Network Working Group                                        S. Krishnan
Request for Comments: 5722                                      Ericsson
Updates: 2460                                              December 2009
Category: Standards Track


                Handling of Overlapping IPv6 Fragments

Abstract

  The fragmentation and reassembly algorithm specified in the base IPv6
  specification allows fragments to overlap.  This document
  demonstrates the security issues associated with allowing overlapping
  fragments and updates the IPv6 specification to explicitly forbid
  overlapping fragments.

Status of This Memo

  This document specifies an Internet standards track protocol for the
  Internet community, and requests discussion and suggestions for
  improvements.  Please refer to the current edition of the "Internet
  Official Protocol Standards" (STD 1) for the standardization state
  and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

  Copyright (c) 2009 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
  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 BSD License.













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Table of Contents

  1. Introduction ....................................................2
     1.1. Conventions Used in This Document ..........................2
  2. Overlapping Fragments ...........................................2
  3. The Attack ......................................................3
  4. Node Behavior ...................................................5
  5. Security Considerations .........................................5
  6. Acknowledgements ................................................5
  7. References ......................................................6
     7.1. Normative References .......................................6
     7.2. Informative References .....................................6

1.  Introduction

  Fragmentation is used in IPv6 when the IPv6 packet will not fit
  inside the path MTU to its destination.  When fragmentation is
  performed, an IPv6 node uses a fragment header, as specified in
  Section 4.5 of the IPv6 base specification [RFC2460], to break down
  the datagram into smaller fragments that will fit in the path MTU.
  The destination node receives these fragments and reassembles them.
  The algorithm specified for fragmentation in [RFC2460] does not
  prevent the fragments from overlapping, and this can lead to some
  security issues with firewalls [RFC4942].  This document explores the
  issues that can be caused by overlapping fragments and updates the
  IPv6 specification to explicitly forbid overlapping fragments.

1.1.  Conventions Used in This Document

  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 [RFC2119].

2.  Overlapping Fragments

  Commonly used firewalls use the algorithm specified in [RFC1858] to
  weed out malicious packets that try to overwrite parts of the
  transport-layer header in order to bypass inbound connection checks.
  [RFC1858] prevents an overlapping fragment attack on an upper-layer
  protocol (in this case, TCP) by recommending that packets with a
  fragment offset of 1 be dropped.  While this works well for IPv4
  fragments, it will not work for IPv6 fragments.  This is because the
  fragmentable part of the IPv6 packet can contain extension headers
  before the TCP header, making this check less effective.







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3.  The Attack

  This attack describes how a malicious node can bypass a firewall
  using overlapping fragments.  Consider a sufficiently large IPv6
  packet that needs to be fragmented.

  +------------------+--------------------//-----------------------+
  |  Unfragmentable  |                 Fragmentable                |
  |       Part       |                     Part                    |
  +------------------+--------------------//-----------------------+

                    Figure 1: Large IPv6 Packet

  This packet is split into several fragments by the sender so that the
  packet can fit inside the path MTU.  Let's say the packet is split
  into two fragments.

  +------------------+--------+--------------------+
  |  Unfragmentable  |Fragment|       first        |
  |       Part       | Header |      fragment      |
  +------------------+--------+--------------------+

  +------------------+--------+--------------------+
  |  Unfragmentable  |Fragment|       second       |
  |       Part       | Header |      fragment      |
  +------------------+--------+--------------------+

          Figure 2: Fragmented IPv6 Packet

  Consider the first fragment.  Let's say it contains a destination
  options header (DOH) 80 octets long and is followed by a TCP header.




















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RFC 5722         Handling of Overlapping IPv6 Fragments    December 2009


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
|NextHdr=DOH(60)|   Reserved    |   FragmentOffset = 0    |Res|1|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                Identification=aaaabbbb                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==DOH
|NextHdr=TCP(6) | HdrExtLen = 9 |                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
|                                                               |
.                                                               .
.                            Options                            .
.                                                               .
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
|        Source Port            |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Sequence Number                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset| Reserved  |U|A|P|R|S|F|           Window              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 3: First Fragment

  The TCP header has the following values of the flags: S(YN)=1 and
  A(CK)=1.  This may make an inspecting stateful firewall think that it
  is a response packet for a connection request initiated from the
  trusted side of the firewall.  Hence, it will allow the fragment to
  pass.  It will also allow the following fragments with the same
  Fragment Identification value in the fragment header to pass through.

  A malicious node can form a second fragment with a TCP header that
  changes the flags and sets S(YN)=1 and A(CK)=0.  This can change the
  packet on the receiving end to consider the packet as a connection
  request instead of a response.  By doing this, the malicious node has
  bypassed the firewall's access control to initiate a connection
  request to a node protected by a firewall.














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RFC 5722         Handling of Overlapping IPv6 Fragments    December 2009


+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==FH
|NextHdr=DOH(60)|   Reserved    |   FragmentOffset = 10   |Res|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                Identification=aaaabbbb                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<==TCP
|        Source Port            |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                       Sequence Number                         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Offset| Reserved  |U|A|P|R|S|F|           Window              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 4: Second Fragment

  Note that this attack is much more serious in IPv6 than in IPv4.  In
  IPv4, the overlapping part of the TCP header does not include the
  source and destination ports.  In IPv6, the attack can easily work to
  replace the source or destination port with an overlapping fragment.

4.  Node Behavior

  IPv6 nodes transmitting datagrams that need to be fragmented MUST NOT
  create overlapping fragments.  When reassembling an IPv6 datagram, if
  one or more its constituent fragments is determined to be an
  overlapping fragment, the entire datagram (and any constituent
  fragments, including those not yet received) MUST be silently
  discarded.

  Nodes MAY also provide mechanisms to track the reception of such
  packets, for instance, by implementing counters or alarms relating to
  these events.

5.  Security Considerations

  This document discusses an attack that can be used to bypass IPv6
  firewalls using overlapping fragments.  It recommends disallowing
  overlapping fragments in order to prevent this attack.

6.  Acknowledgements

  The author would like to thank Thomas Narten, Doug Montgomery,
  Gabriel Montenegro, Remi Denis-Courmont, Marla Azinger, Arnaud
  Ebalard, Seiichi Kawamura, Behcet Sarikaya, Vishwas Manral, Christian
  Vogt, Bob Hinden, Carl Wallace, Jari Arkko, Pasi Eronen, Francis





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RFC 5722         Handling of Overlapping IPv6 Fragments    December 2009


  Dupont, Neville Brownlee, Dan Romascanu, Lars Eggert, Cullen
  Jennings, and Alfred Hoenes for their reviews and suggestions that
  made this document better.

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.

7.2.  Informative References

  [RFC1858]  Ziemba, G., Reed, D., and P. Traina, "Security
             Considerations for IP Fragment Filtering", RFC 1858,
             October 1995.

  [RFC4942]  Davies, E., Krishnan, S., and P. Savola, "IPv6
             Transition/Co-existence Security Considerations", RFC
             4942, September 2007.

Author's Address

  Suresh Krishnan
  Ericsson
  8400 Blvd Decarie
  Town of Mount Royal, Quebec
  Canada

  EMail: [email protected]


















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