Handling of Overlapping IPv6 Fragments
RFC 5722

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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.

Krishnan                    Standards Track                     [Page 1]
RFC 5722         Handling of Overlapping IPv6 Fragments    December 2009

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.

Krishnan                    Standards Track                     [Page 2]
RFC 5722         Handling of Overlapping IPv6 Fragments    December 2009

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