Internet DRAFT - draft-ietf-6man-ipv6-atomic-fragments
draft-ietf-6man-ipv6-atomic-fragments
IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft Huawei Technologies
Updates: 2460, 5722 (if approved) March 21, 2013
Intended status: Standards Track
Expires: September 22, 2013
Processing of IPv6 "atomic" fragments
draft-ietf-6man-ipv6-atomic-fragments-04
Abstract
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 updates RFC 2460 and RFC 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 Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on September 22, 2013.
Copyright Notice
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Copyright (c) 2013 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Generation of IPv6 'atomic fragments' . . . . . . . . . . . . 6
4. Updating RFC 2460 and RFC 5722 . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . . 12
Appendix A. Survey of processing of IPv6 atomic fragments by
different operating systems . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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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, 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] forbid overlapping fragments, specifying that when
overlapping fragments are detected, all the fragments corresponding
to that packet must 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 in all
subsequent packets sent to that destination. The resulting packets
will thus *not* be actually fragmented into several pieces, but 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
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[I-D.gont-6man-predictable-fragment-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.
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2. 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 in RFC 2119 [RFC2119].
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3. 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 [I-D.gont-6man-predictable-fragment-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).
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 in Section 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 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.
o Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
error messages, the Destination Cache [RFC4861] is usually updated
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to reflect that any subsequent packets to such 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 such destination.
o Some implementations employ predictable Fragment Identification
values, thus greatly improving the chances of an attacker of
successfully performing fragmentation-based attacks
[I-D.gont-6man-predictable-fragment-id].
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4. Updating RFC 2460 and RFC 5722
Section 4.5 of [RFC2460] and Section 4 of [RFC5722] are updated as
follows:
A host that receives an IPv6 packet which includes a Fragment
Header with the "Fragment Offset" equal to 0 and the "M" flag
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}. A received "atomic fragments" 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.
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.
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5. 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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6. 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 [I-D.gont-6man-predictable-fragment-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.
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7. 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).
Fernando Gont would like to thank CPNI (http://www.cpni.gov.uk) for
their continued support.
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8. References
8.1. 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.
[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.
8.2. 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).
[I-D.gont-6man-predictable-fragment-id]
Gont, F., "Security Implications of Predictable Fragment
Identification Values",
draft-gont-6man-predictable-fragment-id-03 (work in
progress), January 2013.
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Appendix 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 in 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 a MTU smaller
than 1280 bytes.
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Author's Address
Fernando Gont
Huawei Technologies
Evaristo Carriego 2644
Haedo, Provincia de Buenos Aires 1706
Argentina
Phone: +54 11 4650 8472
Email: fgont@si6networks.com
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