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)
draft-savola-pim-lasthop-threats-01.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 15, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
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.
Savola & Lingard Expires July 15, 2005 [Page 1]
Internet-Draft Last-hop PIM Threats January 2005
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
Savola & Lingard Expires July 15, 2005 [Page 2]
Internet-Draft Last-hop PIM Threats January 2005
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
issues.
It is assumed that the reader is familiar with the basic concepts of
PIM.
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
router.
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.
Savola & Lingard Expires July 15, 2005 [Page 3]
Internet-Draft Last-hop PIM Threats January 2005
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
router.
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
LAN.
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
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 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
Savola & Lingard Expires July 15, 2005 [Page 4]
Internet-Draft Last-hop PIM Threats January 2005
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
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
DR.
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].
Savola & Lingard Expires July 15, 2005 [Page 5]
Internet-Draft Last-hop PIM Threats January 2005
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.
Savola & Lingard Expires July 15, 2005 [Page 6]
Internet-Draft Last-hop PIM Threats January 2005
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
[I-D.atwood-pim-sm-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). 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
IPsec.
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.
Savola & Lingard Expires July 15, 2005 [Page 7]
Internet-Draft Last-hop PIM Threats January 2005
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
approaches.
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.
Savola & Lingard Expires July 15, 2005 [Page 8]
Internet-Draft Last-hop PIM Threats January 2005
8. References
8.1 Normative References
[I-D.ietf-mboned-mroutesec]
Savola, P., Lehtonen, R. and D. Meyer, "PIM-SM Multicast
Routing Security Issues and Enhancements",
draft-ietf-mboned-mroutesec-04 (work in progress), October
2004.
[I-D.ietf-pim-sm-v2-new]
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
2004.
8.2 Informative References
[I-D.atwood-pim-sm-linklocal]
Atwood, J., "Security Issues in PIM-SM Link-local
Messages", draft-atwood-pim-sm-linklocal-00 (work in
progress), October 2004.
[I-D.daley-magma-smld-prob]
Daley, G. and G. Kurup, "Trust Models and Security in
Multicast Listener Discovery",
draft-daley-magma-smld-prob-00 (work in progress), July
2004.
[I-D.hayashi-igap]
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.
Savola & Lingard Expires July 15, 2005 [Page 9]
Internet-Draft Last-hop PIM Threats January 2005
Authors' Addresses
Pekka Savola
CSC - Scientific Computing Ltd.
Espoo
Finland
EMail: psavola@funet.fi
James Lingard
Data Connection Ltd
100 Church Street
Enfield EN2 6BQ
United Kingdom
EMail: james.lingard@dataconnection.com
Savola & Lingard Expires July 15, 2005 [Page 10]
Internet-Draft Last-hop PIM Threats January 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Savola & Lingard Expires July 15, 2005 [Page 11]