Network Working Group                                          P. Savola
Request for Comments: 5294                                     CSC/FUNET
Category: Informational                                       J. Lingard
                                                                 Arastra
                                                             August 2008

          Host Threats to Protocol Independent Multicast (PIM)

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Abstract

   This memo complements the list of multicast infrastructure security
   threat analysis documents by describing Protocol Independent
   Multicast (PIM) threats specific to router interfaces connecting
   hosts.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Host-Interface PIM Vulnerabilities . . . . . . . . . . . . . .  2
     2.1.  Nodes May Send Illegitimate PIM Register Messages  . . . .  3
     2.2.  Nodes May Become Illegitimate PIM Neighbors  . . . . . . .  3
     2.3.  Routers May Accept PIM Messages from Non-Neighbors . . . .  3
     2.4.  An Illegitimate Node May Be Elected as the PIM DR or DF  .  3
       2.4.1.  PIM-SM Designated Router Election  . . . . . . . . . .  3
       2.4.2.  BIDIR-PIM Designated Forwarder Election  . . . . . . .  4
     2.5.  A Node May Become an Illegitimate PIM Asserted
           Forwarder  . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.6.  BIDIR-PIM Does Not Use RPF Check . . . . . . . . . . . . .  4
   3.  On-Link Threats  . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Denial-of-Service Attack on the Link . . . . . . . . . . .  5
     3.2.  Denial-of-Service Attack on the Outside  . . . . . . . . .  6
     3.3.  Confidentiality, Integrity, or Authorization Violations  .  6
   4.  Mitigation Methods . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Passive Mode for PIM . . . . . . . . . . . . . . . . . . .  7
     4.2.  Use of IPsec among PIM Routers . . . . . . . . . . . . . .  7
     4.3.  IP Filtering PIM Messages  . . . . . . . . . . . . . . . .  8
     4.4.  Summary of Vulnerabilities and Mitigation Methods  . . . .  8
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 10
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10

1.  Introduction

   There has been some analysis of the security threats to the multicast
   routing infrastructures [RFC4609], some work on implementing
   confidentiality, integrity, and authorization in the multicast
   payload [RFC3740], and also some analysis of security threats in
   Internet Group Management Protocol/Multicast Listener Discovery
   (IGMP/MLD) [DALEY-MAGMA], but no comprehensive analysis of security
   threats to PIM at the host-connecting (typically "Local Area
   Network") links.

   We define these PIM host threats to include:

   o  Nodes using PIM to attack or deny service to hosts on the same
      link,

   o  Nodes using PIM to attack or deny service to valid multicast
      routers on the link, or

   o  Nodes using PIM (Register messages) to bypass the controls of
      multicast routers on the link.

   The attacking node is typically a host or a host acting as an
   illegitimate router.

   A node originating multicast data can disturb existing receivers of
   the group on the same link, but this issue is not PIM-specific so it
   is out of scope.  Subverting legitimate routers is out of scope.
   Security implications on multicast routing infrastructure are
   described in [RFC4609].

   This document analyzes the PIM host-interface vulnerabilities,
   formulates a few specific threats, proposes some potential ways to
   mitigate these problems, and analyzes how well those methods
   accomplish fixing the issues.

   It is assumed that the reader is familiar with the basic concepts of
   PIM.

   Analysis of PIM-DM [RFC3973] is out of scope of this document.

2.  Host-Interface PIM Vulnerabilities

   This section briefly describes the main attacks against host-
   interface PIM signaling, before we get to the actual threats and
   mitigation methods in the next sections.

   The attacking node may be either a malicious host or an illegitimate
   router.

2.1.  Nodes May Send Illegitimate PIM Register Messages

   PIM Register messages are sent unicast, and contain encapsulated
   multicast data packets.  Malicious hosts or routers could also send
   Register messages themselves, for example, to get around rate-limits
   or to interfere with foreign Rendezvous Points (RPs), as described in
   [RFC4609].

   The Register message can be targeted to any IP address, whether in or
   out of the local PIM domain.  The source address may be spoofed,
   unless spoofing has been prevented [RFC3704], to create arbitrary
   state at the RPs.

2.2.  Nodes May Become Illegitimate PIM Neighbors

   When PIM has been enabled on a router's host interface, any node can
   also become a PIM neighbor using PIM Hello messages.  Having become a
   PIM neighbor in this way, the node is able to send other PIM messages
   to the router and may use those messages to attack the router.

2.3.  Routers May Accept PIM Messages from Non-Neighbors

   The PIM-SM (Sparse Mode) specification recommends that PIM messages
   other than Hellos should not be accepted, except from valid PIM
   neighbors.  The Bidirectional-PIM (BIDIR-PIM) specification specifies
   that packets from non-neighbors "SHOULD NOT" be accepted; see Section
   5.2 of [RFC5015].  However, the specification does not mandate this,
   so some implementations may be susceptible to attack from PIM
   messages sent by non-neighbors.

2.4.  An Illegitimate Node May Be Elected as the PIM DR or DF

2.4.1.  PIM-SM Designated Router Election

   In PIM-SM, the Designated Router (DR) on a Local Area Network (LAN)
   is responsible for Register-encapsulating data from new sources on
   the LAN, and for generating PIM Join/Prune messages on behalf of
   group members on the LAN.

   A node that can become a PIM neighbor can also cause itself to be
   elected DR, whether or not the DR Priority option is being used in
   PIM Hello messages on the LAN.

2.4.2.  BIDIR-PIM Designated Forwarder Election

   In BIDIR-PIM [RFC5015], a Designated Forwarder (DF) is elected per
   link.  The DF is responsible for forwarding data downstream onto the
   link, and also for forwarding data from its link upstream.

   A node that can become a BIDIR-PIM neighbor (this is just like
   becoming a PIM neighbor, except that the PIM Hello messages must
   include the Bidirectional Capable PIM-Hello option) can cause itself
   to be elected DF by sending DF Offer messages with a better metric
   than its neighbors.

   There are also some other BIDIR-PIM attacks related to DF election,
   including spoofing DF Offer and DF Winner messages (e.g., using a
   legitimate router's IP address), making all but the impersonated
   router believe that router is the DF.  Also, an attacker might
   prevent the DF election from converging by sending an infinite
   sequence of DF Offer messages.

   For further discussion of BIDIR-PIM threats, we refer to the Security
   Considerations section in [RFC5015].

2.5.  A Node May Become an Illegitimate PIM Asserted Forwarder

   With a PIM Assert message, a router can be elected to be in charge of
   forwarding all traffic for a particular (S,G) or (*,G) onto the LAN.
   This overrides DR behavior.

   The specification says that Assert messages should only be accepted
   from known PIM neighbors, and "SHOULD" be discarded otherwise.  So,
   either the node must be able to spoof an IP address of a current
   neighbor, form a PIM adjacency first, or count on these checks being
   disabled.

   The Assert Timer, by default, is 3 minutes; the state must be
   refreshed or it will be removed automatically.

   As noted before, it is also possible to spoof an Assert (e.g., using
   a legitimate router's IP address) to cause a temporary disruption on
   the LAN.

2.6.  BIDIR-PIM Does Not Use RPF Check

   PIM protocols do not perform Reverse Path Forwarding (RPF) check on
   the shared tree (e.g., in PIM-SM from the RP to local receivers).  On
   the other hand, RPF check is performed, e.g., on stub host
   interfaces.  Because all forwarding in BIDIR-PIM is based on the
   shared tree principle, it does not use RPF check to verify that the

   forwarded packets are being received from a "topologically correct"
   direction.  This has two immediately obvious implications:

   1.  A node may maintain a forwarding loop until the Time to Live
       (TTL) runs out by passing packets from interface A to B. This is
       not believed to cause significant new risk as with a similar ease
       such a node could generate original packets that would loop back
       to its other interface.

   2.  A node may spoof source IP addresses in multicast packets it
       sends.  Other PIM protocols drop such packets when performing the
       RPF check.  BIDIR-PIM accepts such packets, allowing easier
       Denial-of-Service (DoS) attacks on the multicast delivery tree
       and making the attacker less traceable.

3.  On-Link Threats

   The previous section described some PIM vulnerabilities; this section
   gives an overview of the more concrete threats exploiting those
   vulnerabilities.

3.1.  Denial-of-Service Attack on the Link

   The easiest attack is to deny the multicast service on the link.
   This could mean either not forwarding all (or parts of) multicast
   traffic from upstream onto the link, or not registering or forwarding
   upstream the multicast transmissions originated on the link.

   These attacks can be done in multiple ways: the most typical one
   would be becoming the DR through becoming a neighbor with Hello
   messages and winning the DR election.  After that, one could, for
   example:

   o  Not send any PIM Join/Prune messages based on the IGMP reports, or

   o  Not forward or register any sourced packets.

   Sending PIM Prune messages may also be an effective attack vector
   even if the attacking node is not elected DR, since PIM Prune
   messages are accepted from downstream interfaces even if the router
   is not a DR.

   An alternative mechanism is to send a PIM Assert message, spoofed to
   come from a valid PIM neighbor or non-spoofed if a PIM adjacency has
   already been formed.  For the particular (S,G) or (*,G) from the
   Assert message, this creates the same result as getting elected as a
   DR.  With BIDIR-PIM, similar attacks can be done by becoming the DF
   or by preventing the DF election from converging.

3.2.  Denial-of-Service Attack on the Outside

   It is also possible to perform Denial-of-Service attacks on nodes
   beyond the link, especially in environments where a multicast router
   and/or a DR is considered to be a trusted node.

   In particular, if DRs perform some form of rate-limiting, for
   example, on new Join/Prune messages, becoming a DR and sending those
   messages yourself allows one to subvert these restrictions;
   therefore, rate-limiting functions need to be deployed at multiple
   layers, as described in [RFC4609].

   In addition, any host can send PIM Register messages on their own, to
   whichever RP it wants; further, if unicast RPF (Reverse Path
   Forwarding) mechanisms [RFC3704] have not been applied, the packet
   may be spoofed.  This can be done to get around rate-limits, and/or
   to attack remote RPs, and/or to interfere with the integrity of an
   ASM group.  This attack is also described in [RFC4609].

   Also, BIDIR-PIM does not prevent nodes from using topologically
   incorrect addresses (source address spoofing) making such an attack
   more difficult to trace.

3.3.  Confidentiality, Integrity, or Authorization Violations

   Contrary to unicast, any node is able to legitimately receive all
   multicast transmission on the link by just adjusting the appropriate
   link-layer multicast filters.  Confidentiality (if needed) must be
   obtained by cryptography.

   If a node can become a DR, it is able to violate the integrity of any
   data streams sent by sources on the LAN, by modifying (possibly in
   subtle, unnoticeable ways) the packets sent by the sources before
   Register-encapsulating them.

   If a node can form a PIM neighbor adjacency or spoof the IP address
   of a current neighbor, then if it has external connectivity by some
   other means other than the LAN, the node is able to violate the
   integrity of any data streams sent by external sources onto the LAN.
   It would do this by sending an appropriate Assert message onto the
   LAN to prevent the genuine PIM routers forwarding the valid data,
   obtaining the multicast traffic via its other connection, and
   modifying those data packets before forwarding them onto the LAN.

   In either of the above two cases, the node could operate as normal
   for some traffic, while violating integrity for some other traffic.

   A more elaborate attack is on authorization.  There are some very
   questionable models [HAYASHI] where the current multicast
   architecture is used to provide paid multicast service, and where the
   authorization/authentication is added to the group management
   protocols such as IGMP.  Needless to say, if a host would be able to
   act as a router, it might be possible to perform all kinds of
   attacks: subscribe to multicast service without using IGMP (i.e.,
   without having to pay for it), deny the service for the others on the
   same link, etc.  In short, to be able to ensure authorization, a
   better architecture should be used instead (e.g., [RFC3740]).

4.  Mitigation Methods

   This section lists some ways to mitigate the vulnerabilities and
   threats listed in previous sections.

4.1.  Passive Mode for PIM

   The current PIM specification seems to mandate running the PIM Hello
   protocol on all PIM-enabled interfaces.  Most implementations require
   PIM to be enabled on an interface in order to send PIM Register
   messages for data sent by sources on that interface or to do any
   other PIM processing.

   As described in [RFC4609], running full PIM, with Hello messages and
   all, is unnecessary for those stub networks for which only one router
   is providing multicast service.  Therefore, such implementations
   should provide an option to specify that the interface is "passive"
   with regard to PIM: no PIM packets are sent or processed (if
   received), but hosts can still send and receive multicast on that
   interface.

4.2.  Use of IPsec among PIM Routers

   Instead of passive mode, or when multiple PIM routers exist on a
   single link, one could also use IPsec to secure the PIM messaging, to
   prevent anyone from subverting it.  The actual procedures have been
   described in [RFC4601] and [LINKLOCAL].

   However, it is worth noting that setting up IPsec Security
   Associations (SAs) manually can be a very tedious process, and the
   routers might not even support IPsec; further automatic key
   negotiation may not be feasible in these scenarios either.  A Group
   Domain of Interpretation (GDOI) [RFC3547] server might be able to
   mitigate this negotiation.

4.3.  IP Filtering PIM Messages

   To eliminate both the unicast and multicast PIM messages, in similar
   scenarios to those for which PIM passive mode is applicable, it might
   be possible to block IP protocol 103 (all PIM messages) in an input
   access list.  This is more effective than PIM passive mode, as this
   also blocks Register messages.

   This is also acceptable when there is more than one PIM router on the
   link if IPsec is used (because the access-list processing sees the
   valid PIM messages as IPsec AH/ESP packets).  In this case, unicast
   Register messages must also be protected with IPsec or the routing
   topology must be such that the link is never used to originate, or
   transit unicast Register messages.

   When multiple routers exist on a link, IPsec is not required if it is
   possible to prevent hosts from sending PIM messages at the Ethernet
   switch (or equivalent) host ports.  This could be accomplished in at
   least two ways:

   1.  Use IP access lists on the stub routers to allow PIM messages
       from the valid neighbor IP addresses only, and implement IP
       spoofing prevention at the Ethernet-switch-port level using
       proprietary mechanisms, or

   2.  Filter out all PIM messages at configured host ports on Ethernet
       switches instead of doing it on the routers.

   The main benefit of this approach is that multiple stub routers can
   still communicate through the LAN without IPsec but hosts are not
   able to disturb the PIM protocol.  The drawback is that Ethernet
   switches need to implement much finer-grained IP layer filtering, and
   the operational requirements of carefully maintaining these filters
   could be significant.

4.4.  Summary of Vulnerabilities and Mitigation Methods

   This section summarizes the vulnerabilities, and how well the
   mitigation methods are able to cope with them.

   Summary of vulnerabilities and mitigations:

     +-----+---------------------+-----------------+-----------------+
     | Sec | Vulnerability       | One stub router | >1 stub routers |
     |     |                     | PASV|IPsec|Filt | PASV|IPsec|Filt |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+
     | 2.1 | Hosts Registering   |  N  | N+  |  Y  |  N  | N+  | Ysw |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+
     | 2.2 | Invalid Neighbor    |  Y  |  Y  |  Y  |  *  |  Y  | Ysw |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+
     | 2.3 | Adjacency Not Reqd  |  Y  |  Y  |  Y  |  *  |  Y  | Ysw |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+
     | 2.4 | Invalid DR /DF      |  Y  |  Y  |  Y  |  *  |  Y  | Ysw |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+
     | 2.5 | Invalid Forwarder   |  Y  |  Y  |  Y  |  *  |  Y  | Ysw |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+
     | 2.6 | No RPF Check (BIDIR)|  x  |  x  |  x  |  x  |  x  |  x  |
     +-----+---------------------+-----+-----+-----+-----+-----+-----+

                                 Figure 1

   "*" means Yes if IPsec is used in addition; No otherwise.

   "Ysw" means Yes if IPsec is used in addition or IP filtering is done
   on Ethernet switches on all host ports; No otherwise.

   "N+" means that the use of IPsec between the on-link routers does not
   protect from this; IPsec would have to be used at RPs.

   "x" means that, with BIDIR-PIM, IP access lists or RPF mechanisms
   need to be applied in stub interfaces to prevent originating packets
   with topologically incorrect source addresses.  This needs to be done
   in addition to any other chosen approach.

   To summarize, IP protocol filtering for all PIM messages appears to
   be the most complete solution when coupled with the use of IPsec
   between the real stub routers when there are more than one of them.
   However, IPsec is not required if PIM message filtering or a certain
   kind of IP spoofing prevention is applied on all the host ports on
   Ethernet switches.  If hosts performing registering is not considered
   a serious problem, IP protocol filtering and passive-mode PIM seem to
   be equivalent approaches.  Additionally, if BIDIR-PIM is used,
   ingress filtering will need to be applied in stub interfaces to
   multicast packets, as well as unicast, to prevent hosts using wrong
   source addresses.

5.  Acknowledgements

   Greg Daley and Gopi Durup wrote an excellent analysis of MLD security
   issues [DALEY-MAGMA], which gave inspiration in exploring the on-link
   PIM threats problem space.

   Ayan Roy-Chowdhury, Beau Williamson, Bharat Joshi, Dino Farinacci,
   John Zwiebel, Stig Venaas, Yiqun Cai, and Eric Gray provided good
   feedback for this memo.

6.  Security Considerations

   This memo analyzes the threats to the PIM multicast routing protocol
   on host interfaces and proposes some possible mitigation techniques.

7.  References

7.1.  Normative References

   [RFC4601]      Fenner, B., Handley, M., Holbrook, H., and I.
                  Kouvelas, "Protocol Independent Multicast - Sparse
                  Mode (PIM-SM): Protocol Specification (Revised)",
                  RFC 4601, August 2006.

   [RFC4609]      Savola, P., Lehtonen, R., and D. Meyer, "Protocol
                  Independent Multicast - Sparse Mode (PIM-SM) Multicast
                  Routing Security Issues and Enhancements", RFC 4609,
                  October 2006.

   [RFC5015]      Handley, M., Kouvelas, I., Speakman, T., and L.
                  Vicisano, "Bidirectional Protocol Independent
                  Multicast (BIDIR-PIM)", RFC 5015, October 2007.

7.2.  Informative References

   [DALEY-MAGMA]  Daley, G. and J. Combes, "Securing Neighbour Discovery
                  Proxy Problem Statement", Work in Progress,
                  February 2008.

   [HAYASHI]      Hayashi, T., "Internet Group membership Authentication
                  Protocol (IGAP)", Work in Progress, August 2003.

   [LINKLOCAL]    Atwood, J., Islam, S., and M. Siami, "Authentication
                  and Confidentiality in PIM-SM Link-local Messages",
                  Work in Progress, February 2008.

   [RFC3547]      Baugher, M., Weis, B., Hardjono, T., and H. Harney,
                  "The Group Domain of Interpretation", RFC 3547,
                  July 2003.

   [RFC3704]      Baker, F. and P. Savola, "Ingress Filtering for
                  Multihomed Networks", BCP 84, RFC 3704, March 2004.

   [RFC3740]      Hardjono, T. and B. Weis, "The Multicast Group
                  Security Architecture", RFC 3740, March 2004.

   [RFC3973]      Adams, A., Nicholas, J., and W. Siadak, "Protocol
                  Independent Multicast - Dense Mode (PIM-DM): Protocol
                  Specification (Revised)", RFC 3973, January 2005.

Authors' Addresses

   Pekka Savola
   CSC - Scientific Computing Ltd.
   Espoo
   Finland

   EMail: psavola@funet.fi


   James Lingard
   Arastra, Inc.
   P.O. Box 10905
   Palo Alto, CA  94303
   USA

   EMail: jchl@arastra.com

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