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Internet Engineering Task Force (IETF)                           F. GontRequest for Comments: 6946                           Huawei TechnologiesUpdates:2460,5722                                             May 2013Category: Standards TrackISSN: 2070-1721Processing of IPv6 "Atomic" FragmentsAbstract   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 updatesRFC 2460 andRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6946.Gont                         Standards Track                    [Page 1]

RFC 6946                  IPv6 Atomic Fragments                 May 2013Copyright Notice   Copyright (c) 2013 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 Simplified BSD License.Table of Contents1. Introduction ....................................................22. Terminology .....................................................43. Generation of IPv6 Atomic Fragments .............................44. UpdatingRFC 2460 andRFC 5722 ..................................55. Security Considerations .........................................66. Acknowledgements ................................................67. References ......................................................77.1. Normative References .......................................77.2. Informative References .....................................7Appendix A. Survey of Processing of IPv6 Atomic Fragments by               Different Operating Systems ............................81.  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 inSection 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 packetsGont                         Standards Track                    [Page 2]

RFC 6946                  IPv6 Atomic Fragments                 May 2013   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.Gont                         Standards Track                    [Page 3]

RFC 6946                  IPv6 Atomic Fragments                 May 20132.  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 inRFC 2119 [RFC2119].3.  Generation of IPv6 Atomic FragmentsSection 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).Gont                         Standards Track                    [Page 4]

RFC 6946                  IPv6 Atomic Fragments                 May 2013   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 inSection 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.  UpdatingRFC 2460 andRFC 5722Section 4.5 of [RFC2460] andSection 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.Gont                         Standards Track                    [Page 5]

RFC 6946                  IPv6 Atomic Fragments                 May 2013         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.Gont                         Standards Track                    [Page 6]

RFC 6946                  IPv6 Atomic Fragments                 May 20137.  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.   [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.Gont                         Standards Track                    [Page 7]

RFC 6946                  IPv6 Atomic Fragments                 May 2013Appendix 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.Gont                         Standards Track                    [Page 8]

RFC 6946                  IPv6 Atomic Fragments                 May 2013Author's Address   Fernando Gont   Huawei Technologies   Evaristo Carriego 2644   Haedo, Provincia de Buenos Aires  1706   Argentina   Phone: +54 11 4650 8472   EMail: fgont@si6networks.comGont                         Standards Track                    [Page 9]