Processing of IPv6 "Atomic" Fragments
RFC 6946

Internet Engineering Task Force (IETF)                           F. Gont
Request for Comments: 6946                           Huawei Technologies
Updates: 2460, 5722                                             May 2013
Category: Standards Track
ISSN: 2070-1721

                 Processing of IPv6 "Atomic" Fragments

Abstract

   The IPv6 specification allows packets to contain a Fragment Header
   without the packet being actually fragmented into multiple pieces (we
   refer to these packets as "atomic fragments").  Such packets are
   typically sent by hosts that have received an ICMPv6 "Packet Too Big"
   error message that advertises a Next-Hop MTU smaller than 1280 bytes,
   and are currently processed by some implementations as normal
   "fragmented traffic" (i.e., they are "reassembled" with any other
   queued fragments that supposedly correspond to the same original
   packet).  Thus, an attacker can cause hosts to employ atomic
   fragments by forging ICMPv6 "Packet Too Big" error messages, and then
   launch any fragmentation-based attacks against such traffic.  This
   document discusses the generation of the aforementioned atomic
   fragments and the corresponding security implications.  Additionally,
   this document formally updates RFC 2460 and RFC 5722, such that IPv6
   atomic fragments are processed independently of any other fragments,
   thus completely eliminating the aforementioned attack vector.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6946.

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Copyright Notice

   Copyright (c) 2013 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
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................4
   3. Generation of IPv6 Atomic Fragments .............................4
   4. Updating RFC 2460 and RFC 5722 ..................................5
   5. Security Considerations .........................................6
   6. Acknowledgements ................................................6
   7. References ......................................................7
      7.1. Normative References .......................................7
      7.2. Informative References .....................................7
   Appendix A. Survey of Processing of IPv6 Atomic Fragments by
               Different Operating Systems ............................8

1.  Introduction

   [RFC2460] specifies the IPv6 fragmentation mechanism, which allows
   IPv6 packets to be fragmented into smaller pieces such that they fit
   in the Path-MTU to the intended destination(s).  [RFC2460] allows
   fragments to overlap, thus leading to ambiguity in the result of the
   reassembly process, which could be leveraged by attackers to bypass
   firewall rules and/or evade Network Intrusion Detection Systems
   (NIDS) [RFC5722].

   [RFC5722] forbids overlapping fragments, specifying that when
   overlapping fragments are detected, all the fragments corresponding
   to that packet must be silently discarded.

   As specified in Section 5 of [RFC2460], when a host receives an
   ICMPv6 "Packet Too Big" message advertising a "Next-Hop MTU" smaller
   than 1280 (the minimum IPv6 MTU), it is not required to reduce the
   assumed Path-MTU, but must simply include a Fragment Header in all
   subsequent packets sent to that destination.  The resulting packets

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   will thus not actually be fragmented into several pieces but will
   just include a Fragment Header with both the "Fragment Offset" and
   the "M" flag set to 0 (we refer to these packets as "atomic
   fragments").  IPv6/IPv4 translators employ the Fragment
   Identification information found in the Fragment Header to select an
   appropriate Fragment Identification value for the resulting IPv4
   fragments.

   While these packets are really atomic fragments (they can be
   processed by the IPv6 module and handed to the upper-layer protocol
   without waiting for any other fragments), many IPv6 implementations
   process them as regular fragments.  Namely, they try to perform IPv6
   fragment reassembly with the atomic fragment and any other fragments
   already queued with the same set {IPv6 Source Address, IPv6
   Destination Address, Fragment Identification}.  For example, in the
   case of IPv6 implementations that have been updated to support
   [RFC5722], if a fragment with the same {IPv6 Source Address, IPv6
   Destination Address, Fragment Identification} is already queued for
   reassembly at a host when an atomic fragment is received with the
   same set {IPv6 Source Address, IPv6 Destination Address, Fragment
   Identification}, and both fragments overlap, all the fragments will
   be silently discarded.

   Processing of IPv6 atomic fragments as regular fragmented packets
   clearly provides an unnecessary vector to perform fragmentation-based
   attacks against non-fragmented traffic (i.e., IPv6 datagrams that are
   not really split into multiple pieces but that just include a
   Fragment Header).

   IPv6 fragmentation attacks have been discussed in great detail in
   [PREDICTABLE-ID] and [CPNI-IPv6], and [RFC5722] describes a specific
   firewall-circumvention attack that could be performed by leveraging
   overlapping fragments.  The possible IPv6 fragmentation-based attacks
   are, in most cases, "ports" of the IPv4 fragmentation attacks
   discussed in [RFC6274].

   Section 3 describes the generation of IPv6 atomic fragments and how
   they can be remotely "triggered" by a remote attacker.  Section 4
   formally updates [RFC2460] and [RFC5722] such that the aforementioned
   attack vector is eliminated.  Appendix A contains a survey of the
   generation and processing of IPv6 atomic fragments in different
   versions of a number of popular IPv6 implementations.

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2.  Terminology

   IPv6 atomic fragments:
      IPv6 packets that contain a Fragment Header with the Fragment
      Offset set to 0 and the M flag set to 0.

   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].

3.  Generation of IPv6 Atomic Fragments

   Section 5 of [RFC2460] states:

      "In response to an IPv6 packet that is sent to an IPv4 destination
      (i.e., a packet that undergoes translation from IPv6 to IPv4), the
      originating IPv6 node may receive an ICMP Packet Too Big message
      reporting a Next-Hop MTU less than 1280.  In that case, the IPv6
      node is not required to reduce the size of subsequent packets to
      less than 1280, but must include a Fragment header in those
      packets so that the IPv6-to-IPv4 translating router can obtain a
      suitable Identification value to use in resulting IPv4 fragments.
      Note that this means the payload may have to be reduced to 1232
      octets (1280 minus 40 for the IPv6 header and 8 for the Fragment
      header), and smaller still if additional extension headers are
      used."

   This means that any ICMPv6 "Packet Too Big" message advertising a
   "Next-Hop MTU" smaller than 1280 could trigger the generation of the
   so-called "atomic fragments" (i.e., IPv6 datagrams that include a
   Fragment Header but that are composed of a single fragment, with both
   the "Fragment Offset" and the "M" fields of the Fragment Header set
   to 0).  This can be leveraged to perform a variety of fragmentation-
   based attacks [PREDICTABLE-ID] [CPNI-IPv6].

      For example, an attacker could forge IPv6 fragments with an
      appropriate {IPv6 Source Address, IPv6 Destination Address,
      Fragment Identification} tuple, such that these malicious
      fragments are incorrectly "reassembled" by the destination host
      together with some of the legitimate fragments of the original
      packet, thus leading to packet drops (and a potential denial of
      service).

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   From a security standpoint, this situation is exacerbated by the
   following factors:

   o  Many implementations fail to perform validation checks on the
      received ICMPv6 error messages, as recommended in Section 5.2 of
      [RFC4443] and documented in [RFC5927].  It should be noted that in
      some cases, such as when an ICMPv6 error message has (supposedly)
      been elicited by a connectionless transport protocol (or some
      other connectionless protocol being encapsulated in IPv6), it may
      be virtually impossible to perform validation checks on the
      received ICMPv6 error messages.

   o  Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
      error messages, the Destination Cache [RFC4861] is usually updated
      to reflect that any subsequent packets to that destination should
      include a Fragment Header.  This means that a single ICMPv6
      "Packet Too Big" error message might affect multiple communication
      instances (e.g., TCP connections) with that IPv6 destination
      address.

   o  Some implementations employ predictable Fragment Identification
      values, thus greatly improving the chances of an attacker
      successfully performing fragmentation-based attacks
      [PREDICTABLE-ID].

4.  Updating RFC 2460 and RFC 5722

   Section 4.5 of [RFC2460] and Section 4 of [RFC5722] are updated as
   follows:

      A host that receives an IPv6 packet that includes a Fragment
      Header with the "Fragment Offset" equal to 0 and the "M" flag
      equal to 0 MUST process that packet in isolation from any other
      packets/fragments, even if such packets/fragments contain the same
      set {IPv6 Source Address, IPv6 Destination Address, Fragment
      Identification}.  A received atomic fragment should be
      "reassembled" from the contents of that sole fragment.

         The Unfragmentable Part of the reassembled packet consists of
         all headers up to, but not including, the Fragment Header of
         the received atomic fragment.

         The Next Header field of the last header of the Unfragmentable
         Part of the reassembled packet is obtained from the Next Header
         field of the Fragment Header of the received atomic fragment.

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         The Payload Length of the reassembled packet is obtained by
         subtracting the length of the Fragment Header (that is, 8) from
         the Payload Length of the received atomic fragment.

      Additionally, if any fragments with the same set {IPv6 Source
      Address, IPv6 Destination Address, Fragment Identification} are
      present in the fragment reassembly queue when the atomic fragment
      is received, such fragments MUST NOT be discarded upon receipt of
      the "colliding" IPv6 atomic fragment, since IPv6 atomic fragments
      MUST NOT interfere with "normal" fragmented traffic.

5.  Security Considerations

   This document describes how the traditional processing of IPv6 atomic
   fragments enables the exploitation of fragmentation-based attacks
   (such as those described in [PREDICTABLE-ID] and [CPNI-IPv6]).  This
   document formally updates [RFC2460] and [RFC5722], such that IPv6
   atomic fragments are processed independently of any other fragments,
   thus completely eliminating the aforementioned attack vector.

6.  Acknowledgements

   The author would like to thank (in alphabetical order) Tore Anderson,
   Ran Atkinson, Remi Despres, Stephen Farrell, Brian Haberman, Timothy
   Hartrick, Steinar Haug, Philip Homburg, Simon Josefsson, Simon
   Perreault, Sean Turner, Florian Weimer, and Bjoern A. Zeeb for
   providing valuable comments on earlier versions of this document.
   Additionally, the author would like to thank Alexander Bluhm, who
   implemented this specification for OpenBSD.

   This document is based on the technical report "Security Assessment
   of the Internet Protocol version 6 (IPv6)" [CPNI-IPv6], authored by
   Fernando Gont on behalf of the UK Centre for the Protection of
   National Infrastructure (CPNI).

   Finally, the author wishes to thank Nelida Garcia and Guillermo Gont
   for their love and 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.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC5722]  Krishnan, S., "Handling of Overlapping IPv6 Fragments",
              RFC 5722, December 2009.

7.2.  Informative References

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

   [PREDICTABLE-ID]
              Gont, F., "Security Implications of Predictable Fragment
              Identification Values", Work in Progress, March 2013.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

   [RFC6274]  Gont, F., "Security Assessment of the Internet Protocol
              Version 4", RFC 6274, July 2011.

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Appendix A.  Survey of Processing of IPv6 Atomic Fragments by Different
             Operating Systems

   This section includes a survey of the support of IPv6 atomic
   fragments in popular operating systems, as tested on October 30,
   2012.

   +---------------------+---------------------+-----------------------+
   |   Operating System  |   Generates atomic  |    Implements this    |
   |                     |      fragments      |     specification     |
   +---------------------+---------------------+-----------------------+
   |     FreeBSD 8.0     |          No         |           No          |
   +---------------------+---------------------+-----------------------+
   |     FreeBSD 8.2     |         Yes         |           No          |
   +---------------------+---------------------+-----------------------+
   |     FreeBSD 9.0     |         Yes         |           No          |
   +---------------------+---------------------+-----------------------+
   |    Linux 3.0.0-15   |         Yes         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |      NetBSD 5.1     |          No         |           No          |
   +---------------------+---------------------+-----------------------+
   |    NetBSD-current   |          No         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |   OpenBSD-current   |         Yes         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |      Solaris 11     |         Yes         |          Yes          |
   +---------------------+---------------------+-----------------------+
   |    Windows XP SP2   |         Yes         |           No          |
   +---------------------+---------------------+-----------------------+
   |    Windows Vista    |         Yes         |           No          |
   |     (Build 6000)    |                     |                       |
   +---------------------+---------------------+-----------------------+
   |    Windows 7 Home   |         Yes         |           No          |
   |       Premium       |                     |                       |
   +---------------------+---------------------+-----------------------+

      Table 1: Processing of IPv6 Atomic Fragments by Different OSes

      In the table above, "generates atomic fragments" notes whether an
      implementation generates atomic fragments in response to received
      ICMPv6 "Packet Too Big" error messages that advertise an MTU
      smaller than 1280 bytes.

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Author's Address

   Fernando Gont
   Huawei Technologies
   Evaristo Carriego 2644
   Haedo, Provincia de Buenos Aires  1706
   Argentina

   Phone: +54 11 4650 8472
   EMail: fgont@si6networks.com

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