IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft UK CPNI
Intended status: Informational June 8, 2011
Expires: December 10, 2011
IPv6 Router Advertisement Guard (RA-Guard) Evasion
draft-gont-v6ops-ra-guard-evasion-01
Abstract
The IPv6 Router Advertisement Guard (RA-Guard) mechanism is commonly
employed to mitigate attack vectors based on forged ICMPv6 Router
Advertisement messages. Many existing IPv6 deployments rely on RA-
Guard as the first line of defense against the aforementioned attack
vectors. This document describes possible ways in which current RA-
Guard implementations can be circumvented, and discusses possible
mitigations.
Status of this Memo
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Router Advertisement Guard (RA Guard) Evasion Vulnerability . 4
2.1. Attack Vector based on IPv6 Extension Headers . . . . . . 4
2.2. Attack vector based on IPv6 fragmentation . . . . . . . . 4
3. Mitigations . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Other Implications . . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Changes from previous versions of the draft (to
be removed by the RFC Editor before publication
of this document as a RFC . . . . . . . . . . . . . . 13
A.1. Changes from draft-gont-v6ops-ra-guard-evasion-00 . . . . 13
Appendix B. Assessment tools . . . . . . . . . . . . . . . . . . 14
Appendix C. Advice and guidance to vendors . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique
for attack vectors based on ICMPv6 Router Advertisement messages.
[RFC6104] describes the problem statement of "Rogue IPv6 Router
Advertisements", and [RFC6105] specifies the "IPv6 Router
Advertisement Guard" functionality.
The basic concept behind RA-Guard is that a layer-2 device filters
ICMPv6 Router Advertisement messages, according to a number of
different criteria. The most basic filtering criterion is that
Router Advertisement messages are discarded by the layer-2 device
unless they are received on a specified port of the layer-2 device.
Clearly, the effectiveness of the RA Guard mitigation relies on the
ability of the layer-2 device to identify ICMPv6 Router Advertisement
messages.
As part of the project "Security Assessment of the Internet Protocol
version 6 (IPv6)" [CPNI-IPv6], we have devised two techniques for
circumventing the RA-Guard protection, which are described in the
following sections of this document. These techniques, and the
corresponding tools to assess their effectiveness, had so far been
made available only to vendors, in the hopes that they could
implement counter-measures before they were publicly disclosed.
However, since there has been some public discussion about these
issues, it was deemed as appropriate to publish the present document.
It should be noted that the aforementioned techniques could also be
exploited to evade network monitoring tools such as NDPMon [NDPMon],
ramond [ramond], and rafixd [rafixd], and could probably be exploited
to perform stealth DHCPv6 attacks.
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 RFC 2119 [RFC2119].
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2. Router Advertisement Guard (RA Guard) Evasion Vulnerability
The following subsections describe two different vectors for evading
the RA-Guard protection. Section 2.1 describes an attack vector
based on the use of IPv6 Extension Headers with the ICMPv6 Router
Advertisement messages, which may be used to circumvent the RA-Guard
protection of those implementations that fail to process an entire
IPv6 header chain when trying to identify the ICMPv6 Router
Advertisement messages. Section 2.2 describes an attack method based
on the use of IPv6 fragmentation, possibly in conjunction with the
use of IPv6 Extension Headers. This later vector is expected to be
effective with all existing implementations of the RA-Guard
functionality.
2.1. Attack Vector based on IPv6 Extension Headers
While there is currently no legitimate use for IPv6 Extension Headers
in ICMPv6 Router Advertisement messages, Neighbor Discovery
implementations allow the use of Extension Headers with these
messages, by simply ignoring the received options. We believe that
some implementations may simply try to identify ICMPv6 Router
Advertisement messages by looking at the "Next Header" field of the
fixed IPv6 header, rather than following the entire header chain. As
a result, these implementations would fail to identify any ICMPv6
Router Advertisement messages that include any Extension Headers (for
example, Hop by Hop Options header, Destination Options Header,
etc.).
The following figure illustrates the structure of ICMPv6 Router
Advertisement messages that implement this RA-Guard evasion
technique:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=60| |NH=58| | |
+-+-+-+ +-+-+-+ + +
| IPv6 header | Dst Opt Hdr | ICMPv6 Router Advertisement |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.2. Attack vector based on IPv6 fragmentation
While the attack vector described in Section 2.1 may be effective
with implementations that fail to process the entire header chain, it
can easily be mitigated by an RA-Guard implementation, since all the
information needed to identify ICMPv6 Router Advertisement messages
is present in the attack packets.
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This section presents a different attack vector, which aims at making
it virtually impossible for a layer-2 device to identify ICMPv6
Router Advertisements by leveraging the IPv6 Fragment Header. The
basic idea behind this attack vector is that if the forged ICMPv6
Router Advertisement is fragmented into at least two fragments, the
layer-2 device implementing "RA-Guard" would be unable to identify
the attack packet, and would thus fail to block it.
A first variant of this attack vector would be an original ICMPv6
Router Advertisement message preceded with a Destination Options
Header, that results in two fragments. The following figure
illustrates the "original" attack packet, prior to fragmentation, and
the two resulting fragments which are actually sent as part of the
attack.
Original packet:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=60| |NH=58| | |
+-+-+-+ +-+-+-+ + +
| IPv6 header | Dst Opt Hdr | ICMPv6 RA |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| |NH=58| |
+-+-+-+ +-+-+-+ +-+-+-+ +
| IPv6 Header | Frag Hdr | Dst Opt Hdr |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| | | |
+-+-+-+ +-+-+-+ + + +
| IPv6 header | Frag Hdr | Dst Opt Hdr | ICMPv6 RA |
+ + + + +
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
It should be noted that the "Hdr Ext Len" field of the Destination
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Options Header is present in the first fragment (rather than the
second). Therefore, it would be impossible for a device processing
only the second fragment to locate the ICMPv6 header contained in
that fragment, since it is unknown how many bytes should be "skipped"
to get to the next header following the Destination Options Header.
Thus, by leveraging the use of the Fragment Header together with the
use of the Destination Options header, the attacker is able to
conceal the type and contents of the ICMPv6 message he is sending (an
ICMPv6 Router Advertisement in this example). Unless the layer-2
device were to implement IPv6 fragment reassembly, it would be
impossible for the device to identify the ICMPv6 type of the message.
A layer-2 device could, however, at least detect that that an
ICMPv6 message (or some type) is being sent, since the "Next
Header" field of the Destination Options header contained in the
first fragment is set to "58" (ICMPv6).
It is possible to take this idea further, such that it is also
impossible for the layer-2 device to detect that the attacker is
sending an ICMPv6 message in the first place. This can be achieved
with an original ICMPv6 Router Advertisement message preceded with
two Destination Options Headers, that results in two fragments. The
following figure illustrates the "original" attack packet, prior to
fragmentation, and the two resulting packets which are actually sent
as part of the attack.
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Original packet:
+-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=60| |NH=60| |NH=58| | |
+-+-+-+ +-+-+-+ +-+-+-+ + +
| IPv6 header | Dst Opt Hdr | Dst Opt Hdr | ICMPv6 RA |
+ + + + +
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-//+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| |NH=60| |
+-+-+-+ +-+-+-+ +-+-+-+ +
| IPv6 header | Frag Hdr | Dst Opt Hdr |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Second fragment:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|NH=44| |NH=60| | |NH=58| | |
+-+-+-+ +-+-+-+ + +-+-+-+ + +
| IPv6 header | Frag Hdr | Dst O Hdr | Dst Opt Hdr | ICMPv6 RA |
+ + + + + +
| | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In this variant, the "Next Header" field of the Destination Options
header contained in the first fragment is set "60" (Destination
Options header), and thus it is impossible for a device processing
only the first fragment to detect that an ICMPv6 message is being
sent in the first place.
The second fragment presents the same challenges as the second
fragment of the previous variant. That is, it would be impossible
for a device processing only the second fragment to locate the second
Destination Options header (and hence the ICMPv6 header), since the
"Hdr Ext Len" field of the first Destination Options header is
present in the first fragment (rather than the second).
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3. Mitigations
The most effective and efficient mitigation for the RA-Guard evasion
vulnerability discussed in this document would be to prohibit the use
of IPv6 Extension Headers in Neighbor Discovery messages, as proposed
in [I-D.gont-6man-nd-extension-headers].
Nevertheless, an administrator might want to mitigate these
vulnerabilities by deploying more advanced filtering. The following
filtering rules could be implemented as part of an "RA-Guard"
implementation, such that the vulnerabilities discussed in this
document can be mitigated:
o When trying to identify an ICMPv6 Router Advertisement message,
follow the IPv6 header chain, enforcing a limit on the maximum
number of Extension Headers that is allowed for each packet. If
such limit is exceeded, block the packet.
o If the layer-2 device is unable to identify whether the packet is
an ICMPv6 Router Advertisement message or not (i.e., the packet is
a fragment, and the necessary information is missing), and the
IPv6 Source Address of the packet is a link-local address or the
unspecified address (::), block the packet.
o In all other cases, pass the packet as usual.
This filtering policy assumes that host implementations require that
the IPv6 Source Address of ICMPv6 Router Advertisement messages be a
link-local address, and that they discard the packet if this check
fails, as required by the current IETF specifications [RFC4861].
Unfortunately, it should be noted that the aforementioned filtering
policy might be inefficient to implement (if at all possible), and
might also result (at least in theory) in false positives.
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4. Other Implications
A similar concept to that of "RA-Guard" has been implemented for
protecting against forged DHCPv6 messages. Such protection can be
circumvented with the same techniques discussed in this document, and
the counter-measures for such evasion attack are analogous to those
described in Section 3 of this document.
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5. Security Considerations
This document describes a number of techniques to circumvent a
mechanism known as "RA-Guard", which many organizations deploy as a
"first line of defense" against attacks based on forged Router
Advertisements.
The most effective and efficient mitigation for these attacks would
be to prohibit the use of IPv6 extension headers (as proposed by
[I-D.gont-6man-nd-extension-headers]), such that the RA-Guard
protection cannot be easily circumvented. However, since this
mitigation requires an update to existing implementations, in the
short term some network administrators might want to mitigate these
issues by implemented the more advanced filtering policy described in
Section 3.
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6. Acknowledgements
The author would like to thank Karl Auer, Robert Downie, David
Farmer, Marc Heuse, and Arturo Servin, for providing valuable
comments on earlier versions of this document.
This document resulted from the project "Security Assessment of the
Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI). The author would like to thank the
UK CPNI, for their continued support.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
7.2. Informative References
[RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, February 2011.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[]
Gont, F. and U. CPNI, "Security Implications of the Use of
IPv6 Extension Headers with IPv6 Neighbor Discovery",
draft-gont-6man-nd-extension-headers-00 (work in
progress), May 2011.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (to be published).
[NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor",
<http://ndpmon.sourceforge.net/>.
[rafixd] "rafixd", <http://www.kame.net/dev/cvsweb2.cgi/kame/kame/
kame/rafixd/>.
[ramond] "ramond", <http://ramond.sourceforge.net/>.
[THC-IPV6]
"THC-IPV6", <http://www.thc.org/thc-ipv6/>.
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Appendix A. Changes from previous versions of the draft (to be removed
by the RFC Editor before publication of this document as a
RFC
A.1. Changes from draft-gont-v6ops-ra-guard-evasion-00
o Minor editorial changes
o The discussion of the challenge represented by a combination of
fragmentation and Destination Options headers was improved/
clarified.
o In Section 2.2, in the illustration of the second variant of the
attack (fragmentation combined with two Destination Optios
headers), the figure corresponding to the "first fragment" was
corrected.
o Clarified the filtering rules in Section 3.
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Appendix B. Assessment tools
CPNI has produced assessment tools, which have not yet been made
publicly available. If you think that you would benefit from these
tools to assess the security of your network or of your RA-Guard
implementation, we might be able to provide a copy of the tools
(please contact Fernando Gont at fernando@gont.com.ar).
[THC-IPV6] is a publicly-available set of tools that implements some
of the techniques described in this document.
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Appendix C. Advice and guidance to vendors
Vendors are urged to contact CSIRTUK (csirt@cpni.gsi.gov.uk) if they
think they may be affected by the issues described in this document.
As the lead coordination centre for these issues, CPNI is well placed
to give advice and guidance as required.
CPNI works extensively with government departments and agencies,
commercial organisations and the academic community to research
vulnerabilities and potential threats to IT systems especially where
they may have an impact on Critical National Infrastructure's (CNI).
Other ways to contact CPNI, plus CPNI's PGP public key, are available
at http://www.cpni.gov.uk.
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Author's Address
Fernando Gont
Centre for the Protection of National Infrastructure
Email: fernando@gont.com.ar
URI: http://www.gont.com.ar
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