingly
   participate in a DDoS attack, among many other risks.  Finally, the
   document also describes security risks that may relate to the
   particular methods of communicating a notification to Internet users.
   Bot networks and bot infections pose extremely serious security
   risks, so readers should review this document carefully.

   In addition, regarding notifications as described in Section 5, care
   should be taken to assure users that notifications have been provided
   by a trustworthy site and/or party, so that the notification is more
   difficult for phishers and/or malicious parties using social
   engineering tactics to mimic.  Otherwise, care should be taken to
   ensure that the user has some level of trust that the notification is
   valid and/or that the user has some way to verify via some other
   mechanism or step that the notification is valid.

10. Privacy Considerations

   This document describes at a high level the activities to which ISPs
   should be sensitive, i.e., where the collection or communication of
   PII may be possible.  In addition, when performing notifications to
   end users (see Section 5), those notifications should not include
   PII.

   As noted in Section 8, any sharing of data from the user to the ISP
   and/or authorized third parties should be done on an opt-in basis.
   Additionally the ISP and or authorized third parties should clearly
   state what data will be shared and with whom the data will be shared.

   Lastly, as noted in other sections, there may be legal requirements
   in particular legal jurisdictions concerning how long any subscriber-
   related or other data is retained.  An ISP operating in such a
   jurisdiction should be aware of these requirements and should comply
   with them.

11. Acknowledgements

   The authors wish to acknowledge the following individuals and groups
   for performing a detailed review of this document and/or providing
   comments and feedback that helped to improve and evolve this
   document:

   Mark Baugher

   Richard Bennett

   James Butler

   Vint Cerf

   Alissa Cooper

   Jonathan Curtis

   Jeff Chan

   Roland Dobbins

   Dave Farber

   Stephen Farrell

   Eliot Gillum

   Joel Halpern

   Joel Jaeggli

   Scott Keoseyan

   Murray S. Kucherawy

   The Messaging Anti-Abuse Working Group (MAAWG)

   Jose Nazario

   Gunter Ollmann

   David Reed

   Roger Safian

   Donald Smith

   Joe Stewart

   Forrest Swick

   Sean Turner

   Robb Topolski

   Maxim Weinstein

   Eric Ziegast

12. Informative References

   [BIOS]     Sacco, A. and A. Ortega, "Persistent BIOS Infection",
              March 2009, <http://www.coresecurity.com/files/
              attachments/Persistent_BIOS_Infection_CanSecWest09.pdf>.

   [Combat-Zone]
              Alshech, E., "Cyberspace as a Combat Zone: The Phenomenon
              of Electronic Jihad", February 2007, <http://
              www.memrijttm.org/content/en/report.htm?report=1822>.

   [Conficker]
              Porras, P., Saidi, H., and V. Yegneswaran, "An Analysis of
              Conficker's Logic and Rendezvous Points", March 2009,
              <http://mtc.sri.com/Conficker/>.

   [DDoS]     Saafan, A., "Distributed Denial of Service Attacks:
              Explanation, Classification and Suggested Solutions",
              March 2009, <www.exploit-db.com/download_pdf/14738/>.

   [Dragon]   Nagaraja, S. and R. Anderson, "The snooping dragon:
              social-malware surveillance of the Tibetan movement",
              March 2009,
              <http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-746.pdf>.

   [Estonia]  Evron, G., "Battling Botnets and Online Mobs: Estonia's
              Defense Efforts during the Internet War", 2008, <http://
              journal.georgetown.edu/wp-content/uploads/9.1-Evron.pdf>.

   [Gh0st]    Vallentin, M., Whiteaker, J., and Y. Ben-David, "The Gh0st
              in the Shell: Network Security in the Himalayas",
              February 2010, <http://www.infowar-monitor.net/wp-content/
              uploads/2010/02/cs294-28-paper.pdf>.

   [RFC1459]  Oikarinen, J. and D. Reed, "Internet Relay Chat Protocol",
              RFC 1459, May 1993.

   [RFC2142]  Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
              FUNCTIONS", RFC 2142, May 1997.

   [RFC3954]  Claise, B., "Cisco Systems NetFlow Services Export Version
              9", RFC 3954, October 2004.

   [RFC5070]  Danyliw, R., Meijer, J., and Y. Demchenko, "The Incident
              Object Description Exchange Format", RFC 5070,
              December 2007.

   [RFC5965]  Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              August 2010.

   [RFC6108]  Chung, C., Kasyanov, A., Livingood, J., Mody, N., and B.
              Van Lieu, "Comcast's Web Notification System Design",
              RFC 6108, February 2011.

   [Snort]    Roesch, M., "Snort Home Page", March 2009,
              <http://www.snort.org/>.

   [Spamalytics]
              Kanich, C., Kreibich, C., Levchenko, K., Enright, B.,
              Voelker, G., Paxson, V., and S. Savage, "Spamalytics: An
              Empirical Analysis of Spam Marketing Conversion",
              October 2008, <http://www.icir.org/christian/publications/
              2008-ccs-spamalytics.pdf>.

   [Threat-Report]
              Ahamad, M., Amster, D., Barret, M., Cross, T., Heron, G.,
              Jackson, D., King, J., Lee, W., Naraine, R., Ollman, G.,
              Ramsey, J., Schmidt, H., and P. Traynor, "Emerging Cyber
              Threats Report for 2009: Data, Mobility and Questions of
              Responsibility will Drive Cyber Threats in 2009 and
              Beyond", October 2008, <http://smartech.gatech.edu/
              bitstream/1853/26301/1/CyberThreatsReport2009.pdf>.

   [Whiz-Kid] Berinato, S., "Case Study: How a Bookmaker and a Whiz Kid
              Took On a DDOS-based Online Extortion Attack", May 2005,
              <http://www.csoonline.com/article/220336/
              How_a_Bookmaker_and_a_Whiz_Kid_Took_On_a_DDOS_based_Online
              _Extortion_Attack>.

Appendix A. Examples of Third-Party Malware Lists

   As noted in Section 4, there are many potential third parties that
   may be willing to share lists of infected hosts.  This list is for
   example purposes only, is not intended to be either exclusive or
   exhaustive, and is subject to change over time.

   o  Arbor - Atlas, see http://atlas.arbor.net/

   o  Internet Systems Consortium - Secure Information Exchange (SIE),
      see https://sie.isc.org/

   o  Microsoft - Smart Network Data Services (SNDS), see
      https://postmaster.live.com/snds/

   o  SANS Institute / Internet Storm Center - DShield Distributed
      Intrusion Detection System, see http://www.dshield.org/about.html

   o  ShadowServer Foundation, see http://www.shadowserver.org/

   o  Spamhaus - Policy Block List (PBL), see
      http://www.spamhaus.org/pbl/

   o  Spamhaus - Exploits Block List (XBL), see
      http://www.spamhaus.org/xbl/

   o  Team Cymru - Community Services, see http://www.team-cymru.org/

Authors' Addresses

   Jason Livingood
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103
   USA

   EMail: jason_livingood@cable.comcast.com
   URI:   http://www.comcast.com

   Nirmal Mody
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103
   USA

   EMail: nirmal_mody@cable.comcast.com
   URI:   http://www.comcast.com

   Mike O'Reirdan
   Comcast Cable Communications
   One Comcast Center
   1701 John F. Kennedy Boulevard
   Philadelphia, PA  19103
   USA

   EMail: michael_oreirdan@cable.comcast.com
   URI:   http://www.comcast.com