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The Network Endpoint Assessment (NEA) Asokan Attack Analysis

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6813.
Authors Joseph A. Salowey , Steve Hanna
Last updated 2015-10-14 (Latest revision 2012-10-19)
Replaces draft-salowey-nea-asokan
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Informational
Additional resources Mailing list discussion
Stream WG state WG Document
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IESG IESG state Became RFC 6813 (Informational)
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Responsible AD Stephen Farrell
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Network Working Group                                        J. Salowey
Internet Draft                                            Cisco Systems
Intended status: Informational                                 S. Hanna
Expires: April 2013                                    Juniper Networks
                                                       October 19, 2012

                        NEA Asokan Attack Analysis


   The Network Endpoint Assessment protocols are subject to a subtle
   forwarding attack that has become known as the NEA Asokan Attack.
   This document describes the attack and countermeasures that may be

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 19, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors. All rights reserved.

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   ( in effect on the date of
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Table of Contents

   1. Introduction...................................................2
   2. NEA Asokan Attack Explained....................................2
   3. Lying Endpoints................................................4
   4. Countermeasures Against The NEA Asokan Attack..................4
      4.1. Identity Binding..........................................4
      4.2. Cryptographic Binding.....................................5
         4.2.1. Binding Options......................................5
   5. Conclusions....................................................6
   6. IANA Considerations............................................6
   7. Security Considerations........................................6
   8. References.....................................................6
      8.1. Informative References....................................6
   9. Acknowledgments................................................7

1. Introduction

   The Network Endpoint Assessment [2] protocols are subject to a
   subtle forwarding attack that has become known as the NEA Asokan
   Attack. This document describes the attack and countermeasures that
   may be mounted. The posture transport (PT) protocols developed by
   the NEA working group, PT-TLS [5] and PT-EAP [6], include mechanisms
   that can provide cryptographic binding and identity binding

2. NEA Asokan Attack Explained

   The NEA Asokan Attack is a variation on an attack described in a
   2002 paper written by Asokan, Niemi, and Nyberg [1]. Figure 1
   depicts one version of the original Asokan attack. This attack
   involves tricking an authorized user into authenticating to a decoy
   AAA server, which forwards the authentication protocol from one
   tunnel to another, tricking the real AAA server into believing these

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   messages originated from the attacker controlled machine.  As a
   result the real AAA server grants access to the attacker controlled

                            +-------------+ ========== +----------+
                            |   Attacker  |-AuthProto--|AAA Server|
                            +-------------+ ========== +----------+
   +--------------+ ========== +----------------+
   |AuthorizedUser|-AuthProto--|Decoy AAA Server|
   +--------------+ ========== +----------------+

         Figure 1: One Example of Original Asokan Attack

   As described in the NEA Overview [2], the NEA Reference Model is
   composed of several nested protocols. The PA protocol is nested in
   the PB protocol, which is nested in the PT protocol. When used
   together successfully, these protocols allow a NEA Server to assess
   the security posture of an endpoint. The NEA Server may use this
   information to decide whether network access should be granted or
   for other purposes.

   Figure 2 illustrates a NEA Asokan Attack. The attacker wants to
   trick GoodServer into believing that DirtyEndpoint has good security
   posture. This might allow the attacker to bring an infected machine
   onto a network and infect others, for example. To accomplish this
   goal, the attacker forwards PA messages from CleanEndpoint through
   BadServer to DirtyEndpoint, which sends them on to GoodServer.
   GoodServer is tricked into thinking that the PA messages came from
   DirtyEndpoint and therefore considers DirtyEndpoint to be clean.

                            +-------------+ ========== +----------+
                            +-------------+ ========== +----------+
   +-------------+ ========== +---------+
   +-------------+ ========== +---------+

         Figure 2: NEA Asokan Attack

   Countermeasures against a NEA Asokan Attack are described in section

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3. Lying Endpoints

   Some may argue that there are other attacks against NEA systems that
   are simpler than the Asokan attack, such as lying endpoint attacks.
   That is true. It's easy for an endpoint to simply lie about its
   posture. But there are defenses against lying endpoint attacks, such
   as using an external measurement agent (EMA).

   An EMA is hardware, software, or firmware designed to accurately
   report on endpoint configuration but to be especially secure and
   hard to compromise. The EMA observes and reports on critical aspects
   of endpoint posture such as which security-relevant firmware and
   software has been loaded.

   When an EMA is used for NEA, the PA messages that reliably and
   securely establish endpoint posture are exchanged between the EMA
   itself and a Posture Validator on the NEA Server. The Posture
   Collector on the endpoint and any other intermediaries between the
   EMA and the Posture Validator on the NEA Server are not trusted.
   They just pass messages along as untrusted intermediaries.

   To ensure that the EMA's messages are accurately conveyed to the
   Posture Validator even if the Posture Collector or other
   intermediaries have been compromised, these PA messages must provide
   integrity protection, replay protection, and source authentication
   between the EMA and the Posture Validator. Confidentiality
   protection is not needed, at least with respect to the software on
   the endpoint. But integrity protection should include protection
   against message deletion and session truncation. Organizations that
   have developed EMAs have typically developed remote attestation
   protocols that provide these properties (e.g. TCG's PTS Protocol
   Binding to IF-M [7]). While the development of lying endpoint
   detection technologies is out of scope for NEA, these technologies
   must be supported by the NEA protocols. Therefore, the NEA protocols
   must support countermeasures against the NEA Asokan Attack.

4. Countermeasures Against The NEA Asokan Attack

4.1. Identity Binding

   One way to mitigate the Asokan attack is to bind the identities used
   in tunnel establishment into a cryptographic exchange at the PA
   layer.  While this can go a long way to preventing the attack it
   does not bind the exchange to a specific TLS exchange, which is
   desirable.  In addition, there is no standard way to extract an
   identity from a TLS session, which could make implementation

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4.2. Cryptographic Binding

   One way to thwart the NEA Asokan Attack is for the PA exchange to be
   cryptographically bound to the PT exchange and to any keying
   material or privileges granted as a result of these two exchanges.
   This allows the NEA Server to ensure that the PA messages pertain to
   the same endpoint as the party terminating the PT exchange and that
   no other party gains any access or advantage from this exchange.

4.2.1. Binding Options

   This section discusses binding protocol solution options and
   provides analysis.  Since PT-TLS and PT-EAP involve TLS, this
   document focuses on TLS based solutions that can work with either
   transport. Information from the TLS Tunnel

   The TLS handshake establishes cryptographic state between the TLS
   client and TLS server.  There are several mechanisms that can be
   used to export information derived from this state.  The client and
   server independently include this information in calculations to
   bind the instance of the tunnel into the PA protocol.

   Keying Material Export - RFC 5705 [8] defines Keying Material
   Exporters for TLS that allow additional secret key material to be
   extracted from the TLS master secret.

   tls-unique Channel Binding Data - RFC 5929 [9] defines several
   quantities that can be extracted from the TLS session to bind the
   TLS session to other protocols.  The tls-unique binding consists of
   data extracted from the TLS handshake finished message. TLS Cipher Suites

   In order to eliminate the possibility of a man-in-the-middle and
   thwart the Asokan attack it is important that neither TLS endpoint
   be in sole control of the TLS pre-master secret.   Cipher suites
   based on key transport such as RSA cipher suites do not meet this
   requirement, instead Diffie-Hellman Cipher Suites, such as RSA-DHE,
   are required when this mechanism is employed. Using Additional Key Material from TLS

   In some cases key material is extracted from the TLS tunnel and used
   to derive ciphering keys used in another protocol.  For example,
   EAP-TLS [10] uses key material extracted from TLS in lower layer

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   ciphering.  In this case the extracted keys must not be under the
   control of a single party so the considerations in the previous
   section are important.  EMA assumptions

   The EMA needs to obtain the binding data from the TLS exchange and
   prove knowledge of the binding data in an exchange that has
   integrity protection, source authentication and replay protection.

5. Conclusions

   The recommendations for addressing the NEA Asokan Attack are as

   1. Protocols should make use of cryptographic binding, in addition
     binding identities of the tunnel endpoints in the EMA may be
   2. In particular, L2 and L3 TLS-based PT transports (e.g. PT-TLS and
     PT-EAP) should use the same cryptographic binding mechanism
   3. The preferred approach is to use the tls-unique channel binding
     data from RFC 5929 [9].  The tls-unique value will be made
     available to the EMA that will use it.   This approach can utilize
     any TLS cipher suite based on a strong cipher algorithm.

6. IANA Considerations

   This document has no actions for IANA.

7. Security Considerations

   This document is primarily concerned with analyzing and proposing
   countermeasures for the NEA Asokan Attack. That does not mean that
   it covers all the possible attacks against the NEA protocols or
   against the NEA Reference Model. For a broader security analysis,
   see the Security Considerations section of the NEA Overview [2], PA-
   TNC [3], PB-TNC [4], PT-TLS [5], and PT-EAP [6].

8. References

8.1. Informative References

   [1]   N. Asokan, Valtteri Niemi, Kaisa Nyberg, "Man in the Middle
         Attacks in Tunneled Authentication Protocols", Nokia Research
         Center, Finland, Nov. 11, 2002,

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   [2]   Sangster, P., Khosravi, H., Mani, M., Narayan, K., and J.
         Tardo, "Network Endpoint Assessment (NEA): Overview and
         Requirements", RFC 5209, June 2008.

   [3]   Sangster, P., and K. Narayan, "PA-TNC: A Posture Attribute
         (PA) Protocol Compatible with Trusted Network Connect (TNC)",
         RFC 5792, March 2010.

   [4]   Sahita, R., Hanna, S., Hurst, R., and K. Narayan, "PB-TNC: A
         Posture Broker (PB) Protocol Compatible with Trusted Network
         Connect (TNC)", RFC 5793, March 2010.

   [5]   Sangster, P., N. Cam-Winget, and J. Salowey, "PT-TLS: A TCP-
         based Posture Transport (PT) Protocol", draft-ietf-nea-pt-tls-
         07.txt (work in progress), August 2012.

   [6]   Cam-Winget, N. and P. Sangster, "PT-EAP: Posture Transport
         (PT) Protocol For EAP Tunnel Methods", draft-ietf-nea-pt-eap-
         03.txt (work in progress), July 2012.

   [7]   Trusted Computing Group, "TCG Attestation PTS Protocol:
         Binding to TNC IF-M", Version 1.0, Revision 27, August 2011.

   [8]   Rescorla, E., "Keying Material Exporters for Transport Layer
         Security (TLS)", RFC 5705, March 2010.

   [9]   Altman, J., Williams, N., and L. Zhu, "Channel Bindings for
         TLS", RFC 5929, July 2010.

   [10]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
         Authentication Protocol", RFC 5216, March 2008.

9. Acknowledgments

   The members of the NEA Asokan Design Team were critical to the
   development of this document: Nancy Cam-Winget, Steve Hanna, Joe
   Salowey, and Paul Sangster.

   The authors would also like to recognize N. Asokan, Valtteri Niemi,
   and Kaisa Nyberg who published the original paper on this type of
   attack and Pasi Eronen who extended this attack to NEA protocols.

   This document was prepared using

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Authors' Addresses

   Joseph Salowey
   Cisco Systems, Inc.
   2901 3rd. Ave
   Seattle, WA  98121

   Steve Hanna
   Juniper Networks, Inc.
   79 Parsons Street
   Brighton, MA  02135

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