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IPv6 Router Advertisement Guard (RA-Guard) Evasion

IPv6 Router Advertisement Guard (RA-Guard) Evasion
Posted Jun 1, 2011
Authored by Fernando Gont | Site ietf.org

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.

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IPv6 Router Advertisement Guard (RA-Guard) Evasion

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IPv6 Operations Working Group (v6ops) F. Gont
Internet-Draft UK CPNI
Intended status: Informational May 31, 2011
Expires: December 2, 2011


IPv6 Router Advertisement Guard (RA-Guard) Evasion
draft-gont-v6ops-ra-guard-evasion-00

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

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. This document may not be modified,
and derivative works of it may not be created, and it may not be
published except as an Internet-Draft.

Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

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."

This Internet-Draft will expire on December 2, 2011.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect



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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.


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. Assessment tools . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14






























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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.
describes the problem statement of "Rogue IPv6 Router
Advertisements", and 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 criteria 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 of identifying ICMPv6 Router
Advertisement messages.

As part of the project "Security Assessment of the Internet Protocol
version 6 (IPv6)" [CPNI-IPv6], we devised a number of 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 appropiate 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].

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 for IPv6 Extension Headers in
ICMPv6 Router Advertisement messages, implementations allow the use
of Extension Headers included in these messages, by simple 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 implements 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 those would fail do 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 |
+ + + + +
| | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

In this variant, by leveraging the use of the Fragment Header



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together with the use of the Destination Options header, the attacker
is able to conceal the type of ICMPv6 error message he is sending.
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.

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.


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=58| |
+-+-+-+ +-+-+-+ +-+-+-+ +
| 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 |
+ + + + + +
| | | | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

It should be obvious that even if the layer-2 device tried to follow
the entire IPv6 header chain, it would still be unable to identify



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that the fragments above transport an ICMPv6 message.


















































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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 [draft-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), then, if
the IPv6 Source Address of the packet is a link-local 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
[draft-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

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.

[draft-gont-6man-nd-extension-headers]
Gont, F., "Security Implications of the Use of IPv6
Extension Headers with IPv6 Neighbor Discovery", IETF
Internet Draft, draft-gont-6man-nd-extension-headers, 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. 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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Author's Address

Fernando Gont
Centre for the Protection of National Infrastructure

Email: fernando@gont.com.ar
URI: http://www.gont.com.ar












































Gont Expires December 2, 2011 [Page 14]

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