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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
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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.
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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.
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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
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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.
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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.
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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.
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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.
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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+ | 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.
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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.
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[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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