Internet Engineering Task Force Francis Dupont
INTERNET DRAFT ENST Bretagne
Expires in December 2002 June 2002
Transient pseudo-NAT attacks or
how NATs are even more evil than you believed
<draft-dupont-transient-pseudonat-00.txt>
Status of this Memo
This document is an Internet Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
This document is an Internet-Draft. Internet-Drafts are working
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Distribution of this memo is unlimited.
Abstract
When a "NAT traversal" capability is added to a class of signaling
protocols which can control some traffic aggregation points,
a new attack based on a temporary access to the path followed
by messages.
Mobile IP [1] with NAT traversal [5] or IKE [2] with NAT
traversal [6], including the IKEv2 [7] proposal, are potential
victims of this kind of attacks.
This document claims the vulnerability is an intrinsic property
of the NAT traversal capability, so is a new point where the
usage of NATs is very damaging.
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1. Introduction
A Network Address Translator (NAT [8]) is a router which rewrites
the source address or/and destination address as well as usually
the transport protocol ports. There are many kinds of NATs [9]
but in this document a NAT is any device which modifies at least
one of the IP header addresses (a pseudo-NAT when this is done
for an attack).
NAT traversal capability consists in a NAT resilient transport,
usually UDP, and in address "agility", i.e., addresses in the
header of packets are taken as they are, especially the source
address (packets with a fake destination address likely don't
reach their intended recipient).
A traffic aggregation point where traffic from many sources and/or
many destinations are aggregated and sent to the same destination
and usually from the same source (the traffic aggregation point)
through a tunnel. Home agents in Mobile IP and security gateways
in IPsec [3] are typical examples of such traffic aggregation
points (which are not necessary for the attack but increase its
impact).
2. The Transient Pseudo-NAT Attack
An attacker acting as a NAT (i.e., a pseudo-NAT) may:
- redirect packets to an accomplice
- make the intended recipient not receive packets to it
(first form of Denial-of-Service (DoS) attack)
- flood a third party by the hijacked packets
(second form of DoS attack, perhaps the most dangerous)
To perform the attack, the attacker must be on the path of packets
during the attack.
When there is a traffic aggregation point, the effects of the
attack are amplified when the attack is done "at the exit" of
the aggregation point.
When a signaling protocol manages the direction followed by the
traffic, the attacker can only spoof the addresses in headers
of some messages of the protocol in order to hijack the traffic
during a long period (i.e., until an error is detected and the
correct path re-established). As the attacker has to stay on
the path only a short moment, at the limit only for one packet,
this attack is named the "transient" pseudo-NAT attack.
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3. Attack Examples
3.1 Mobile IP
For Mobile IP the traffic aggregation point for choice is the
home agent and the target signaling protocol is the binding update -
binding acknowledgment exchange. If the NAT traversal capability
is enabled, the care-of address of the mobile may not be protected
therefore may be easily spoofed.
If no binding acknowledgment is required the attack can be reduced
to the modification in transit of only one packet so we recommend
to always require acknowledgment when NAT traversal is enabled
(as a weak form of return-routability check).
3.2 IKE
The attack against IKE is worse because IKE is supposed to ensure
a very high level of security, unfortunately defeated by NAT
traversal which is the first short-term work item of the IETF
ipsec working group charter [4]...
The attack follows the same scheme: addresses in headers of IKE
exchange messages are spoofed and the traffic, for instance between
two security gateways, is hijacked.
Any improvement of the IKE protocol makes the attack easier (a
very unpleasant property of this attack). For instance if an
implementation supports an address change between two "phases"
(something desirable and supported via the SPI of the phase one)
then to spoof the two or three messages of a quick mode exchange is
enough, or in IKEv2 only one packet of a CREATE-CHILD-SA exchange.
Again there is no easy defense which keeps the NAT traversal
capability. For instance the protection of the header addresses
(very easy to provide in the IKE framework) is effective against
both the vulnerability and the NAT traversal capability...
4. Security Considerations
The Mobile IP NAT traversal new document has a long description
of this attack [10,5]. We believe the ipsec working group will
examine in details what features can help mobility or/and NAT
traversal and what are their consequences for security.
The architectural implications of NAT document [11] does not
describe this attack but it can be considered as a result of
the violation of the end-to-end principle on the trust model.
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5. Acknowledgments
Maryline Maknavicius-Laurent drew my attention on this attack at
the IP Cellular Network 2002 conference. Phil Roberts encouraged
me to point out this attack in the IETF mobileip WG mailing-list
ASAP. I'd like to thank a well known NAT hater who'd like to stay
anonymous for his help to write this document.
6. Normative References
[1] C. Perkins (ed.), "IP Mobility Support for IPv4", RFC 3220,
January 2002.
[2] D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[3] S. Kent, R. Atkinson, "Security Architecture for the Internet
Protocol", RFC 2401, November 1998.
[4] http://www.ietf.org/html.charters/ipsec-charter.html
7. Informative References
[5] H. Levkowetz, S. Vaarala, "Mobile IP NAT/NAPT Traversal using
UDP Tunnelling", draft-ietf-mobileip-nat-traversal-04.txt,
May 2002.
[6] A. Huttunen & all, "UDP Encapsulation of IPsec Packets",
draft-ietf-ipsec-udp-encaps-02.txt, April 2002.
[7] D. Harkins & all, "Proposal for the IKEv2 Protocol",
draft-ietf-ipsec-ikev2-02.txt, April 2002.
[8] K. Egevang, P. Francis, "The IP Network Address Translator
(NAT)", RFC 1631, May 1994.
[9] P. Srisuresh, M. Holdrege, "IP Network Address Translator
(NAT) Terminology and Considerations", RFC 2663, August 1999.
[10] S. Vaarala, public communication in the mobileip mailing-list,
<E2EFC3D881823A4CA24022D163D2C4AE2391AB@server.netseal.com>,
May 2002.
[11] T. Hain, "Architectural Implications of NAT", RFC 2993,
November 2000.
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8. Author's Address
Francis Dupont
ENST Bretagne
Campus de Rennes
2, rue de la Chataigneraie
BP 78
35512 Cesson-Sevigne Cedex
FRANCE
Fax: +33 2 99 12 70 30
EMail: Francis.Dupont@enst-bretagne.fr
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