





Internet Engineering Task Force (IETF)                         S. Winter
Request for Comments: 7057                                       RESTENA
Updates: 3748                                                 J. Salowey
Category: Standards Track                                          Cisco
ISSN: 2070-1721                                            December 2013


         Update to the Extensible Authentication Protocol (EAP)
                      Applicability Statement for
      Application Bridging for Federated Access Beyond Web (ABFAB)

Abstract

   This document updates the Extensible Authentication Protocol (EAP)
   applicability statement from RFC 3748 to reflect recent usage of the
   EAP protocol in the Application Bridging for Federated Access Beyond
   web (ABFAB) architecture.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7057.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





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Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................2
   2. Uses of EAP for Application-Layer Access ........................2
      2.1. Retransmission .............................................4
      2.2. Re-authentication ..........................................5
   3. Revised EAP Applicability Statement .............................5
   4. Security Considerations .........................................6
   5. Acknowledgements ................................................6
   6. References ......................................................6
      6.1. Normative References .......................................6
      6.2. Informative References .....................................6

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 reasons for the recommendation against usage of EAP
   for bulk data transport are 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
   Section 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",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT
   RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
   interpreted as described in [RFC2119].

2.  Uses of EAP for Application-Layer Access

   Ongoing work in the IETF specifies the use of EAP over Generic
   Security Service Application Program Interface (GSS-API) for generic
   application layer access [RFC7055].  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.






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   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.

   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.

   In environments where EAP is used for purposes other than network
   access authentication:

   o  All EAP servers and all application access EAP peers MUST support
      channel bindings.  All network access EAP peers SHOULD support
      channel bindings.

   o  Channel binding MUST be used for all application authentication.
      The EAP server MUST require that either the correct EAP lower-
      layer attribute or another attribute indicating the purpose of the
      authentication be present in the channel binding data for
      application authentication.

   o  Channel binding SHOULD be used for all network access
      authentication, and when channel binding data is present, the EAP
      server MUST require that it contain the correct EAP lower-layer
      attribute to explicitly identify the reason for authentication.

   o  Any new usage of EAP MUST use channel bindings including the EAP
      lower-layer attribute to prevent confusion with network access
      usage.






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   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 service other than network access.  A
   device can be created that impersonates a Network Access Service
   (NAS) to peers, but actually proxies the authentication to the new
   application service that accepts EAP authentications.  This 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 that both the EAP
   peer and EAP authenticator possess the EAP Master Session Key (MSK).
   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
   channel binding attributes that are sufficient to validate that the
   application service is being correctly represented to the peer.

2.1.  Retransmission

   In EAP, the authenticator is responsible for retransmission.  By
   default, EAP assumes that the lower layer (the application in this
   context) is unreliable.  The authenticator can send a packet whenever
   its retransmission timer triggers.  In this mode, applications need
   to be able to receive and process EAP messages at any time during the
   authentication conversation.

   Alternatively, EAP permits a lower layer to set the retransmission
   timer to infinite.  When this happens, the lower layer becomes
   responsible for reliable delivery of EAP messages.  Applications that
   use a lock-step or client-driven authentication protocol might
   benefit from this approach.

   In addition to retransmission behavior, applications need to deal
   with discarded EAP messages.  For example, whenever some EAP methods
   receive erroneous input, these methods discard the input rather than
   generating an error response.  If the erroneous input was generated
   by an attacker, legitimate input can sometimes be received after the
   erroneous input.  Applications MUST handle discarded EAP messages,
   although the specific way in which discarded messages will be handled
   depends on the characteristics of the application.  Options include
   failing the authentication at the application level, requesting an
   EAP retransmit and waiting for additional EAP input.

   Applications designers that incorporate EAP into their application
   need to determine how retransmission and message discards are
   handled.




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2.2.  Re-authentication

   EAP lower layers MAY provide a mechanism for re-authentication to
   happen within an existing session [RFC3748].  Re-authentication
   permits security associations to be updated without establishing a
   new session.  For network access, this can be important because
   interrupting network access can disrupt connections and media.

   Some applications might not need re-authentication support.  For
   example, if sessions are relatively short-lived or if sessions can be
   replaced without significant disruption, re-authentication might not
   provide value.  Protocols like HyperText Transfer Protocol (HTTP)
   [RFC2616] and Simple Mail Transport Protocol (SMTP) [RFC5321] are
   examples of protocols where establishing a new connection to update
   security associations is likely to be sufficient.

   Re-authentication is likely to be valuable if sessions or connections
   are long-lived or if there is a significant cost to disrupting them.

   Another factor may make re-authentication important.  Some protocols
   only permit one party in a protocol (for example, the client) to
   establish a new connection.  If another party in the protocol needs
   the security association refreshed, then re-authentication can
   provide a mechanism to do so.

   Application designers need to determine whether re-authentication
   support is needed and which parties can initiate it.

3.  Revised EAP Applicability Statement

   The following text is appended to the EAP applicability statement in
   [RFC3748].

   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.  The protocol
   carrying EAP MUST prove possession of the EAP MSK between the EAP
   peer and EAP authenticator.  In the context of EAP for application
   access the application is providing the EAP lower layer.
   Applications protocols vary so their specific behavior and transport
   characteristics needs to be considered when determining their
   retransmission and re-authentication behavior.  Circumstances might
   require that applications need to perform conversion of identities
   from an application specific character set to UTF-8 or another




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   character set required by a particular EAP method.  See also
   [RADEXT-NAI], Section 2.6, for information about normalization of
   identifiers.

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 [RFC7029].  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.  Acknowledgements

   Large amounts of helpful text and insightful thoughts were
   contributed by Sam Hartman, Painless Security.  David Black
   contributed to the text clarifying channel bindings usage.

6.  References

6.1.  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.

6.2.  Informative References

   [RADEXT-NAI] DeKok, A., "The Network Access Identifier", Work in
                Progress, November 2013.

   [RFC2616]    Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
                Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
                Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.




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   [RFC5321]    Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
                October 2008.

   [RFC7029]    Hartman, S., Wasserman, M., and D. Zhang, "Extensible
                Authentication Protocol (EAP) Mutual Cryptographic
                Binding", RFC 7029, October 2013.

   [RFC7055]    Hartman, S., Ed. and J. Howlett, "A GSS-API Mechanism
                for the Extensible Authentication Protocol", RFC 7055,
                December 2013.

Authors' Addresses

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

   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
   USA

   EMail: jsalowey@cisco.com



















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