Internet Engineering Task Force                                P. Savola
Internet-Draft                                                 CSC/FUNET
Expires: July 15, 2005                                        J. Lingard
                                                     Data Connection Ltd
                                                        January 14, 2005

        Last-hop Threats to Protocol Independent Multicast (PIM)

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   An analysis of security threats has been done for some parts of the
   multicast infrastructure, but the threats specific to the last-hop
   attacks by hosts on the PIM routing protocol have not been well
   described in the past.  This memo aims to fill that gap.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Last-hop PIM Vulnerabilities . . . . . . . . . . . . . . . . .  3
     2.1   Nodes May Send Unauthorized PIM Register Messages  . . . .  3
     2.2   Nodes May Become Unauthorized PIM Neighbors  . . . . . . .  4
     2.3   Routers May Accept PIM Messages From Non-Neighbors . . . .  4
     2.4   An Unauthorized Node May Be Elected as the PIM DR  . . . .  4
     2.5   A Node May Become an Unauthorized PIM Asserted
           Forwarder  . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  On-link Threats  . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1   Denial-of-Service Attack on the Link . . . . . . . . . . .  5
     3.2   Denial-of-Service Attack on the Outside  . . . . . . . . .  5
     3.3   Confidentiality, Integrity or Authorization Violations . .  6
   4.  Mitigation Methods . . . . . . . . . . . . . . . . . . . . . .  6
     4.1   Passive Mode for PIM . . . . . . . . . . . . . . . . . . .  6
     4.2   Use of IPsec among PIM Routers . . . . . . . . . . . . . .  7
     4.3   IP Filtering PIM Messages  . . . . . . . . . . . . . . . .  7
     4.4   Summary of Vulnerabilities and Mitigation Methods  . . . .  7
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   8.1   Normative References . . . . . . . . . . . . . . . . . . . .  9
   8.2   Informative References . . . . . . . . . . . . . . . . . . .  9
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 10
       Intellectual Property and Copyright Statements . . . . . . . . 11

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1.  Introduction

   There has been some analysis of the security threats to the multicast
   routing infrastructures [I-D.ietf-mboned-mroutesec], some work on
   implementing confidentiality, integrity and authorization in the
   multicast payload [RFC3740], and also some analysis of security
   threats in IGMP/MLD [I-D.daley-magma-smld-prob], but no comprehensive
   analysis of security threats to PIM at the last-hop links.

   We define "PIM last-hop threats" to include:

   o  Nodes -- hosts or unauthorized routers -- 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.

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

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

2.  Last-hop PIM Vulnerabilities

   This section describes briefly the main attacks against last-hop PIM
   signalling, 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 unauthorized

2.1  Nodes May Send Unauthorized PIM Register Messages

   PIM Register messages are sent by 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).

   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.

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2.2  Nodes May Become Unauthorized 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

2.3  Routers May Accept PIM Messages From Non-Neighbors

   The PIM-SM specification recommends that PIM messages other than
   Hellos should not be accepted except from valid PIM neighbors.
   However, the specification does not mandate this, and so some
   implementations may be susceptible to attack from PIM messages sent
   by non-neighbors.

2.4  An Unauthorized Node May Be Elected as the PIM DR

   The Designated Router (DR) on a 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

   A node which 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.

2.5  A Node May Become an Unauthorized 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 behaviour.

   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

   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 on someone
   else's behalf to cause a temporary disruption on the LAN.

3.  On-link Threats

   The last section described some PIM vulnerabilities; this section

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   gives an overview of the more concrete threats exploiting those

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
   from upstream onto the link, or not registering or forwarding
   upstream the multicast transmissions originated on the link.

   These attacks can be done 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,

   o  Not forward or register any sourced packets, or

   o  Send PIM Prune messages to cut off existing transmissions because
      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

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
   oneself allows one to subvert these restrictions: therefore
   rate-limiting functions need to be deployed at multiple layers as
   described in [I-D.ietf-mboned-mroutesec].

   In addition, any host can send PIM Register messages on their own, to
   whichever RP it wants; further, if unicast RPF 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 [I-D.ietf-mboned-mroutesec].

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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 get to be 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 is 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 [I-D.hayashi-igap] 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 of 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.

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   As described in [I-D.ietf-mboned-mroutesec], 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 for 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 [I-D.ietf-pim-sm-v2-new] and

   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).  However, this presumes
   that the link is not used to transit unicast packets between the PIM
   routers, or that the Register messages are also being sent with

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.

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   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+  |  *  |
      | 2.2 | Invalid Neighbor   |  Y  |  Y  |  Y  |  *  |  Y  |  *  |
      | 2.3 | Adjacency Not Reqd |  Y  |  Y  |  Y  |  *  |  Y  |  *  |
      | 2.4 | Invalid DR         |  Y  |  Y  |  Y  |  *  |  Y  |  *  |
      | 2.5 | Invalid Forwarder  |  Y  |  Y  |  Y  |  *  |  Y  |  *  |

                                Figure 1

   "*" means Yes if IPsec is used in addition; 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.

   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.
   If hosts performing registering is not considered a serious problem,
   IP protocol filtering and passive-mode PIM seem to be equivalent

5.  Acknowledgements

   Greg Daley and Gopi Durup wrote an excellent analysis of MLD security
   issues [I-D.daley-magma-smld-prob], which gave inspiration in
   exploring the on-link PIM threats problem space.

   Ayan Roy-Chowdhury and Beau Williamson provided good feedback for
   this memo.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   This memo analyzes the threats to the PIM multicast routing protocol
   at the last-hop, and proposes some possible mitigation techniques.

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8.  References

8.1  Normative References

              Savola, P., Lehtonen, R. and D. Meyer, "PIM-SM Multicast
              Routing Security Issues and Enhancements",
              draft-ietf-mboned-mroutesec-04 (work in progress), October

              Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode PIM-SM):
              Protocol Specification  (Revised)",
              draft-ietf-pim-sm-v2-new-11 (work in progress), October

8.2  Informative References

              Atwood, J., "Security Issues in PIM-SM Link-local
              Messages", draft-atwood-pim-sm-linklocal-00 (work in
              progress), October 2004.

              Daley, G. and G. Kurup, "Trust Models and Security in
              Multicast Listener Discovery",
              draft-daley-magma-smld-prob-00 (work in progress), July

              Hayashi, T., "Internet Group membership Authentication
              Protocol (IGAP)", draft-hayashi-igap-03 (work in
              progress), August 2003.

   [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.

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Authors' Addresses

   Pekka Savola
   CSC - Scientific Computing Ltd.


   James Lingard
   Data Connection Ltd
   100 Church Street
   Enfield  EN2 6BQ
   United Kingdom


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