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Network Working Group                                        S. KrishnanRequest for Comments: 5722                                      EricssonUpdates:2460                                              December 2009Category: Standards TrackHandling of Overlapping IPv6 FragmentsAbstract   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 toBCP 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.Krishnan                    Standards Track                     [Page 1]

RFC 5722         Handling of Overlapping IPv6 Fragments    December 2009Table of Contents1. Introduction ....................................................21.1. Conventions Used in This Document ..........................22. Overlapping Fragments ...........................................23. The Attack ......................................................34. Node Behavior ...................................................55. Security Considerations .........................................56. Acknowledgements ................................................57. References ......................................................67.1. Normative References .......................................67.2. Informative References .....................................61.  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 inSection 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.Krishnan                    Standards Track                     [Page 2]

RFC 5722         Handling of Overlapping IPv6 Fragments    December 20093.  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.Krishnan                    Standards Track                     [Page 3]

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.Krishnan                    Standards Track                     [Page 4]

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, FrancisKrishnan                    Standards Track                     [Page 5]

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.  References7.1.  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",RFC4942, September 2007.Author's Address   Suresh Krishnan   Ericsson   8400 Blvd Decarie   Town of Mount Royal, Quebec   Canada   EMail: suresh.krishnan@ericsson.comKrishnan                    Standards Track                     [Page 6]