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Network Working Group                                      M. Jones, Ed.
Internet-Draft                                                 Microsoft
Intended status: Standards Track                                D. Hardt
Expires: June 4, 2011                                        independent
                                                             D. Recordon
                                                                Facebook
                                                        December 1, 2010


                 The OAuth 2.0 Protocol: Bearer Tokens
                     draft-ietf-oauth-v2-bearer-01

Abstract

   This specification describes how to use bearer tokens when accessing
   OAuth 2.0 protected resources.

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 4, 2011.

Copyright Notice

   Copyright (c) 2010 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Notational Conventions . . . . . . . . . . . . . . . . . .  3
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Authenticated Requests . . . . . . . . . . . . . . . . . . . .  4
     2.1.  The Authorization Request Header Field . . . . . . . . . .  5
     2.2.  Form-Encoded Body Parameter  . . . . . . . . . . . . . . .  5
     2.3.  URI Query Parameter  . . . . . . . . . . . . . . . . . . .  6
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  7
     3.1.  Security Threats . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Threat Mitigation  . . . . . . . . . . . . . . . . . . . .  7
     3.3.  Summary of Recommendations . . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   5.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     5.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Acknowledgements  . . . . . . . . . . . . . . . . . . 10
   Appendix B.  Document History  . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11






























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1.  Introduction

   OAuth enables clients to access protected resources by obtaining an
   access token, which is defined in [OAuth2] as "a string representing
   an access authorization issued to the client", rather than using the
   resource owner's credentials.

   Tokens are issued to clients by an authorization server with the
   approval of the resource owner.  The client uses the access token to
   access the protected resources hosted by the resource server.  This
   specification describes how to make protected resource requests by
   treating an OAuth access token as a bearer token.

   This specification defines the use of bearer tokens with OAuth over
   HTTP [RFC2616] using TLS [RFC2818].  Other specifications may extend
   it for use with other transport protocols.

1.1.  Notational Conventions

   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 [RFC2119].

   This document uses the Augmented Backus-Naur Form (ABNF) notation of
   [I-D.ietf-httpbis-p1-messaging].  Additionally, the following rules
   are included from [RFC2617]: auth-param; and from
   [I-D.ietf-httpbis-p1-messaging]: RWS.

   Unless otherwise noted, all the protocol parameter names and values
   are case sensitive.

1.2.  Terminology

   All terms are as defined in The OAuth 2.0 Protocol [OAuth2].

1.3.  Overview

   OAuth provides a method for clients to access a protected resource on
   behalf of a resource owner.  Before a client can access a protected
   resource, it must first obtain authorization (access grant) from the
   resource owner, then exchange the access grant for an access token
   (representing the grant's scope, duration, and other attributes).
   The client accesses the protected resource by presenting the access
   token to the resource server.

   The access token provides an abstraction layer, replacing different
   authorization constructs (e.g. username and password, assertion) for
   a single token understood by the resource server.  This abstraction



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   enables issuing access tokens valid for a short time period, as well
   as removing the resource server's need to understand a wide range of
   authentication schemes.

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

                     Figure 1: Abstract Protocol Flow

   The abstract flow illustrated in Figure 1 describes the overall OAuth
   2.0 protocol architecture.  The following steps are specified within
   this document:

      E) The client makes a protected resource request to the resource
      server by presenting the access token.

      F) The resource server validates the access token, and if valid,
      serves the request.


2.  Authenticated Requests

   Clients make authenticated token requests using the "Authorization"
   request header field.  Resource servers MUST accept authenticated
   requests using the "OAuth2" HTTP authentication scheme as described
   in Section 2.1, and MAY support additional methods.

   Alternatively, clients MAY attempt to include the access token in the
   HTTP body when using the "application/x-www-form-urlencoded" content
   type as described in Section 2.2 or using the HTTP request URI in the
   query component as described in Section 2.3.  Resource servers MAY
   support these alternative methods.




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   Clients SHOULD only use the request body or URI when the
   "Authorization" request header field is not available, and MUST NOT
   use more than one method to transport the token in each request.
   Because of the Security Considerations (Section 3) associated with
   the URI method, it SHOULD only be used if no other method is
   feasible.

2.1.  The Authorization Request Header Field

   The "Authorization" request header field is used by clients to make
   authenticated token requests.  The client uses the "OAuth2"
   authentication scheme to include the access token in the request.

   For example:

   GET /resource HTTP/1.1
   Host: server.example.com
   Authorization: OAuth2 vF9dft4qmT

   The "Authorization" header field uses the framework defined by
   [RFC2617] as follows:

   credentials    = "OAuth2" RWS access-token [ RWS 1#auth-param ]
   access-token   = 1*( quoted-char / <"> )

   quoted-char    =   "!" / "#" / "$" / "%" / "&" / "'" / "("
                    / ")" / "*" / "+" / "-" / "." / "/" / DIGIT
                    / ":" / "<" / "=" / ">" / "?" / "@" / ALPHA
                    / "[" / "]" / "^" / "_" / "`" / "{" / "|"
                    / "}" / "~" / "\" / "," / ";"

2.2.  Form-Encoded Body Parameter

   When including the access token in the HTTP request entity-body, the
   client adds the access token to the request body using the
   "oauth_token" parameter.  The client can use this method only if the
   following REQUIRED conditions are met:

   o  The HTTP request entity-body is single-part.

   o  The entity-body follows the encoding requirements of the
      "application/x-www-form-urlencoded" content-type as defined by
      [W3C.REC-html401-19991224].

   o  The HTTP request entity-header includes the "Content-Type" header
      field set to "application/x-www-form-urlencoded".





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   o  The HTTP request method is one for which a body is permitted to be
      present in the request.  In particular, this means that the "GET"
      method MAY NOT be used.

   The entity-body can include other request-specific parameters, in
   which case, the "oauth_token" parameters SHOULD be appended following
   the request-specific parameters, properly separated by an "&"
   character (ASCII code 38).

   For example, the client makes the following HTTP request using
   transport-layer security:

   POST /resource HTTP/1.1
   Host: server.example.com
   Content-Type: application/x-www-form-urlencoded

   oauth_token=vF9dft4qmT

   The "application/x-www-form-urlencoded" method should typically only
   be used in application contexts where participating browsers do not
   have access to the "Authorization" request header field.

2.3.  URI Query Parameter

   When including the access token in the HTTP request URI, the client
   adds the access token to the request URI query component as defined
   by [RFC3986] using the "oauth_token" parameter.

   For example, the client makes the following HTTP request using
   transport-layer security:

   GET /resource?oauth_token=vF9dft4qmT HTTP/1.1
   Host: server.example.com

   The HTTP request URI query can include other request-specific
   parameters, in which case, the "oauth_token" parameters SHOULD be
   appended following the request-specific parameters, properly
   separated by an "&" character (ASCII code 38).

   For example:

   http://example.com/resource?x=y&oauth_token=vF9dft4qmT

   Because of the Security Considerations (Section 3) associated with
   the URI method, it SHOULD only be used if no other method is
   feasible.





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3.  Security Considerations

   This section describes the relevant security threats regarding token
   handling when using bearer tokens and describes how to mitigate these
   threats.

3.1.  Security 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].  Since this document builds
   on the OAuth 2.0 specification, we exclude a discussion of threats
   that are described there or in related documents.

   Token manufacture/modification:  An attacker may generate a bogus
      token or modify the token contents (such as the authentication or
      attribute statements) of an existing token, causing the resource
      server to grant inappropriate access to the client.  For example,
      an attacker may modify the token to extend the validity period; a
      malicious client may modify the assertion to gain access to
      information that they should not be able to view.

   Token disclosure:  Tokens may contain authentication and attribute
      statements that include sensitive information.

   Token redirect:  An attacker uses the token generated for consumption
      by 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 that resource server in the past.

3.2.  Threat Mitigation

   A large range of threats can be mitigated by protecting the contents
   of the token by using a digital signature or a keyed message digest.
   Alternatively, the contents of the token could be passed by reference
   rather than by value (requiring a separate message exchange to
   resolve the reference to the token contents).

   This document does not specify the encoding or the contents of the
   token; hence detailed recommendations for token integrity protection
   are outside the scope of this document.  We assume that the token
   integrity protection is sufficient to prevent the token from being
   modified.

   To deal with token redirect, it is important for the authorization
   server to include the identity of the intended recipients, namely a
   single resource server (or a list of resource servers).  Restricting



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   the use of the token to a specific scope is also recommended.

   To provide protection against token disclosure, confidentiality
   protection is applied via TLS with a ciphersuite that offers
   confidentiality protection.  This requires that the communication
   interaction between the client and the authorization server, as well
   as the interaction between the client and the resource server,
   utilize confidentiality protection.  Encrypting the token contents is
   another alternative.  Since TLS is mandatory to implement and to use
   with this specification, it is the preferred approach for preventing
   token disclosure via the communication channel.  For those rare cases
   where the client is prevented from observing the contents of the
   token, token encryption has to be applied in addition to the usage of
   TLS protection.

   To deal with token reuse, the following recommendations are made:
   First, the lifetime of the token has to be limited by putting a
   validity time field inside the protected part of the token.  Note
   that using short-lived (one hour or less) tokens significantly
   reduces the impact of one of them being leaked.  Second,
   confidentiality protection of the exchanges between the client and
   the authorization server and between the client and the resource
   server MUST be applied.  As a consequence, no eavesdropper along the
   communication path is able to observe the token exchange.
   Consequently, such an on-path adversary cannot replay the token.
   Furthermore, the resource server MUST ensure that it only hands out
   tokens to clients it has authenticated first and authorized.  For
   this purpose, the client MUST be authenticated and authorized by the
   resource server.  The authorization server MUST also receive a
   confirmation (the consent of the resource owner) prior to providing a
   token to the client.  Furthermore, when presenting the token to a
   resource server, the client MUST verify the identity of that resource
   server.  Note that the client MUST validate the TLS certificate chain
   when making these requests to protected resources.  Presenting the
   token to an unauthenticated and unauthorized resource server or
   failing to validate the certificate chain will allow adversaries to
   steal the token and gain unauthorized access to protected resources.

3.3.  Summary of Recommendations

   Safeguard bearer tokens  Client implementations MUST ensure that
      bearer tokens are not leaked to unintended parties, as they will
      be able to use them to gain access to protected resources.  This
      is the primary security consideration when using bearer tokens
      with OAuth and underlies all the more specific recommendations
      that follow.





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   Validate SSL certificate chains  The client must validate the TLS
      certificate chain when making requests to protected resources.
      Failing to do so may enable DNS hijacking attacks to steal the
      token and gain unintended access.

   Always use TLS (https)  Clients MUST always use TLS (https) when
      making requests with bearer tokens.  Failing to do so exposes the
      token to numerous attacks that could give attackers unintended
      access.

   Don't store bearer tokens in cookies  As cookies are generally sent
      in the clear, implementations MUST NOT store bearer tokens within
      them.

   Issue short-lived bearer tokens  Using short-lived (one hour or less)
      bearer tokens can reduce the impact of one of them being leaked.
      The User-Agent flow should only issue short lived access tokens.

   Don't pass bearer tokens in page URLs  Browsers may not adequately
      secure URLs in the browser history.  If bearer tokens are passed
      in page URLs (typically as query string parameters), attackers
      might be able to steal them from the history data.  Instead, pass
      browser tokens in message bodies for which confidentiality
      measures are taken.


4.  IANA Considerations

   This document neither establishes new IANA registries nor adds new
   values to existing registries.


5.  References

5.1.  Normative References

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
              Masinter, L., Leach, P., Berners-Lee, T., and J. Reschke,
              "HTTP/1.1, part 1: URIs, Connections, and Message
              Parsing", draft-ietf-httpbis-p1-messaging-09 (work in
              progress), March 2010.

   [OAuth2]   Hammer-Lahav, E., Ed., Recordon, D., and D. Hardt, "The
              OAuth 2.0 Protocol", 2010.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.



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   [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.

   [RFC2617]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
              Leach, P., Luotonen, A., and L. Stewart, "HTTP
              Authentication: Basic and Digest Access Authentication",
              RFC 2617, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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

   [W3C.REC-html401-19991224]
              Raggett, D., Hors, A., and I. Jacobs, "HTML 4.01
              Specification", World Wide Web Consortium
              Recommendation REC-html401-19991224, December 1999,
              <http://www.w3.org/TR/1999/REC-html401-19991224>.

5.2.  Informative References

   [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.


Appendix A.  Acknowledgements

   The following people contributed to preliminary versions of this
   document: Blaine Cook (BT), Brian Eaton (Google), Yaron Goland
   (Microsoft), Brent Goldman (Facebook), Raffi Krikorian (Twitter),
   Luke Shepard (Facebook), and Allen Tom (Yahoo!).  The content and
   concepts within are a product of the OAuth community, WRAP community,
   and the OAuth Working Group.

   The OAuth Working Group has dozens of very active contributors who
   proposed ideas and wording for this document, including: [[ If your
   name is missing or you think someone should be added here, please
   send Mike Jones a note - don't be shy! ]]

   Michael Adams, Andrew Arnott, Dirk Balfanz, Brian Campbell, Leah
   Culver, Bill de hOra, Brian Ellin, Igor Faynberg, George Fletcher,



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   Tim Freeman, Evan Gilbert, Justin Hart, John Kemp, Eran Hammer-Lahav,
   Chasen Le Hara, Michael B. Jones, Torsten Lodderstedt, Eve Maler,
   James Manger, Laurence Miao, Chuck Mortimore, Justin Richer, Peter
   Saint-Andre, Nat Sakimura, Rob Sayre, Marius Scurtescu, Naitik Shah,
   Justin Smith, Jeremy Suriel, Christian Stuebner, Paul Tarjan, and
   Franklin Tse.


Appendix B.  Document History

   [[ to be removed by RFC editor before publication as an RFC ]]

   -01

   o  First public draft, which incorporates feedback received on -00
      including enhanced Security Considerations content.  This version
      is intended to accompany OAuth 2.0 draft 11.

   -00

   o  Initial draft based on preliminary version of OAuth 2.0 draft 11.


Authors' Addresses

   Michael B. Jones (editor)
   Microsoft

   Email: mbj@microsoft.com
   URI:   http://self-issued.info/


   Dick Hardt
   independent

   Email: dick.hardt@gmail.com
   URI:   http://dickhardt.org/


   David Recordon
   Facebook

   Email: davidrecordon@facebook.com
   URI:   http://www.davidrecordon.com/







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