[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01

OAuth                                                      H. Tschofenig
Internet-Draft                                    Nokia Siemens Networks
Intended status: Informational                                   P. Hunt
Expires: June 19, 2013                                Oracle Corporation
                                                       December 16, 2012


             OAuth 2.0 Security: Going Beyond Bearer Tokens
                 draft-tschofenig-oauth-security-01.txt

Abstract

   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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

Copyright Notice

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





Tschofenig & Hunt        Expires June 19, 2013                  [Page 1]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Security and Privacy Threats . . . . . . . . . . . . . . . . .  3
   4.  Threat Mitigation  . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Confidentiality Protection . . . . . . . . . . . . . . . .  5
     4.2.  Sender Constraint  . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Key Confirmation . . . . . . . . . . . . . . . . . . . . .  6
     4.4.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     6.1.  Access to an 'Unprotected' Resource  . . . . . . . . . . . 12
     6.2.  Offering Application Layer End-to-End Security . . . . . . 13
     6.3.  Preventing Access Token Re-Use by the Resource Server  . . 13
     6.4.  TLS Channel Binding Support  . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  Next Steps . . . . . . . . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 16
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17

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 [RFC6749] generalized the protocol and
   the Bearer Token security specification [RFC6750] 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.






Tschofenig & Hunt        Expires June 19, 2013                  [Page 2]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012



        +--------+                               +---------------+
        |        |--(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.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 [RFC6749].  In particular,
   the terms Client, Resource Server, Authorization Server, and Access
   Token are used.

3.  Security and Privacy Threats






Tschofenig & Hunt        Expires June 19, 2013                  [Page 3]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   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
   itself.

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

4.  Threat Mitigation


Tschofenig & Hunt        Expires June 19, 2013                  [Page 4]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


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

   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
   Server.

   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











Tschofenig & Hunt        Expires June 19, 2013                  [Page 5]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012

   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 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
      credentials

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



Tschofenig & Hunt        Expires June 19, 2013                  [Page 6]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   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:

   Confidentiality Protection:










Tschofenig & Hunt        Expires June 19, 2013                  [Page 7]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012

                               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.

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
   protection.








Tschofenig & Hunt        Expires June 19, 2013                  [Page 8]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   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
   4962.

   These requirements include

   Cryptographic Algorithm Independent:

                                        The key management protocol MUST
      be cryptographic algorithm independent.

   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.

   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 dialogue.

   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.

   Authorization:


Tschofenig & Hunt        Expires June 19, 2013                  [Page 9]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


                  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 material.

   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 hierarchy.

   Bind Key to its Context:


Tschofenig & Hunt        Expires June 19, 2013                 [Page 10]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012

                            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.

   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.

   Collusion:

              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:

Tschofenig & Hunt        Expires June 19, 2013                 [Page 11]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


                                    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?]

   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?]

6.  Use Cases

   This section lists use cases that provide additional requirements and
   constrain the solution space.

6.1.  Access to an 'Unprotected' Resource

   This use case is for a web client that needs to access a resource
   where no integrity and confidentiality protection is provided for the
   exchange of data using TLS following the OAuth-based request.  In
   accessing the resource, the request, which includes the access token,
   must be protected against replay, and modification.









Tschofenig & Hunt        Expires June 19, 2013                 [Page 12]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   While it is possible to utilize bearer tokens in this scenario, as
   described in  [RFC6750], with TLS protection when the request to the
   protected resource is made there may be the desire to avoid using TLS
   between the client and the resource server at all.  In such a case
   the bearer token approach is not possible since it relies on TLS for
   ensuring integrity and confidentiality protection of the access token
   exchange since otherwise replay attacks are possible: First, an
   eavesdropper may steal an access token and represent it at a
   different resource server.  Second, an eavesdropper may steal an
   access token and replay it against the same resource server at a
   later point in time.  In both cases, if the attack is successful, the
   adversary gets access to the resource owners data or may perform an
   operation selected by the adversary (e.g., sending a message).  Note
   that the adversary may obtain the access token (if the
   recommendations in [RFC6749] and [RFC6750] are not followed) using a
   number of ways, including eavesdropping the communication on the
   wireless link.

   Consequently, the important assumption in this use case is that a
   resource server does not have TLS support and the security solution
   should work in such a scenario.  Furthermore, it may not be necessary
   to provide authentication of the resource server towards the client.

6.2.  Offering Application Layer End-to-End Security

   In Web deployments resource servers are often placed behind load
   balancers.  Note that the load balancers are deployed by the same
   organization that operates the resource servers.  These load
   balancers may terminate Transport Layer Security (TLS) and the
   resulting HTTP traffic may be transmitted in clear from the load
   balancer to the resource server.  With application layer security
   independent of the underlying TLS security it is possible to allow
   application servers to perform cryptographic verification on an end-
   to-end basis.

   The key aspect in this use case is therefore to offer end-to-end
   security in the presence of load balancers via application layer
   security.

6.3.  Preventing Access Token Re-Use by the Resource Server

   Imagine a scenario where a resource server that receives a valid
   access token re-uses it with other resource server.  The reason for
   re-use may be malicious or may well be legimiate.  In a legimiate use
   case consider a case where the resource server needs to consult third
   party resource servers to complete the requested operation.  In both
   cases it may be assumed that the scope of the access token is
   sufficiently large that it allows such a re-use.  For example,
   imagine a case where a company operates email services as well as
   picture sharing services and that company had decided to issue access
   tokens with a scope that allows access to both services.



Tschofenig & Hunt        Expires June 19, 2013                 [Page 13]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   With this use case the desire is to prevent such access token re-use.
   This also implies that the legimiate use cases require additional
   enhancements for request chaining.

6.4.  TLS Channel Binding Support

   In this use case we consider the scenario where an OAuth 2.0 request
   to a protected resource is secured using TLS but the client and the
   resource server demand that the underlying TLS exchange is bound to
   additional application layer security to prevent cases where the TLS
   connection is terminated at a load balancer or a TLS proxy is used
   that splits the TLS connection into two separate connections.

   In this use case additional information is conveyed to the resource
   server to ensure that no entity entity has tampered with the TLS
   connection.

7.  Security Considerations

   The main focus of this document is on security.

8.  Next Steps

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

   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
       minor.

   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].






Tschofenig & Hunt        Expires June 19, 2013                 [Page 14]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   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:

   Threats:

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

   Requirements:

                 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
      development?

9.  IANA Considerations

   This document does not require actions by IANA.

10.  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.


Tschofenig & Hunt        Expires June 19, 2013                 [Page 15]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


11.  References

11.1.  Normative References

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

   [RFC6749]  Hardt, D., "The OAuth 2.0 Authorization Framework", RFC
              6749, October 2012.

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

   [I-D.ietf-oauth-json-web-token]
              Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", Internet-Draft draft-ietf-oauth-json-web-token-05,
              November 2012.

11.2.  Informative References

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

   [I-D.iab-privacy-considerations]
              Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", Internet-Draft
              draft-iab-privacy-considerations-03, 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.

   [I-D.hardjono-oauth-kerberos]
              Hardjono, T., "OAuth 2.0 support for the Kerberos V5
              Authentication Protocol", Internet-Draft draft-hardjono-
              oauth-kerberos-01, 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.



Tschofenig & Hunt        Expires June 19, 2013                 [Page 16]

Internet-Draft        Enhancing OAuth 2.0 Security         December 2012


   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750, October 2012.

   [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.

   [I-D.ietf-oauth-v2-http-mac]
              Richer, J., Mills, W., and H. Tschofenig, "OAuth 2.0
              Message Authentication Code (MAC) Tokens", Internet-Draft
              draft-ietf-oauth-v2-http-mac-02, November 2012.

   [I-D.tschofenig-oauth-hotk]
              Bradley, J., Hunt, P., Nadalin, A., and H. Tschofenig,
              "The OAuth 2.0 Authorization Framework: Holder-of-the-Key
              Token Usage", Internet-Draft draft-tschofenig-oauth-
              hotk-01, July 2012.

   [NIST800-63]
              Burr, W., Dodson, D., Perlner, R., Polk, T., Gupta, S.,
              and E. Nabbus, "NIST Special Publication 800-63-1,
              INFORMATION SECURITY", December 2008.

Authors' Addresses

   Hannes Tschofenig
   Nokia Siemens Networks
   Linnoitustie 6
   Espoo  02600
   Finland

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


   Phil Hunt
   Oracle Corporation

   Email: phil.hunt@yahoo.com











Tschofenig & Hunt        Expires June 19, 2013                 [Page 17]


Html markup produced by rfcmarkup 1.107, available from http://tools.ietf.org/tools/rfcmarkup/