Network Working Group                                   M. Stillman, Ed.
Request for Comments: 5355                                         Nokia
Category: Informational                                         R. Gopal
                                                  Nokia Siemens Networks
                                                              E. Guttman
                                                        Sun Microsystems
                                                             S. Sengodan
                                                  Nokia Siemens Networks
                                                             M. Holdrege
                                                          September 2008

Threats Introduced by Reliable Server Pooling (RSerPool)
and Requirements for Security in Response to Threats

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

   Reliable Server Pooling (RSerPool) is an architecture and set of
   protocols for the management and access to server pools supporting
   highly reliable applications and for client access mechanisms to a
   server pool.  This document describes security threats to the
   RSerPool architecture and presents requirements for security to
   thwart these threats.

   1. Introduction ....................................................3
      1.1. Definitions ................................................3
      1.2. Conventions ................................................4
   2. Threats .........................................................4
      2.1. PE Registration/De-Registration Flooding --
           Non-Existent PE ............................................4
      2.2. PE Registration/De-Registration Flooding --
           Unauthorized PE ............................................5
      2.3. PE Registration/De-Registration Spoofing ...................6
      2.4. PE Registration/De-Registration Unauthorized ...............6
      2.5. Malicious ENRP Server Joins the Group of Legitimate
           ENRP Servers ...............................................7
      2.6. Registration/De-Registration with Malicious ENRP Server ....7
      2.7. Malicious ENRP Handlespace Resolution ......................8
      2.8. Malicious Node Performs a Replay Attack ....................9
      2.9. Re-Establishing PU-PE Security during Failover .............9
      2.10. Integrity ................................................10
      2.11. Data Confidentiality .....................................10
      2.12. ENRP Server Discovery ....................................11
      2.13. Flood of Endpoint-Unreachable Messages from the
            PU to the ENRP Server ....................................12
      2.14. Flood of Endpoint Keep-Alive Messages from the
            ENRP Server to a PE ......................................12
      2.15. Security of the ENRP Database ............................13
      2.16. Cookie Mechanism Security ................................13
      2.17. Potential Insider Attacks from Legitimate ENRP Servers ...14
   3. Security Considerations ........................................15
   4. Normative References ...........................................17

1. Introduction

   The RSerPool architecture [RFC5351] supports high-availability and
   load balancing by enabling a pool user to identify the most
   appropriate server from the server pool at a given time.  The
   architecture is defined to support a set of basic goals.  These
   include application-independent protocol mechanisms, separation of
   server naming from IP addressing, the use of the end-to-end principle
   to avoid dependencies on intermediate equipment, separation of
   session availability/failover functionality from the application
   itself, the ability to facilitate different server selection
   policies, the ability to facilitate a set of application-independent
   failover capabilities, and a peer-to-peer structure.

   RSerPool provides a session layer for robustness.  The session layer
   function may redirect communication transparently to upper layers.
   This alters the direct one-to-one association between communicating
   endpoints that typically exists between clients and servers.  In
   particular, secure operation of protocols often relies on assumptions
   at different layers regarding the identity of the communicating party
   and the continuity of the communication between endpoints.  Further,
   the operation of RSerPool itself has security implications and risks.
   The session layer operates dynamically, which imposes additional
   concerns for the overall security of the end-to-end application.

   This document explores the security implications of RSerPool, both
   due to its own functions and due to its being interposed between
   applications and transport interfaces.

   This document is related to the RSerPool Aggregate Server Access
   Protocol (ASAP) [RFC5352] and Endpoint Name Resolution Protocol
   (ENRP) [RFC5353] documents, which describe, in their Security
   Consideration sections, the mechanisms for meeting the security
   requirements in this document.  TLS [RFC5246] is the security
   mechanism for RSerPool that was selected to meet all the requirements
   described in this document.  The Security Considerations sections of
   ASAP and ENRP describe how TLS is actually used to provide the
   security that is discussed in this document.

1.1. Definitions

   This document uses the following terms:

   Endpoint Name Resolution Protocol (ENRP):
      Within the operational scope of RSerPool, ENRP[RFC5353] defines
      the procedures and message formats of a distributed fault-tolerant
      registry service for storing, bookkeeping, retrieving, and
      distributing pool operation and membership information.

   Aggregate Server Access Protocol (ASAP):
      ASAP [RFC5352] is a session layer protocol that uses ENRP to
      provide a high-availability handlespace.  ASAP is responsible for
      the abstraction of the underlying transport technologies, load
      distribution management, fault management, as well as the
      presentation to the upper layer (i.e., the ASAP User) of a unified
      primitive interface.

   Operational scope:
      The part of the network visible to pool users by a specific
      instance of the Reliable Server Pooling protocols.

   Pool (or server pool):
      A collection of servers providing the same application
      functionality.

   Pool handle:
      A logical pointer to a pool.  Each server pool will be
      identifiable in the operational scope of the system by a unique
      pool handle.

   ENRP handlespace (or handlespace):
      A cohesive structure of pool names and relations that may be
      queried by a client.  A client in this context is an application
      that accesses another remote application running on a server using
      a network.

   Pool element (PE):  A server entity having registered to a pool.

   Pool user (PU):  A server pool user.

1.2. Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2. Threats

2.1. PE Registration/De-Registration Flooding -- Non-Existent PE

2.1.1. Threat

   A malicious node could send a stream of false registrations/de-
   registrations on behalf of non-existent PEs to ENRP servers at a very
   rapid rate and thereby create unnecessary state in an ENRP server.

2.1.2. Effect

   The malicious node will corrupt the pool registrar database and/or
   disable the RSerPool discovery and database function.  This
   represents a denial-of-service attack, as the PU would potentially
   get an IP address of a non-existent PE in response to an ENRP query.

2.1.3. Requirement

   An ENRP server that receives a registration/de-registration MUST NOT
   create or update state information until it has authenticated the PE.
   TLS with a pre-shared-key (PSK) is mandatory to implement as the
   authentication mechanism.  For PSK, having a pre-shared-key
   constitutes authorization.  The network administrators of a pool need
   to decide which nodes are authorized to participate in the pool.  The
   justification for PSK is that we assume that one administrative
   domain will control and manage the server pool.  This allows for PSK
   to be implemented and managed by a central security administrator.

2.2. PE Registration/De-Registration Flooding -- Unauthorized PE

2.2.1. Threat

   A malicious node or PE could send a stream of registrations/de-
   registrations that are unauthorized to register/de-register to ENRP
   servers at a very rapid rate and thereby create unnecessary state in
   an ENRP server.

2.2.2. Effect

   This attack will corrupt the pool registrar database and/or disable
   the RSerPool discovery and database function.  There is the potential
   for two types of attacks: denial of service and data interception.
   In the denial-of-service attack, the PU gets an IP address of a rogue
   PE in response to an ENRP query, which might not provide the actual
   service.  In addition, a flood of message could prevent legitimate
   PEs from registering.  In the data interception attack, the rogue PE
   does provide the service as a man in the middle (MITM), which allows
   the attacker to collect data.

2.2.3. Requirement

   An ENRP server that receives a registration/de-registration MUST NOT
   create or update state information until the authentication
   information of the registering/de-registering entity is verified.

   TLS is used as the authentication mechanism between the ENRP server
   and PE.  TLS with PSK is mandatory to implement as the authentication
   mechanism.  For PSK, having a pre-shared-key constitutes
   authorization.  The network administrators of a pool need to decide
   which nodes are authorized to participate in the pool.

2.3. PE Registration/De-Registration Spoofing

2.3.1. Threat

   A malicious node could send false registrations/de-registrations to
   ENRP servers concerning a legitimate PE, thereby creating false state
   information in the ENRP servers.

2.3.2. Effect

   This would generate misinformation in the ENRP server concerning a PE
   and would be propagated to other ENRP servers, thereby corrupting the
   ENRP database.  Distributed Denial of Service (DDoS) could result: if
   a PE that is a target for a DDoS attack for some popular high-volume
   service, then the attacker can register a PE to which a lot of PUs
   will try to connect.  This allows man-in-the-middle or masquerade
   attacks on the service provided by the legitimate PEs.  If an
   attacker registers its server address as a PE and handles the
   requests, he can eavesdrop on service data.

2.3.3. Requirement

   An ENRP server that receives a registration/de-registration MUST NOT
   create or update state information until it has authenticated the PE.
   TLS is used as the authentication mechanism between the ENRP server
   and the PE.  TLS with PSK is mandatory to implement as the
   authentication mechanism.  For PSK, having a pre-shared-key
   constitutes authorization.  The network administrators of a pool need
   to decide which nodes are authorized to participate in the pool.  A
   PE can register only for itself and cannot register on behalf of
   other PEs.

2.4. PE Registration/De-Registration Unauthorized

2.4.1. Threat

   A PE that is not authorized to join a pool could send registrations/
   de-registrations to ENRP servers, thereby creating false state
   information in the ENRP servers.

2.4.2. Effect

   This attack would generate misinformation in the ENRP server
   concerning a PE and would be propagated to other ENRP servers thereby
   corrupting the ENRP database.  This allows man-in-the-middle or
   masquerade attacks on the service provided by the legitimate PEs.  If
   an attacker registers its server address as a PE and handles the
   requests, he can eavesdrop on service data.

2.4.3. Requirement

   An ENRP server that receives a registration/de-registration MUST NOT
   create or update state information until it has authorized the
   requesting entity.  TLS is used as the authentication mechanism.  TLS
   with PSK is mandatory to implement as the authentication mechanism.
   For PSK, having a pre-shared-key constitutes authorization.  The
   network administrators of a pool need to decide which nodes are
   authorized to participate in the pool.

2.5. Malicious ENRP Server Joins the Group of Legitimate ENRP Servers

2.5.1. Threat

   A malicious ENRP server joins the group of legitimate ENRP servers
   with the intent of propagating inaccurate updates to corrupt the ENRP
   database.  The attacker sets up an ENRP server and attempts to
   communicate with other ENRP servers.

2.5.2. Effect

The result would be Inconsistent ENRP database state.

2.5.3. Requirement

   ENRP servers MUST perform mutual authentication.  This would prevent
   the attacker from joining its ENRP server to the pool.  TLS is used
   as the mutual authentication mechanism.  TLS with PSK is mandatory to
   implement as the authentication mechanism.  For PSK, having a
   pre-shared-key constitutes authorization.  The network administrators
   of a pool need to decide which nodes are authorized to participate in
   the pool.

2.6. Registration/De-Registration with Malicious ENRP Server

2.6.1. Threat

A PE unknowingly registers/de-registers with a malicious ENRP server.

2.6.2. Effect

   The registration might not be properly processed or it might be
   ignored.  A rogue ENRP server has the ability to return any address
   to a user requesting service; this ability could result in denial of
   service or connection to a rogue PE that is the attacker's choice for
   service.

2.6.3. Requirement

   The PE MUST authenticate the ENRP server.  TLS is the mechanism used
   for the authentication.  TLS with PSK is mandatory to implement as
   the authentication mechanism.  For PSK, having a pre-shared-key
   constitutes authorization.  The network administrators of a pool need
   to decide which nodes are authorized to participate in the pool.
   This requirement prevents malicious outsiders and insiders from
   adding their own ENRP server to the pool.

2.7. Malicious ENRP Handlespace Resolution

2.7.1. Threat

   The ASAP protocol receives a handlespace resolution response from an
   ENRP server, but the ENRP server is malicious and returns random IP
   addresses or an inaccurate list in response to the pool handle.

2.7.2. Effect

   The PU application communicates with the wrong PE or is unable to
   locate the PE since the response is incorrect in saying that a PE
   with that handle did not exist.  A rogue ENRP server has the ability
   to return any address to ASAP requesting an address list that could
   result in denial of service or connection to a rogue PE of the
   attacker's choice for service.  From the PE, the attacker could
   eavesdrop or tamper with the application.

2.7.3. Requirement

   ASAP SHOULD authenticate the ENRP server.  TLS with certificates is
   the mandatory-to-implement mechanism used for authentication.  The
   administrator uses a centralized Certificate Authority (CA) to
   generate and sign certificates.  The certificate is stored on the
   ENRP server.  A CA trusted root certification authority certificate
   is sent to the client out of band, and the certificate signature on
   the ENRP server certificate is checked for validity during the TLS
   handshake.  This authentication prevents malicious outsiders and
   insiders from adding an ENRP server to the pool that may be accessed
   by ASAP.

2.8. Malicious Node Performs a Replay Attack

2.8.1. Threat

   A malicious node could replay the entire message previously sent by a
   legitimate entity.  This could create false/unnecessary state in the
   ENRP servers when the replay is for registration/de-registration or
   update.

2.8.2. Effect

   The result is that false/extra state is maintained by ENRP servers.
   This would most likely be used as a denial-of-service attack if the
   replay is used to de-register all PEs.

2.8.3. Requirement

The protocol MUST prevent replay attacks. The replay attack
prevention mechanism in TLS meets this requirement.

2.9. Re-Establishing PU-PE Security during Failover

2.9.1. Threat

   The PU fails over from PE A to PE B.  In the case that the PU had a
   trusted relationship with PE A, the PU will likely not have the same
   relationship established with PE B.

2.9.2. Effect

   If there was a trust relationship involving security context between
   PU and PE A, the equivalent trust relationship will not exist between
   PU and PE B.  This will violate security policy.  For example, if the
   security context with A involves encryption and the security context
   with B does not, then an attacker could take advantage of the change
   in security.

2.9.3. Requirement

   The application SHOULD be notified when failover occurs so the
   application can take appropriate action to establish a trusted
   relationship with PE B.  ENRP has a mechanism to perform this
   function.

2.10. Integrity

2.10.1. Threat

   The following are all instances of the same class of threats, and all
   have similar effects.

   a.  ENRP response to pool handle resolution is corrupted during
       transmission.

   b.  ENRP peer messages are corrupted during transmission.

   c.  PE sends an update for values, and that information is corrupted
       during transmission.

2.10.2. Effect

   The result is that ASAP receives corrupt information for pool handle
   resolution, which the PU believes to be accurate.  This corrupt
   information could be an IP address that does not resolve to a PE so
   the PU would not be able to contact the server.

2.10.3. Requirement

   An integrity mechanism MUST be present.  Corruption of data that is
   passed to the PU means that the PU can't rely on it.  The consequence
   of corrupted information is that the IP addresses passed to the PU
   might be wrong, in which case, it will not be able to reach the PE.
   The interfaces that MUST implement integrity are PE to ENRP server
   and ENRP to ENRP server.  The integrity mechanism in TLS is used for
   this.

2.11. Data Confidentiality

2.11.1. Threat

   An eavesdropper capable of snooping on fields within messages in
   transit may be able to gather information, such as
   topology/location/IP addresses, etc., which may not be desirable to
   divulge.

2.11.2. Effect

   Information that an administrator does not wish to divulge is
   divulged.  The attacker gains valuable information that can be used
   for financial gain or attacks on hosts.

2.11.3. Requirement

A provision for data confidentiality service SHOULD be available.
TLS provides data confidentiality in support of this mechanism.

2.12. ENRP Server Discovery

2.12.1. Threats

   a.  Thwarting successful discovery: When a PE wishes to register with
       an ENRP server, it needs to discover an ENRP server.  An attacker
       could thwart the successful discovery of ENRP server(s), thereby
       inducing the PE to believe that no ENRP server is available.  For
       instance, the attacker could reduce the returned set of ENRP
       servers to null or a small set of inactive ENRP servers.  The
       attacker performs a MITM attack to do this.

   b.  A similar thwarting scenario also applies when an ENRP server or
       ASAP on behalf of a PU needs to discover ENRP servers.

   c.  Spoofing successful discovery: An attacker could spoof the
       discovery by claiming to be a legitimate ENRP server.  When a PE
       wishes to register, it finds the spoofed ENRP server.  An
       attacker can only make such a claim if no security mechanisms are
       used.

   d.  A similar spoofing scenario also applies when an ENRP server or
       ASAP on behalf of a PU needs to discover ENRP servers.

2.12.2. Effects (Letters Correlate with Threats above)

   a.  A PE that could have been in an application server pool does not
       become part of a pool.  The PE does not complete discovery
       operation.  This is a DoS attack.

   b.  An ENRP server that could have been in an ENRP server pool does
       not become part of a pool.  A PU is unable to utilize services of
       ENRP servers.

   c.  This malicious ENRP would either misrepresent, ignore, or
       otherwise hide or distort information about the PE to subvert
       RSerPool operation.

   d.  Same as above.

2.12.3. Requirement

   A provision for authentication MUST be present and a provision for
   data confidentiality service SHOULD be present.  TLS has a mechanism
   for confidentiality.

2.13. Flood of Endpoint-Unreachable Messages from the PU to the ENRP
Server

2.13.1. Threat

   Endpoint-unreachable messages are sent by ASAP to the ENRP server
   when it is unable to contact a PE.  There is the potential that a PU
   could flood the ENRP server intentionally or unintentionally with
   these messages.  The non-malicious case would require an incorrect
   implementation.  The malicious case would be caused by writing code
   to flood the ENRP server with endpoint unreachable messages.

2.13.2. Effect

   The result is a DoS attack on the ENRP server.  The ENRP server would
   not be able to service other PUs effectively and would not be able to
   take registrations from PEs in a timely manner.  Further, it would
   not be able to communicate with other ENRP servers in the pool to
   update the database in a timely fashion.

2.13.3. Requirement

   The number of endpoint unreachable messages sent to the ENRP server
   from the PU SHOULD be limited.  This mechanism is described in the
   ASAP [RFC5352] protocol document.

2.14. Flood of Endpoint Keep-Alive Messages from the ENRP Server to a
PE

2.14.1. Threat

   Endpoint Keep-Alive messages would be sent from the ENRP server to
   the PEs during the process of changing the Home ENRP server for this
   PE.

2.14.2. Effect

   If the ENRP server maliciously sent a flood of endpoint Keep-Alive
   messages to the PE, the PE would not be able to service clients.  The
   result is a DoS attack on the PE.

2.14.3. Requirement

   ENRP MUST limit the frequency of Keep-Alive messages to a given PE to
   prevent overwhelming the PE.  This mechanism is described in the ENRP
   [RFC5353] protocol document.

2.15. Security of the ENRP Database

2.15.1. Threat

   Another consideration involves the security characteristics of the
   ENRP database.  Suppose that some of the PEs register with an ENRP
   server using security and some do not.  In this case, when a client
   requests handlespace resolution information from ENRP, it would have
   to be informed which entries are "secure" and which are not.

2.15.2. Effect

   This would not only complicate the protocol, but actually bring into
   question the security and integrity of such a database.  What can be
   asserted about the security of such a database is a very thorny
   question.

2.15.3. Requirement

   The requirement is that either the entire ENRP server database MUST
   be secure; that is, it has registrations exclusively from PEs that
   have used security mechanisms, or the entire database MUST be
   insecure; that is, registrations are from PEs that have used no
   security mechanisms.  ENRP servers that support security MUST reject
   any PE server registration that does not use the security mechanisms.
   Likewise, ENRP servers that support security MUST NOT accept updates
   from other ENRP servers that do not use security mechanisms.  TLS is
   used as the security mechanism so any information not sent using TLS
   to a secure ENRP server MUST be rejected.

2.16. Cookie Mechanism Security

   The application layer is out of scope for RSerPool.  However, some
   questions have been raised about the security of the cookie
   mechanism, which will be addressed.

   Cookies are passed via the ASAP control channel.  If TCP is selected
   as the transport, then the data and control channel MUST be
   multiplexed.  Therefore, the cases:

   a.  control channel is secured; data channel is not

   b.  data channel is secured; control channel is not

   are not possible, as the multiplexing onto one TCP port results in
   security for both data and control channels or neither.

   The multiplexing requirement results in the following cases:

   1.  the multiplexed control channel-data channel is secure; *or*

   2.  the multiplexed control channel-data channel is not secured.

   This applies to cookies in the sense that, if you choose to secure
   your control-data channel, then the cookies are secured.

   A second issue is that the PE could choose to sign and/or encrypt the
   cookie.  In this case, it must share keys and other information with
   other PEs.  This application-level state sharing is out of scope of
   RSerPool.

2.17. Potential Insider Attacks from Legitimate ENRP Servers

   The previous text does not address all byzantine attacks that could
   arise from legitimate ENRP servers.  True protection against
   misbehavior by authentic (but rogue) servers is beyond the capability
   of TLS security mechanisms.  Authentication using TLS does not
   protect against byzantine attacks, as authenticated ENRP servers
   might have been maliciously hacked.  Protections against insider
   attacks are generally specific to the attack, so more experimentation
   is needed.  For example, the following discusses two insider attacks
   and potential mitigations.

   One issue is that legitimate users may choose not to follow the
   proposed policies regarding the choice of servers (namely, members in
   the pool).  If the "choose a member at random" policy is employed,
   then a pool user can always set its "random choices" so that it picks
   a particular pool member.  This bypasses the "load sharing" idea
   behind the policy.  Another issue is that a pool member (or server)
   may report a wrong policy to a user.

   To mitigate the first attack, the protocol may require the pool user
   to "prove" to the pool member that the pool member was chosen
   "randomly", say by demonstrating that the random choice was the
   result of applying some hash function to a public nonce.  Different
   methods may be appropriate for other member scheduling policies.

   To mitigate the second attack, the protocol might require the PE to
   sign the policy sent to the ENRP server.  During pool handle
   resolution, the signed policy needs to be sent from an ENRP server to
   an ASAP endpoint in a way that will allow the user to later hold the
   server accountable to the policy.

3. Security Considerations

   This informational document characterizes potential security threats
   targeting the RSerPool architecture.  The security mechanisms
   required to mitigate these threats are summarized for each
   architectural component.  It will be noted which mechanisms are
   required and which are optional.

   From the threats described in this document, the security services
   required for the RSerPool protocol suite are given in the following
   table.

   Summary by component:

   RSerPool client --  mandatory-to-implement authentication of the ENRP
      server is required for accurate pool handle resolution.  This is
      to protect against threats from rogue ENRP servers.  In addition,
      confidentiality, integrity, and preventing replay attack are also
      mandatory to implement to protect from eavesdropping and data
      corruption or false data transmission.  Confidentiality is
      mandatory to implement and is used when privacy is required.

   PE to ENRP communications --  mandatory-to-implement mutual
      authentication, integrity, and protection from replay attack is
      required for PE to ENRP communications.  This is to protect the
      integrity of the ENRP handlespace database.  Confidentiality is
      mandatory to implement and is used when privacy is required.

   ENRP to ENRP communications --  mandatory-to-implement mutual
      authentication, integrity, and protection from replay attack is
      required for ENRP to ENRP communications.  This is to protect the
      integrity of the ENRP handlespace database.  Confidentiality is
      mandatory to implement and is used when privacy is required.

4. Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5352]   Stewart, R., Xie, Q., Stillman, M., and M. Tuexen,
              "Aggregate Server Access Protocol (ASAP)", RFC 5352,
              September 2008.

   [RFC5353]  Xie, Q., Stewart, R., Stillman, M., Tuexen, M., and A.
              Silverton, "Endpoint Handlespace Redundancy Protocol
              (ENRP)", RFC 5353, September 2008.

   [RFC5351]  Lei, P., Ong, L., Tuexen, M., and T. Dreibholz, "An
              Overview of Reliable Server Pooling Protocols", RFC 5351,
              September 2008.

Authors' Addresses

   Maureen Stillman, Ed.
   Nokia
   1167 Peachtree Court
   Naperville, IL  60540
   USA

   EMail: maureen.stillman@nokia.com

   Ram Gopal
   Nokia Siemens Networks
   12278 Scripps Summit Drive
   San Diego, CA  92131
   USA

   EMail: ram.gopal@nsn.com

   Erik Guttman
   Sun Microsystems
   Eichhoelzelstrasse 7
   74915 Waibstadt
   DE

   EMail: Erik.Guttman@sun.com

   Senthil Sengodan
   Nokia Siemens Networks
   6000 Connection Drive
   Irving, TX  75039
   USA

   EMail: Senthil.sengodan@nsn.com

   Matt Holdrege

   EMail: Holdrege@gmail.com

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