Internet DRAFT - draft-gont-6man-ipv6-atomic-fragments
draft-gont-6man-ipv6-atomic-fragments
IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft UK CPNI
Updates: 2460, 5722 (if approved) December 15, 2011
Intended status: Standards Track
Expires: June 17, 2012
Processing of IPv6 "atomic" fragments
draft-gont-6man-ipv6-atomic-fragments-00
Abstract
IPv6 allows packets to contain a Fragment Header, without the packet
being actually fragmented into multiple pieces. Such packets
typically result from 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 hosts as "fragmented traffic".
By forging ICMPv6 "Packet Too Big" error messages an attacker can
cause hosts to employ "atomic fragments", and the launch any
fragmentation-based attacks against such traffic. This document
discusses the generation of the aforementioned "atomic fragments",
the corresponding security implications, and formally updates RFC
2460 and RFC 5722 such that the attack vector based on "atomic
fragments" is completely eliminated.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on June 17, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Generation of IPv6 'atomic fragments' . . . . . . . . . . . . 5
3. Updating RFC 2460 and RFC 5722 . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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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, and hence allowed for ambiguity in 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 overlapping fragments
should 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. The
resulting packets will thus *not* be actually fragmented into several
pieces, but only include a Fragment Header with both the "Fragment
Offset" and the "M" bit set to 0.
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), most IPv6 implementations
process them as regular fragments. Namely, they try to perform IPv6
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 are silently
discarded.
Processing an "atomic fragment" as regular fragmented packet 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
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discussed in [RFC6274].
Section 2 describes the generation of IPv6 "atomic fragments", and
how they can be remotely "triggered" by a remote attacker. Section 3
formally updates [RFC2460] and [RFC5722] such that the aforementioned
attack vector is eliminated.
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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2. 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 [PREDICTABLE-ID] [CPNI-IPv6].
From a security standpoint, this situation is exacerbated by the
following factors:
Many implementations fail to perform validation checks on the
received ICMPv6 error messages, as recommended in Section 5.2 of
[RFC4443] and [RFC5927].
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.
Upon receipt of one of the aforementioned ICMPv6 "Packet Too Big"
error messages, the Destinations Cache is usually updated to
reflect that any further packets to such destination should
include a Fragment Header. This means that a single ICMPv6
"Packet Too Big" error message may affect multiple communication
instances (e.g., TCP connections) with such destination.
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Some implementations employ Fragment Identification values that
are predictable by remote attackers, greatly improving the chances
of an attacker of successfully performing a fragmentation-based
attack [PREDICTABLE-ID].
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3. Updating RFC 2460 and RFC 5722
This document updates [RFC2460] and [RFC5722] as follows:
A host that receives an IPv6 packet which includes a Fragment
Header with the "Fragment Offset" equal to 0 and the "M" bit 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}. In other words, the Fragment Header of "atomic
fragments" should be ignored by the receiving host.
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4. 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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5. Security Considerations
This document describes how an attacker can exploit ICMPv6 "Packet
Too Big" error messages to cause further IPv6 packets to include a
Fragment Header, such that he can perform any fragmentation-based
attack against otherwise non-fragmented traffic. This document
updates [RFC2460] and [RFC5722], such that the aforementioned attack
vector is completely eliminated.
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6. Acknowledgements
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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7. References
7.1. 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.
[RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",
RFC 5722, December 2009.
7.2. 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).
[PREDICTABLE-ID]
Gont, F., "Security Implications of Predictable Fragment
Identification Values", Work in Progress, December 2011, <
http://tools.ietf.org/html/
draft-gont-6man-predictable-fragment-id>.
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Author's Address
Fernando Gont
UK CPNI
Email: fgont@si6networks.com
URI: http://www.cpni.gov.uk
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