IPv6 Operations Working Group (v6ops)                            F. Gont
Internet-Draft                                                   UK CPNI
Intended status: Informational                           January 5, 2012
Expires: July 8, 2012


  Implementation Advice for IPv6 Router Advertisement Guard (RA-Guard)
              draft-gont-v6ops-ra-guard-implementation-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.  However, some implementations of RA-Guard have been found
   to be prone to circumvention by employing IPv6 Extension Headers.
   This document describes the evasion techniques that affect the
   aforementioned implementations, and provides advice on the
   implementation of RA-Guard, such that that evasion vectors are
   eliminated.

Status of this Memo

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Copyright Notice

   Copyright (c) 2012 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



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   (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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   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Evasion techniques for some Router Advertisement Guard (RA
       Guard) implementations . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Attack Vector based on IPv6 Extension Headers  . . . . . .  4
     2.2.  Attack vector based on IPv6 fragmentation  . . . . . . . .  4
   3.  RA-Guard implementation advice . . . . . . . . . . . . . . . .  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-01  . . . . 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.

   Some popular RA-Guard implementations have been found to be possible
   to circumvent by employing IPv6 extension headers [CPNI-IPv6].  This
   document describes such evasion techniques, and provides advice to
   RA-Guard implementers such that the aforementioned evasion vectors
   can be eliminated.

   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.  Evasion techniques for some Router Advertisement Guard (RA Guard)
    implementations

   The following subsections describe two different vectors that have
   been found to be effective for the evasion of popular implementations
   of 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 has been found to
   be effective with all existing implementations of the RA-Guard
   mechanism.

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.  Some RA-Guard
   implementations 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,
   such implementations fail to identify any ICMPv6 Router Advertisement
   messages that include any Extension Headers (for example, Hop by Hop
   Options header, Destination Options Header, etc.), and can be easily
   circumvented.

   The following figure illustrates the structure of ICMPv6 Router
   Advertisement messages that implement this evasion technique:


      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |NH=60|       |NH=58|        |                                |
      +-+-+-+       +-+-+-+        +                                +
      | IPv6 header |  Dst Opt Hdr |  ICMPv6 Router Advertisement   |
      +             +              +                                +
      |             |              |                                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

2.2.  Attack vector based on IPv6 fragmentation

   This section presents a different attack vector, which has been found
   to be effective against all implementations of RA-Guard.  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



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   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
   Options Header is present in the first fragment (rather than the
   second).  Therefore, it is 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.



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   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).

   This idea can be taken 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.  RA-Guard implementation advice

   The following filtering rules MUST be implemented as part of an "RA-
   Guard" implementation, such that the possible vulnerabilities
   discussed in this document are eliminated:

   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].




























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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 that have been found
   to be effective to circumvent popular RA-Guard implementations.

   The most effective and efficient mitigation for these attacks would
   be to prohibit the use of some IPv6 extension headers with Router
   Advertisement messages (as proposed by
   [I-D.gont-6man-nd-extension-headers]).  However, since such
   mitigation would require an update to existing implementations, it
   cannot be relied upon in the short or near term.








































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6.  Acknowledgements

   The author would like to thank Ran Atkinson, Karl Auer, Robert
   Downie, David Farmer, Marc Heuse, Arturo Servin, and Gunter van de
   Velde, 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.

   [I-D.gont-6man-nd-extension-headers]
              Gont, F. and U. CPNI, "Security Implications of the Use of
              IPv6 Extension Headers with IPv6 Neighbor Discovery",
              draft-gont-6man-nd-extension-headers-01 (work in
              progress), June 2011.

   [CPNI-IPv6]
              Gont, F., "Security Assessment of the Internet Protocol
              version 6 (IPv6)",  UK Centre for the Protection of
              National Infrastructure, (available on request).

   [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-01

   o  The contents were updated to reflect that the evasion
      vulnerabilities are based on implementation flaws, rather than on
      the RA-Guard "concept" itself.

   o  The I-D now focuses on providing advice to RA-Guard implementers.








































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Appendix B.  Assessment tools

   CPNI has produced assessment tools (which have not yet been made
   publicly available) to assess RA-Guard implementations with respect
   to the issues described in this document.  If you think that you
   would benefit from these tools, 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: fgont@si6networks.com
   URI:   http://www.cpni.gov.uk












































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