ABFAB Working Group                                            S. Winter
Internet-Draft                                                   RESTENA
Intended status: Standards Track                              J. Salowey
Expires: April 17, 2013                                            Cisco
                                                        October 14, 2012

          Update to the EAP Applicability Statement for ABFAB


   This document updates the Extensible Authentication Protocol (EAP)
   applicability statement from RFC3748 to reflect recent usage of the
   EAP protocol in the Application Bridging for Federated Access Beyond
   web (ABFAB) working group.

Status of This Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . . . 3
   2.  Uses of EAP for Application-Layer Access  . . . . . . . . . . . 3
   3.  Revised EAP applicability statement . . . . . . . . . . . . . . 4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 5
     7.2.  Informational References  . . . . . . . . . . . . . . . . . 5

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1.  Introduction

   The EAP applicability statement in [RFC3748] defines the scope of the
   Extensible Authentication Protocol to be "for use in network access
   authentication, where IP layer connectivity may not be available.",
   and states that "Use of EAP for other purposes, such as bulk data
   transport, is NOT RECOMMENDED.".

   While some of the recommendation against usage of EAP for bulk data
   transport is still valid, some of the other provisions in the
   applicability statement have turned out to be too narrow.  Section 2
   describes the example where EAP is used to authenticate application
   layer access.  Section 3 provides new text to update the paragraph
   1.3.  "Applicability" in [RFC3748].

1.1.  Requirements Language

   In this document, several words are used to signify the requirements
   of the specification.  The key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in RFC 2119.  [RFC2119]

2.  Uses of EAP for Application-Layer Access

   Ongoing work in the IETF (abfab working group) specifies the use of
   EAP over GSSAPI for generic application layer access.  In the past,
   using EAP in this context has met resistance due to the lack of
   channel bindings [RFC6677].  Without channel bindings, a peer cannot
   verify if an authenticator is authorized to provide an advertised
   service.  In most network access use cases all access servers that
   are served by a particular EAP server are providing the same or very
   similar types of service.  The peer does not need to differentiate
   between different access network services supported by the same EAP

   However as additional services use EAP for authentication, the
   distinction of which service is being contacted becomes more
   important.  Application services might have different properties.
   Consider an environment with multiple printers some of which provide
   a confidential service to output documents to a controlled location.
   If a peer sent a document to the wrong service then potentially
   sensitive information might be printed in an uncontrolled location
   and be disclosed.  In addition, it might be more likely that a low-
   value service is compromised than some high value service.  If the
   high-value service could be impersonated by a low-value service then
   the security of the overall system would be limited by the security
   of the lower value service.

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   This distinction is present in any environment where peers' security
   depends on which service they reach.  However it is particularly
   acute in a federated environment where multiple organizations are
   involved.  It is very likely that these organizations will have
   different security policies and practices.  It is very likely that
   the goals of these organizations will not entirely be aligned.  In
   many situations one organization could gain value by being able to
   impersonate another.  In this environment, authenticating the EAP
   server is insufficient: the peer must also validate that the
   contacted host is authorized to provide the requested service.

   For these reasons, channel binding MUST be implemented by peers, EAP
   servers and AAA servers in environments where EAP authentication is
   used to access application layer services.  In addition, channel
   binding MUST default to being required by peers for non-network
   authentication.  If the EAP server is aware that authentication is
   for something other than a network service, it too MUST default to
   requiring channel binding.  Operators need to carefully consider the
   security implications before relaxing these requirements.  One
   potentially serious attack exists when channel binding is not
   required and EAP authentication is introduced into an existing non-
   network service.  A device can be created that impersonates a Network
   Access Service to peers, but actually proxies the authentication to
   the service that newly accepts EAP authentications may decrease the
   security of this service even for users who previously used non-EAP
   means of authentication to the service.

   It is REQUIRED for the application layer to prove possession of the
   EAP MSK between the EAP Peer and EAP Authenticator.  Failing to
   validate the possession of the EAP MSK can allow an attacker to
   insert himself into the conversation and impersonate the peer or
   authenticator.  In addition, the application should define an channel
   binding attributes that are sufficient to validate that the
   application service is being correctly represented to the peer.

3.  Revised EAP applicability statement

   The following text is added to the EAP applicability statement in

   In cases where EAP is used for application authentication, support
   for EAP Channel Bindings is REQUIRED on the EAP Peer and EAP Server
   to validate that the host is authorized to provide the services
   requested.  In addition, the application MUST define channel binding
   attributes that are sufficient to validate that the application
   service is being correctly represented to the peer.  It is important
   for the protocol carrying EAP to prove possession of the EAP MSK
   between the EAP Peer and EAP Authenticator.

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4.  Security Considerations

   In addition to the requirements discussed in the main sections of the
   document applications should take into account how server
   authentication is achieved.  Some deployments may allow for weak
   server authentication that is then validated with an additional
   existing exchange that provides mutual authentication.  In order to
   fully mitigate the risk of NAS impersonation when these mechanisms
   are used, it is RECOMMENDED that mutual channel bindings be used to
   bind the authentications together as described in
   [I-D.ietf-emu-crypto-bind].  When doing channel binding it is
   REQUIRED that the authenticator is not able to modify the channel
   binding data passed between the peer to the authenticator as part of
   the authentication process.

5.  IANA Considerations

   This document has no actions for IANA.

6.  Acknowledgements

   Large amounts of helpful text and insightful thoughts were
   contributed by Sam Hartman, Painless Security.

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.

   [RFC3748]                   Aboba, B., Blunk, L., Vollbrecht, J.,
                               Carlson, J., and H. Levkowetz,
                               "Extensible Authentication Protocol
                               (EAP)", RFC 3748, June 2004.

   [RFC6677]                   Hartman, S., Clancy, T., and K. Hoeper,
                               "Channel-Binding Support for Extensible
                               Authentication Protocol (EAP) Methods",
                               RFC 6677, July 2012.

7.2.  Informational References

   [I-D.ietf-emu-crypto-bind]  Hartman, S., Wasserman, M., and D. Zhang,
                               "EAP Mutual Cryptographic Binding",
                               draft-ietf-emu-crypto-bind-00 (work in
                               progress), June 2012.

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Authors' Addresses

   Stefan Winter
   Fondation RESTENA
   6, rue Richard Coudenhove-Kalergi
   Luxembourg  1359

   Phone: +352 424409 1
   Fax:   +352 422473
   EMail: stefan.winter@restena.lu
   URI:   http://www.restena.lu.

   Joseph Salowey
   Cisco Systems
   2901 3rd Ave
   Seattle, Washington  98121

   EMail: jsalowey@cisco.com

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