IPv6 maintenance Working Group (6man) F. Gont
Internet-Draft UK CPNI
Updates: 3971, 4861 (if approved) June 29, 2012
Intended status: Standards Track
Expires: December 31, 2012
Security Implications of the Use of IPv6 Extension Headers with IPv6
Neighbor Discovery
draft-ietf-6man-nd-extension-headers-00
Abstract
This document analyzes the security implications of using IPv6
Extension Headers with Neighbor Discovery (ND) messages. It updates
RFC 4861 such that use of the IPv6 Fragmentation Header is forbidden
in all Neighbor Discovery messages, thus allowing for simple and
effective counter-measures for Neighbor Discovery attacks. Finally,
it discusses the security implications of using IPv6 fragmentation
with SEcure Neighbor Discovery (SEND), and provides advice such that
the aforementioned security implications are mitigated.
Status of this Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Traditional Neighbor Discovery and IPv6 Fragmentation . . . . 5
3. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation . . . 6
4. Specification . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
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1. Introduction
The Neighbor Discovery Protocol (NDP) is specified in RFC 4861
[RFC4861] and RFC 4862 [RFC4862]. It is used by both hosts and
routers. Its functions include Neighbor Discovery (ND), Router
Discovery (RD), Address Autoconfiguration, Address Resolution,
Neighbor Unreachability Detection (NUD), Duplicate Address Detection
(DAD), and Redirection.
Many of the possible attacks against the Neighbor Discovery Protocol
are discussed in detail in [RFC3756]. In order to mitigate the
aforementioned possible attacks, the SEcure Neighbor Discovery (SEND)
was standardized. SEND employs a number of mechanisms to certify the
origin of Neighbor Discovery packets and the authority of routers,
and to protect Neighbor Discovery packets from being the subject of
modification or replay attacks.
However, a number of factors, such as the use of trust anchors and
the unavailability of SEND implementations for many widely-deployed
operating systems, make SEND hard to deploy [Gont-DEEPSEC2011].
Thus, in many general scenarios it may be necessary and/or convenient
to use other mitigation techniques for NDP-based attacks. The
following "lightweight" mitigations are currently available for NDP
attacks:
o Layer-2 filtering of Neighbor Discovery packets (such as RA-Guard
[RFC6105])
o Neighbor Discovery monitoring tools (e.g., such as NDPMon
[NDPMon])
IPv6 Router Advertisement Guard (RA-Guard) is a mitigation technique
for attack vectors based on ICMPv6 Router Advertisement messages. It
is meant to block attack packets at a layer-2 device before the
attack packets actually reach the target nodes. [RFC6104] describes
the problem statement of "Rogue IPv6 Router Advertisements", and
[RFC6105] specifies the "IPv6 Router Advertisement Guard"
functionality.
Tools such as NDPMon [NDPMon] and ramond [ramond] aim at monitoring
Neighbor Discovery traffic in the hopes of detecting possible attacks
when there are discrepancies between the information advertised in
Neighbor Discovery packets and the information stored on a local
database.
A key challenge that these mitigation or monitoring techniques face
is that introduced by IPv6 fragmentation, since it is trivial for an
attacker to conceal his attack by fragmenting his packets into
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multiple fragments. This may limit or even eliminate the
effectiveness of the aforementioned mitigation or monitoring
techniques. Recent work [CPNI-IPv6] indicates that current
implementations of the aforementioned "lightweight" mitigations for
NDP attacks can be trivially evaded. For example, as noted in
[I-D.ietf-v6ops-ra-guard-implementation], current RA-Guard
implementations can be trivially evaded by fragmenting the attack
packets into multiple fragments, such that the layer-2 device cannot
find all the necessary information to perform packet filtering in the
same packet. While Neighbor Discovery monitoring tools could (in
theory implement IPv6 fragment reassembly, this is usually an arms-
race with the attacker (an attacker generate lots of forged fragments
to "confuse" the monitoring tools), and therefore the aforementioned
tools are unreliable for the detection of such attacks.
Section 2 analyzes the use of IPv6 fragmentation in traditional
Neighbor discovery. Section 3 analyzes the use of IPv6 fragmentation
in SEcure Neighbor Discovery (SEND). Section 4 formally updates RFC
4861 such that use of the IPv6 Fragment Header with traditional
Neighbor Discovery is forbidden, and provides advice on the use of
IPv6 fragmentation with SEND.
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. Traditional Neighbor Discovery and IPv6 Fragmentation
The only potential use case for IPv6 fragmentation with traditional
(i.e., non-SEND) IPv6 Neighbor Discovery would be that in which a
Router Advertisement must include a large number of options (Prefix
Information Options, Route Information Options, etc.). However, this
could still be achieved without employing fragmentation, by splitting
the aforementioned information into multiple Router Advertisement
messages.
Some Neighbor Discovery implementations are known to silently
ignore Router Advertisement messages that employ fragmentation.
Therefore, splitting the necessary information into multiple RA
messages (rather than sending a large RA message that is
fragmented into multiple IPv6 fragments) is probably desirable
even from an interoperability point of view.
As a result of the aforementioned considerations, and since avoiding
the use of IPv6 fragmentation in traditional Neighbor Discovery would
greatly simplify and improve the effectiveness of monitoring and
filtering ND, Section 4 specifies that hosts silently ignore
traditional Neighbor Discovery messages (i.e., those specified in
[RFC4861]) that employ IPv6 fragmentation.
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3. SEcure Neighbor Discovery (SEND) and IPv6 Fragmentation
SEND packets typically carry more information than traditional
Neighbor Discovery packets: for example, they include additional
options such as the CGA option and the RSA signature option.
In the case of Neighbor Discovery messages specified in [RFC4861],
the situation is roughly the same: if more information than would fit
in a non-fragmented Neighbor Discovery packet needs to be sent, it
should be split into multiple Neighbor Discovery messages (such that
IPv6 fragmentation is avoided).
However, Certification Path Advertisement messages (specified in
[RFC3971]) pose a different situation, since the Certificate Option
they include contain much more information than any other Neighbor
Discovery option. For example, Appendix C of [RFC3971] reports
Certification Path Advertisement messages from 1050 to 1066 bytes on
an Ethernet link layer. Since the size of CPA messages could
potentially exceed the MTU of the local link, we recommend that
fragmented CPA messages be normally processed (although *sending*
fragmented CPA messages is discouraged).
It should be noted that relying on fragmentation opens the door to a
variety of IPv6 fragmentation-based attacks. In particular, if an
attacker is located on the same broadcast domain as the victim host,
and Certification Path Advertisement messages employ IPv6
fragmentation, it would be trivial for the attacker to forge IPv6
fragment such that they result in "Fragment ID collisions", causing
both the attack fragments and the legitimate fragments to be
discarded by the victim node. This would eventually cause the
Authorization Delegation Discovery to fail, thus leading the host to
fall back (depending to local configuration) either to unsecured
mode, or to reject the corresponding Router Advertisement messages
(possibly resulting in a Denial of Service).
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4. Specification
Nodes MUST NOT employ IPv6 fragmentation for sending any of the
following Neighbor Discovery and SEcure Neighbor Discovery messages:
Neighbor Solicitation, Neighbor Advertisement, Router Solicitation,
Router Advertisement, Redirect, and Certification Path Solicitation.
SEND nodes SHOULD NOT employ IPv6 fragmentation for sending
Certification Path Advertisement messages.
Nodes MUST silently ignore the following Neighbor Discovery and
SEcure Neighbor Discovery messages if the packets carrying them
include an IPv6 Fragmentation Header: Neighbor Solicitation, Neighbor
Advertisement, Router Solicitation, Router Advertisement, Redirect,
and Certification Path Solicitation.
Nodes SHOULD normally process Certification Path Advertisement
messages that employ IPv6 fragmentation.
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5. Security Considerations
The IPv6 Fragmentation Header can be leveraged to circumvent network
monitoring tools and current implementations of mechanisms such as
RA-Guard [I-D.ietf-v6ops-ra-guard-implementation]. By updating the
relevant specifications such that the IPv6 Fragment Header is not
allowed in any Neighbor Discovery messages except "Certification Path
Advertisement", protection of local nodes against Neighbor Discovery
attacks, and monitoring of Neighbor Discovery traffic is greatly
simplified.
[I-D.ietf-v6ops-ra-guard-implementation] discusses an improvement to
the RA-Guard mechanism that can mitigate Neighbor Discovery attacks
that employ IPv6 Fragmentation. However, it should be noted that
unless [RFC4861] is updated (as proposed in this document), Neighbor
Discovery monitoring tools (such as NDPMon [NDPMon]) would remain
unreliable and trivial to circumvent by a skilled attacker.
As noted in Section 3, use of SEND could potentially result in
fragmented "Certification Path Advertisement" messages, thus allowing
an attacker to employ IPv6 fragmentation-based attacks against such
messages. Therefore, to the extent that is possible, such use of
fragmentation should be avoided.
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6. Acknowledgements
The author would like to thank (in alphabetical order) Mikael
Abrahamsson, Ran Atkinson, Ron Bonica, Jean-Michel Combes, David
Farmer, Roque Gagliano, Bob Hinden, Philip Homburg, Ray Hunter,
Arturo Servin, and Mark Smith, for providing valuable comments on
earlier versions of this document.
This document resulted from the project "Security Assessment of the
Internet Protocol version 6 (IPv6)" [CPNI-IPv6], carried out by
Fernando Gont on behalf of the UK Centre for the Protection of
National Infrastructure (CPNI). The author would like to thank the
UK CPNI, for their continued support.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862, September 2007.
7.2. Informative References
[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[RFC6104] Chown, T. and S. Venaas, "Rogue IPv6 Router Advertisement
Problem Statement", RFC 6104, February 2011.
[RFC6105] Levy-Abegnoli, E., Van de Velde, G., Popoviciu, C., and J.
Mohacsi, "IPv6 Router Advertisement Guard", RFC 6105,
February 2011.
[NDPMon] "NDPMon - IPv6 Neighbor Discovery Protocol Monitor",
.
[ramond] "ramond", .
[I-D.ietf-v6ops-ra-guard-implementation]
Gont, F., "Implementation Advice for IPv6 Router
Advertisement Guard (RA-Guard)",
draft-ietf-v6ops-ra-guard-implementation-04 (work in
progress), May 2012.
[CPNI-IPv6]
Gont, F., "Security Assessment of the Internet Protocol
version 6 (IPv6)", UK Centre for the Protection of
National Infrastructure, (available on request).
[Gont-DEEPSEC2011]
Gont, "Results of a Security Assessment of the Internet
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Protocol version 6 (IPv6)", DEEPSEC 2011 Conference,
Vienna, Austria, November 2011, .
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Author's Address
Fernando Gont
Centre for the Protection of National Infrastructure
Email: fernando@gont.com.ar
URI: http://www.cpni.gov.uk
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