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Versions: 00 draft-ietf-6man-ipv6-atomic-fragments

IPv6 maintenance Working Group (6man)                            F. Gont
Internet-Draft                                                   UK CPNI
Updates: 2460, 5722 (if approved)                      December 15, 2011
Intended status: Standards Track
Expires: June 17, 2012


                 Processing of IPv6 "atomic" fragments
                draft-gont-6man-ipv6-atomic-fragments-00

Abstract

   IPv6 allows packets to contain a Fragment Header, without the packet
   being actually fragmented into multiple pieces.  Such packets
   typically result from 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 hosts as "fragmented traffic".
   By forging ICMPv6 "Packet Too Big" error messages an attacker can
   cause hosts to employ "atomic fragments", and the launch any
   fragmentation-based attacks against such traffic.  This document
   discusses the generation of the aforementioned "atomic fragments",
   the corresponding security implications, and formally updates RFC
   2460 and RFC 5722 such that the attack vector based on "atomic
   fragments" is completely eliminated.

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   and derivative works of it may not be created, and it may not be
   published except as an Internet-Draft.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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 17, 2012.

Copyright Notice

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



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   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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   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.  Generation of IPv6 'atomic fragments'  . . . . . . . . . . . .  5
   3.  Updating RFC 2460 and RFC 5722 . . . . . . . . . . . . . . . .  7
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12




























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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] allowed
   fragments to overlap, and hence allowed for ambiguity in the
   reassembly process, which could be leveraged by attackers to bypass
   firewall rules and/or evade Network Intrusion Detection Systems
   (NIDs) [RFC5722].

   [RFC5722] forbid overlapping fragments, specifying that when
   overlapping fragments are detected, all the overlapping fragments
   should 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.  The
   resulting packets will thus *not* be actually fragmented into several
   pieces, but only include a Fragment Header with both the "Fragment
   Offset" and the "M" bit set to 0.

   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), most IPv6 implementations
   process them as regular fragments.  Namely, they try to perform IPv6
   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 are silently
   discarded.

   Processing an "atomic fragment" as regular fragmented packet 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



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   discussed in [RFC6274].

   Section 2 describes the generation of IPv6 "atomic fragments", and
   how they can be remotely "triggered" by a remote attacker.  Section 3
   formally updates [RFC2460] and [RFC5722] such that the aforementioned
   attack vector is eliminated.

   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.  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].

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

      Many implementations fail to perform validation checks on the
      received ICMPv6 error messages, as recommended in Section 5.2 of
      [RFC4443] and [RFC5927].

      In some cases, such as when an ICMPv6 error message has
      (supposedly) been elicited by a connection-less transport protocol
      (or some other connection-less protocol being encapsulated in
      IPv6), it may be virtually impossible to perform validation checks
      on the received ICMPv6 error messages.

      Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
      error messages, the Destinations Cache is usually updated to
      reflect that any further packets to such destination should
      include a Fragment Header.  This means that a single ICMPv6
      "Packet Too Big" error message may affect multiple communication
      instances (e.g., TCP connections) with such destination.







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      Some implementations employ Fragment Identification values that
      are predictable by remote attackers, greatly improving the chances
      of an attacker of successfully performing a fragmentation-based
      attack [PREDICTABLE-ID].















































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3.  Updating RFC 2460 and RFC 5722

   This document updates [RFC2460] and [RFC5722] as follows:

      A host that receives an IPv6 packet which includes a Fragment
      Header with the "Fragment Offset" equal to 0 and the "M" bit equal
      to 0 MUST process such 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}.  In other words, the Fragment Header of "atomic
      fragments" should be ignored by the receiving host.








































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4.  IANA Considerations

   There are no IANA registries within this document.  The RFC-Editor
   can remove this section before publication of this document as an
   RFC.














































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5.  Security Considerations

   This document describes how an attacker can exploit ICMPv6 "Packet
   Too Big" error messages to cause further IPv6 packets to include a
   Fragment Header, such that he can perform any fragmentation-based
   attack against otherwise non-fragmented traffic.  This document
   updates [RFC2460] and [RFC5722], such that the aforementioned attack
   vector is completely eliminated.











































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6.  Acknowledgements

   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).

   Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
   their continued support.










































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7.  References

7.1.  Normative References

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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [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.

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

7.2.  Informative References

   [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.

   [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, December 2011, <
              http://tools.ietf.org/html/
              draft-gont-6man-predictable-fragment-id>.

















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Author's Address

   Fernando Gont
   UK CPNI

   Email: fgont@si6networks.com
   URI:   http://www.cpni.gov.uk












































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