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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
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   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












































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