OAuth                                                      H. Tschofenig
Internet-Draft                                    Nokia Siemens Networks
Intended status: Informational                                   P. Hunt
Expires: March 10, 2013                               Oracle Corporation
                                                       September 6, 2012

             OAuth 2.0 Security: Going Beyond Bearer Tokens


   The OAuth working group has finished work on the OAuth 2.0 core
   protocol as well as the Bearer Token specification.  The Bearer Token
   is a TLS-based solution for ensuring that neither the interaction
   with the Authorization Server (when requesting a token) nor the
   interaction with the Resource Server (for accessing a protected
   resource) leads to token leakage.  There has, however, always been
   the desire to develop a security solution that is "better" than
   Bearer Tokens (or at least different) where the Client needs to show
   possession of some keying material when accessing a Resource Server.
   This document tries to capture the discussion and to come up with
   requirements to process the work on solutions.

   This document aims to discuss threats, security requirements and
   desired design properties of an enhanced OAuth security mechanism.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on March 10, 2013.

Copyright Notice

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

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   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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Security and Privacy Threats . . . . . . . . . . . . . . . . .  5
   4.  Threat Mitigation  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Confidentiality Protection . . . . . . . . . . . . . . . .  7
     4.2.  Sender Constraint  . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Key Confirmation . . . . . . . . . . . . . . . . . . . . .  8
     4.4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   7.  Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22

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1.  Introduction

   OAuth 1.0 [RFC5849] included a mechanism for putting a digital
   signature (when using asymmetric keys) and a keyed message digest
   (when using symmetric keys) to a resource request when presenting the
   OAuth access token.  OAuth 2.0 [I-D.ietf-oauth-v2] generalized the
   protocol and the Bearer Token security specification
   [I-D.ietf-oauth-v2-bearer] is close to publication as an RFC.

   Figure 1 shows the OAuth 2.0 exchange at an abstract level and
   illustrates the main entities.  For most parts of this document the
   focus is on the interaction between the Client and the Authorization
   Server and between the Client and the Resource Server.

        +--------+                               +---------------+
        |        |--(A)- Authorization Request ->|   Resource    |
        |        |                               |     Owner     |
        |        |<-(B)-- Authorization Grant ---|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(C)-- Authorization Grant -->| Authorization |
        | Client |                               |     Server    |
        |        |<-(D)----- Access Token -------|               |
        |        |                               +---------------+
        |        |
        |        |                               +---------------+
        |        |--(E)----- Access Token ------>|    Resource   |
        |        |                               |     Server    |
        |        |<-(F)--- Protected Resource ---|               |
        +--------+                               +---------------+

                  Figure 1: OAuth: Abstract Protocol Flow

   From a security point of view the following aspects of the OAuth 2.0
   specification are worth mentioning:

   o  Standardization of a JSON-based format and the content of the
      access token are still work in progress
      [I-D.ietf-oauth-json-web-token].  The same is true for the JSON-
      based security mechanisms.

   o  The interaction to obtain an access token in step #1 mandates to
      implement and to use TLS with server-side authentication to
      protect the confidentiality of the transmitted information.

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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document uses the terminology defined in RFC 4949 [RFC4949].
   The terms 'keyed hash' and 'keyed message digest' are used
   interchangable.  For privacy related matters we utilize the
   terminology defined in [I-D.iab-privacy-considerations].

   This document uses OAuth 2.0 terminology [I-D.ietf-oauth-v2].  In
   particular, the terms Client, Resource Server, Authorization Server,
   and Access Token are used.

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3.  Security and Privacy Threats

   The following list presents several common threats against protocols
   utilizing some form of tokens.  This list of threats is based on NIST
   Special Publication 800-63 [NIST800-63].  We exclude a discussion of
   threats related to any form of identity proofing and authentication
   of the Resource Owner to the Authorization Server since these
   procedures are not part of the OAuth 2.0 protocol specificaiton

   Token manufacture/modification:

      An attacker may generate a bogus tokens or modify the token
      content (such as authentication or attribute statements) of an
      existing token, causing Resource Server to grant inappropriate
      access to the Client.  For example, an attacker may modify the
      token to extend the validity period.  A Client may modify the
      token to have access to information that they should not be able
      to view.

   Token disclosure:  Tokens may contain personal data, such as real
      name, age or birthday, payment information, etc.

   Token redirect:

      An attacker uses the token generated for consumption by the
      Resource Server to obtain access to another Resource Server.

   Token reuse:

      An attacker attempts to use a token that has already been used
      once with a Resource Server.  The attacker may be an eavesdropper
      who observes the communication exchange or, worse, one of the
      communication end points.  A Client may, for example, leak access
      tokens because it cannot keep secrets confidential.  A Client may
      also re-use access tokens for some other Resource Servers.
      Finally, a Resource Server may use a token it had obtained from a
      Client and use it with another Resource Server that the Client
      interacts with.  A Resource Server, offering relatively
      unimportant application services, may attempt to use an access
      token obtained from a Client to access a high-value service, such
      as a payment service, on behalf of the Client using the same
      access token.

   We excluded one threat from the list, namely 'token repudiation'.
   Token repudiation refers to a property whereby a Resource Server is
   given an assurance that the Authorization Server cannot deny to have
   created a token for the Client.  We believe that such a property is

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   interesting but most deployments prefer to deal with the violation of
   this security property through business actions rather than by using

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4.  Threat Mitigation

   The purpose of this section is to discuss ways to mitigate the
   threats without taking the current working group status into

   A large range of threats can be mitigated by protecting the content
   of the token, using a digital signature or a keyed message digest.
   Alternatively, the content of the token could be passed by reference
   rather than by value (requiring a separate message exchange to
   resolve the reference to the token content).  To simplify the
   subsequent description we assume that the token itself is digitally
   signed by the Authorization Server and therefore cannot be modified.

   To deal with token redirect it is important for the Authorization
   Server to include the identifier of the intended recipient - the
   Resource Server.  A Resource Server must not be allowed to accept
   access tokens that are not meant for its consumption.

   To provide protection against token disclosure two approaches are
   possible, namely (a) not to include sensitive information inside the
   token or (b) to ensure confidentiality protection.  The latter
   approach requires at least the communication interaction between the
   Client and the Authorization Server as well as the interaction
   between the Client and the Resource Server to experience
   confidentiality protection.  As an example, Transport Layer Security
   with a ciphersuite that offers confidentiality protection has to be
   applied.  Encrypting the token content itself is another alternative.
   In our scenario the Authorization Server would, for example, encrypt
   the token content with a symmetric key shared with the Resource

   To deal with token reuse more choices are available.

4.1.  Confidentiality Protection

   In this approach confidentiality protection of the exchange is
   provided on the communication interfaces between the Client and the
   Resource Server, and between the Client and the Authorization Server.
   No eavesdropper on the wire is able to observe the token exchange.
   Consequently, a replay by a third party is not possible.  An
   Authorization Server wants to ensure that it only hands out tokens to
   Clients it has authenticated first and who are authorized.  For this
   purpose, authentication of the Client to the Authorization Server
   will be a requirement to ensure adequate protection against a range
   of attacks.  This is, however, true for the description in
   Section 4.2 and Section 4.3 as well.  Furthermore, the Client has to
   make sure it does not distribute the access token to entities other

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   than the intended the Resource Server.  For that purpose the Client
   will have to authenticate the Resource Server before transmitting the
   access token.

4.2.  Sender Constraint

   Instead of providing confidentiality protection the Authorization
   Server could also put the identifier of the Client into the protected
   token with the following semantic: 'This token is only valid when
   presented by a Client with the following identifer.'  When the access
   token is then presented to the Resource Server how does it know that
   it was provided by the Client?  It has to authenticate the Client!
   There are many choices for authenticating the Client to the Resource
   Server, for example by using client certificates in TLS [RFC5246], or
   pre-shared secrets within TLS [RFC4279].  The choice of the preferred
   authentication mechanism and credential type may depend on a number
   of factors, including

   o  security properties

   o  available infrastructure

   o  library support

   o  credential cost (financial)

   o  performance

   o  integration into the existing IT infrastructure

   o  operational overhead for configuration and distribution of

   This long list hints to the challenge of selecting at least one
   mandatory-to-implement Client authentication mechanism.

4.3.  Key Confirmation

   A variation of the mechanism of sender authentication described in
   Section 4.2 is to replace authentication with the proof-of-possession
   of a specific (session) key, i.e. key confirmation.  In this model
   the Resource Server would not authenticate the Client itself but
   would rather verify whether the Client knows the session key
   associated with a specific access token.  Examples of this approach
   can be found with the OAuth 1.0 MAC token [RFC5849], Kerberos
   [RFC4120] when utilizing the AP_REQ/AP_REP exchange (see also
   [I-D.hardjono-oauth-kerberos] for a comparison between Kerberos and
   OAuth), the OAuth 2.0 MAC token [I-D.ietf-oauth-v2-http-mac], and the

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   Holder-of-the-Key approach [I-D.tschofenig-oauth-hotk].

   To illustrate key confirmation the first examples borrow from
   Kerberos and use symmetric key cryptography.  Assume that the
   Authorization Server shares a long-term secret with the Resource
   Server, called K(Authorization Server-Resource Server).  This secret
   would be established between them in an initial registration phase.
   When the Client requests an access token the Authorization Server
   creates a fresh and unique session key Ks and places it into the
   token encrypted with the long term key K(Authorization Server-
   Resource Server).  Additionally, the Authorization Server attaches Ks
   to the response message to the Client (in addition to the access
   token itself) over a confidentiality protected channel.  When the
   Client sends a request to the Resource Server it has to use Ks to
   compute a keyed message digest for the request (in whatever form or
   whatever layer).  The Resource Server, when receiving the message,
   retrieves the access token, verifies it and extracts K(Authorization
   Server-Resource Server) to obtain Ks.  This key Ks is then used to
   verify the keyed message digest of the request message.

   Note that in this example one could imagine that the mechanism to
   protect the token itself is based on a symmetric key based mechanism
   to avoid any form of public key infrastructure but this aspect is not
   further elaborated in the scenario.

   A similar mechanism can also be designed using asymmetric
   cryptography.  When the Client requests an access token the
   Authorization Server creates an ephemeral public / privacy key pair
   (PK/SK) and places the public key PK into the protected token.  When
   the Authorization Server returns the access token to the Client it
   also provides the PK/SK key pair over a confidentiality protected
   channel.  When the Client sends a request to the Resource Server it
   has to use the privacy key SK to sign the request.  The Resource
   Server, when receiving the message, retrieves the access token,
   verifies it and extracts the public key PK.  It uses this ephemeral
   public key to verify the attached signature.

4.4.  Summary

   As a high level message, there are various ways how the threats can
   be mitigated and while the details of each solution is somewhat
   different they all ultimately accomplish the goal.

   The three approaches are:

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   Confidentiality Protection:

      The weak point with this approach, which is briefly described in
      Section 4.1, is that the Client has to be careful to whom it
      discloses the access token.  What can be done with the token
      entirely depends on what rights the token entitles the presenter
      and what constraints it contains.  A token could encode the
      identifier of the Client but there are scenarios where the Client
      is not authenticated to the Resource Server or where the
      identifier of the Client rather represents an application class
      rather than a single application instance.  As such, it is
      possible that certain deployments choose a rather liberal approach
      to security and that everyone who is in possession of the access
      token is granted access to the data.

   Sender Constraint:

      The weak point with this approach, which is briefly described in
      Section 4.2, is to setup the authentication infrastructure such
      that Clients can be authenticated towards Resource Servers.
      Additionally, Authorization Server must encode the identifier of
      the Client in the token for later verification by the Resource
      Server.  Depending on the chosen layer for providing Client-side
      authentication there may be additional challenges due Web server
      load balancing, lack of API access to identity information, etc.

   Key Confirmation:

      The weak point with this approach, see Section 4.3, is the
      increased complexity: a complete key distribution protocol has to
      be defined.

   In all cases above it has to be ensured that the Client is able to
   keep the credentials secret.

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5.  Requirements

   In an attempt to address the threats described in Section 3 the
   Bearer Token, which corresponds to the description in Section 4.1,
   was standardized and the work on a JSON-based token format has been
   started [I-D.ietf-oauth-json-web-token].  The required capability to
   protected the content of a JSON token using integrity and
   confidentiality mechanisms is currently work in progress in the IETF
   JOSE working group.

   Consequently, the purpose of the remaining document is to provide
   security that goes beyond the Bearer Token offered security

   Luckily this is not the first security protocol that has been
   designed.  In trying to seek guidance the authors found RFC 4962
   [RFC4962], which gives useful guidelines for designers of
   authentication and key management protocols.  While RFC 4962 was
   written with the AAA framework used for network access authentication
   in mind the offered suggestions are useful for the design of other
   key management systems as well.  The following requirements list
   applies OAuth 2.0 terminology to the requirements outlined in RFC

   These requirements include

   Cryptographic Algorithm Independent:

      The key management protocol MUST be cryptographic algorithm

   Strong, fresh session keys:

      Session keys MUST be strong and fresh.  Each session deserves an
      independent session key, i.e., one that is generated specifically
      for the intended use.  In context of OAuth this means that keying
      material is created in such a way that can only be used by the
      combination of a Client instance, protected resource, and
      authorization scope.

   Limit Key Scope:

      Following the principle of least privilege, parties MUST NOT have
      access to keying material that is not needed to perform their
      role.  Any protocol that is used to establish session keys MUST
      specify the scope for session keys, clearly identifying the
      parties to whom the session key is available.

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   Replay Detection Mechanism:

      The key management protocol exchanges MUST be replay protected.
      Replay protection allows a protocol message recipient to discard
      any message that was recorded during a previous legitimate
      dialogue and presented as though it belonged to the current

   Authenticate All Parties:

      Each party in the key management protocol MUST be authenticated to
      the other parties with whom they communicate.  Authentication
      mechanisms MUST maintain the confidentiality of any secret values
      used in the authentication process.  Secrets MUST NOT be sent to
      another party without confidentiality protection.


      Client and Resource Server authorization MUST be performed.  These
      entities MUST demonstrate possession of the appropriate keying
      material, without disclosing it.  Authorization is REQUIRED
      whenever a Client interacts with an Authorization Server.  The
      authorization checking prevents an elevation of privilege attack,
      and it ensures that an unauthorized authorized is detected.

   Keying Material Confidentiality and Integrity:

      While preserving algorithm independence, confidentiality and
      integrity of all keying material MUST be maintained.

   Confirm Cryptographic Algorithm Selection:

      The selection of the "best" cryptographic algorithms SHOULD be
      securely confirmed.  The mechanism SHOULD detect attempted roll-
      back attacks.

   Uniquely Named Keys:

      Key management proposals require a robust key naming scheme,
      particularly where key caching is supported.  The key name
      provides a way to refer to a key in a protocol so that it is clear
      to all parties which key is being referenced.  Objects that cannot
      be named cannot be managed.  All keys MUST be uniquely named, and
      the key name MUST NOT directly or indirectly disclose the keying

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   Prevent the Domino Effect:

      Compromise of a single Client MUST NOT compromise keying material
      held by any other Client within the system, including session keys
      and long-term keys.  Likewise, compromise of a single Resource
      Server MUST NOT compromise keying material held by any other
      Resource Server within the system.  In the context of a key
      hierarchy, this means that the compromise of one node in the key
      hierarchy must not disclose the information necessary to
      compromise other branches in the key hierarchy.  Obviously, the
      compromise of the root of the key hierarchy will compromise all of
      the keys; however, a compromise in one branch MUST NOT result in
      the compromise of other branches.  There are many implications of
      this requirement; however, two implications deserve highlighting.
      First, the scope of the keying material must be defined and
      understood by all parties that communicate with a party that holds
      that keying material.  Second, a party that holds keying material
      in a key hierarchy must not share that keying material with
      parties that are associated with other branches in the key

   Bind Key to its Context:

      Keying material MUST be bound to the appropriate context.  The
      context includes the following.

      *  The manner in which the keying material is expected to be used.

      *  The other parties that are expected to have access to the
         keying material.

      *  The expected lifetime of the keying material.  Lifetime of a
         child key SHOULD NOT be greater than the lifetime of its parent
         in the key hierarchy.

      Any party with legitimate access to keying material can determine
      its context.  In addition, the protocol MUST ensure that all
      parties with legitimate access to keying material have the same
      context for the keying material.  This requires that the parties
      are properly identified and authenticated, so that all of the
      parties that have access to the keying material can be determined.
      The context will include the Client and the Resource Server
      identities in more than one form.

   Authorization Restriction:

      If Client authorization is restricted, then the Client SHOULD be
      made aware of the restriction.

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   Client Identity Confidentiality:

      A Client has identity confidentiality when any party other than
      the Resource Server and the Authorization Server cannot
      sufficiently identify the Client within the anonymity set.  In
      comparison to anonymity and pseudonymity, identity confidentiality
      is concerned with eavesdroppers and intermediaries.  A key
      management protocol SHOULD provide this property.

   Resource Owner Identity Confidentiality:

      Resource servers SHOULD be prevented from knowing the real or
      pseudonymous identity of the Resource Owner, since the
      Authorization Server is the only entity involved in verifying the
      Resource Owner's identity.


      Resource Servers that collude can be prevented from using
      information related to the Resource Owner to track the individual.
      That is, two different Resource Servers can be prevented from
      determining that the same Resource Owner has authenticated to both
      of them.  This requires that each Authorization Server obtains
      different keying material as well as different access tokens with
      content that does not allow identification of the Resource Owner.

   AS-to-RS Relationship Anonymity:

      The Authorization Server can be prevented from knowing which
      Resource Servers a Resource Owner interacts with.  This requires
      avoiding direct communication between the Authorization Server and
      the Resource Server at the time when access to a protected
      resource by the Client is made.  Additionally, the Client must not
      provide information about the Resource Server in the access token
      request.  [QUESTION: Is this a desirable property given that it
      has other implications for security?]

   As an additional requirement a solution MUST enable support for
   channel bindings.  The concept of channel binding, as defined in
   [RFC5056], allows applications to establish that the two end-points
   of a secure channel at one network layer are the same as at a higher
   layer by binding authentication at the higher layer to the channel at
   the lower layer.

   Furthermore, there are performance concerns specifically with the
   usage of asymmetric cryptography.  As such, the requirement can be
   phrases as 'faster is better'.  [QUESTION: How are we trading the
   benefits of asymmetric cryptography against the performance impact?]

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   Finally, there are threats that relate to the experience of the
   software developer as well as operational policies.  For example, a
   frequently raised concern is the absent of verifying that the
   server's presented identity matches its reference identity so it can
   authenticate the communication endpoint and authorize it.  Verifying
   the server identity in TLS is discussed at length in [RFC6125].
   There are also various guesses about what application developers are
   able to implement correctly and easily and to what degree they can
   rely on third party libraries.[QUESTION: How do we reflect these
   requirements in the design?]

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6.  Security Considerations

   The main focus of this document is on security.

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7.  Next Steps

   From this description so far a few observations and next steps can be

   1.  Bearer Tokens are a viable solution for protecting against the
       threats described in Section 3.  Further standardization work on
       OAuth security mechanisms needs to provide additional security
       benefits on top of those provided by the bearer token solution.

   2.  The requirements listed in Section 5 aim to provide a starting
       point for a discussion on a security solution that provides
       additional security and privacy benefits for OAuth 2.0.

   3.  It is likely that implementers will find security solutions hard
       to implement and hard to configure right.  Additional guidance
       and the availability to libraries may help to improve security on
       the Internet for OAuth-based implementations.  Fundamentally,
       there is the question about a design that is based on symmetric
       vs. asymmetric cryptography.  Ideally, only a single solution
       should be developed (or a very small number) since the
       differences between different variations of such as protocol are

   4.  A standardized solution for the token format is needed to
       mitigate a number of attacks and this work is already ongoing
       under the name of JWT [I-D.ietf-oauth-json-web-token].

   To make progress with the above-mentioned items before the next IETF
   meeting in Atlanta I therefore suggest to (a) solicit for document
   reviews regarding the JWT document, and (b) progress the work on the
   extended OAuth security mechanism.  Regarding the latter aspect
   consider the following questions:


      Section 3 lists a few security threats.  Are these the threats you
      care about?  Which threats missing?


      The working group has expressed interest to work on an extended
      OAuth security mechanism.  Assuming that the group wants to
      develop a key distribution protocol (as described in Section 4.3)
      are the requirements listed in Section 5 complete?  Who is
      interested to develop early prototypes of support the standards

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8.  IANA Considerations

   This document does not require actions by IANA.

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9.  Acknowledgments

   The authors would like to thank the OAuth working group for their
   discussion input.  A group of regular OAuth participants met at the
   IETF #82 meeting in Vancouver to discuss this topic in preparation
   for the face-to-face meeting.  The participants were:

   o  John Bradley

   o  Brian Campbell

   o  Phil Hunt

   o  Leif Johansson

   o  Mike Jones

   o  Lucy Lynch

   o  Tony Nadalin

   o  Klaas Wierenga

   This document reuses content from [RFC4962] and the author would like
   thank Russ Housely and Bernard Aboba for their work on that document.

   Finally, I would like to thank Blaine Cook.  This document was
   derived from an earlier draft that Blaine and I wrote.

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10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", March 1997.

              Hardt, D., "The OAuth 2.0 Authorization Framework",
              draft-ietf-oauth-v2-31 (work in progress), August 2012.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              RFC 4949, August 2007.

              Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage",
              draft-ietf-oauth-v2-bearer-23 (work in progress),
              August 2012.

              Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", draft-ietf-oauth-json-web-token-03 (work in
              progress), July 2012.

10.2.  Informative References

   [RFC4962]  Housley, R. and B. Aboba, "Guidance for Authentication,
              Authorization, and Accounting (AAA) Key Management",
              BCP 132, RFC 4962, July 2007.

              Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols",
              draft-iab-privacy-considerations-03 (work in progress),
              July 2012.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279,
              December 2005.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

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              Hardjono, T., "OAuth 2.0 support for the Kerberos V5
              Authentication Protocol", draft-hardjono-oauth-kerberos-01
              (work in progress), December 2010.

   [RFC5849]  Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
              April 2010.

   [RFC5056]  Williams, N., "On the Use of Channel Bindings to Secure
              Channels", RFC 5056, November 2007.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

              Hammer-Lahav, E., "HTTP Authentication: MAC Access
              Authentication", draft-ietf-oauth-v2-http-mac-01 (work in
              progress), February 2012.

              Bradley, J., Hunt, P., Nadalin, A., and H. Tschofenig,
              "The OAuth 2.0 Authorization Framework: Holder-of-the-Key
              Token Usage", draft-tschofenig-oauth-hotk-01 (work in
              progress), July 2012.

              Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
              and E. Nabbus, "NIST Special Publication 800-63-1,
              INFORMATION SECURITY", December 2008.

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Authors' Addresses

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600

   Phone: +358 (50) 4871445
   Email: Hannes.Tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

   Phil Hunt
   Oracle Corporation

   Email: phil.hunt@yahoo.com

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