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RFC 7662

OAuth 2.0 Token Introspection

Pages: 17
Proposed Standard
Errata

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Internet Engineering Task Force (IETF)                    J. Richer, Ed.
Request for Comments: 7662                                  October 2015
Category: Standards Track
ISSN: 2070-1721


                     OAuth 2.0 Token Introspection

Abstract

This specification defines a method for a protected resource to query an OAuth 2.0 authorization server to determine the active state of an OAuth 2.0 token and to determine meta-information about this token. OAuth 2.0 deployments can use this method to convey information about the authorization context of the token from the authorization server to the protected resource. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7662. Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Notational Conventions . . . . . . . . . . . . . . . . . 3 1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 2. Introspection Endpoint . . . . . . . . . . . . . . . . . . . 3 2.1. Introspection Request . . . . . . . . . . . . . . . . . . 4 2.2. Introspection Response . . . . . . . . . . . . . . . . . 6 2.3. Error Response . . . . . . . . . . . . . . . . . . . . . 8 3. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 3.1. OAuth Token Introspection Response Registry . . . . . . . 9 3.1.1. Registration Template . . . . . . . . . . . . . . . . 10 3.1.2. Initial Registry Contents . . . . . . . . . . . . . . 10 4. Security Considerations . . . . . . . . . . . . . . . . . . . 12 5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 14 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 15 6.1. Normative References . . . . . . . . . . . . . . . . . . 15 6.2. Informative References . . . . . . . . . . . . . . . . . 16 Appendix A. Use with Proof-of-Possession Tokens . . . . . . . . 17 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 17 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17

1. Introduction

In OAuth 2.0 [RFC6749], the contents of tokens are opaque to clients. This means that the client does not need to know anything about the content or structure of the token itself, if there is any. However, there is still a large amount of metadata that may be attached to a token, such as its current validity, approved scopes, and information about the context in which the token was issued. These pieces of information are often vital to protected resources making authorization decisions based on the tokens being presented. Since OAuth 2.0 does not define a protocol for the resource server to learn meta-information about a token that it has received from an authorization server, several different approaches have been developed to bridge this gap. These include using structured token formats such as JWT [RFC7519] or proprietary inter-service communication mechanisms (such as shared databases and protected enterprise service buses) that convey token information. This specification defines a protocol that allows authorized protected resources to query the authorization server to determine the set of metadata for a given token that was presented to them by an OAuth 2.0 client. This metadata includes whether or not the token is currently active (or if it has expired or otherwise been revoked), what rights of access the token carries (usually conveyed through OAuth 2.0 scopes), and the authorization context in which the token was granted (including who authorized the token and which client it
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   was issued to).  Token introspection allows a protected resource to
   query this information regardless of whether or not it is carried in
   the token itself, allowing this method to be used along with or
   independently of structured token values.  Additionally, a protected
   resource can use the mechanism described in this specification to
   introspect the token in a particular authorization decision context
   and ascertain the relevant metadata about the token to make this
   authorization decision appropriately.

1.1. Notational Conventions

The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL', 'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'NOT RECOMMENDED', 'MAY', and 'OPTIONAL' in this document are to be interpreted as described in [RFC2119]. Unless otherwise noted, all the protocol parameter names and values are case sensitive.

1.2. Terminology

This section defines the terminology used by this specification. This section is a normative portion of this specification, imposing requirements upon implementations. This specification uses the terms "access token", "authorization endpoint", "authorization grant", "authorization server", "client", "client identifier", "protected resource", "refresh token", "resource owner", "resource server", and "token endpoint" defined by OAuth 2.0 [RFC6749], and the terms "claim names" and "claim values" defined by JSON Web Token (JWT) [RFC7519]. This specification defines the following terms: Token Introspection The act of inquiring about the current state of an OAuth 2.0 token through use of the network protocol defined in this document. Introspection Endpoint The OAuth 2.0 endpoint through which the token introspection operation is accomplished.

2. Introspection Endpoint

The introspection endpoint is an OAuth 2.0 endpoint that takes a parameter representing an OAuth 2.0 token and returns a JSON [RFC7159] document representing the meta information surrounding the token, including whether this token is currently active. The
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   definition of an active token is dependent upon the authorization
   server, but this is commonly a token that has been issued by this
   authorization server, is not expired, has not been revoked, and is
   valid for use at the protected resource making the introspection
   call.

   The introspection endpoint MUST be protected by a transport-layer
   security mechanism as described in Section 4.  The means by which the
   protected resource discovers the location of the introspection
   endpoint are outside the scope of this specification.

2.1. Introspection Request

The protected resource calls the introspection endpoint using an HTTP POST [RFC7231] request with parameters sent as "application/x-www-form-urlencoded" data as defined in [W3C.REC-html5-20141028]. The protected resource sends a parameter representing the token along with optional parameters representing additional context that is known by the protected resource to aid the authorization server in its response. token REQUIRED. The string value of the token. For access tokens, this is the "access_token" value returned from the token endpoint defined in OAuth 2.0 [RFC6749], Section 5.1. For refresh tokens, this is the "refresh_token" value returned from the token endpoint as defined in OAuth 2.0 [RFC6749], Section 5.1. Other token types are outside the scope of this specification. token_type_hint OPTIONAL. A hint about the type of the token submitted for introspection. The protected resource MAY pass this parameter to help the authorization server optimize the token lookup. If the server is unable to locate the token using the given hint, it MUST extend its search across all of its supported token types. An authorization server MAY ignore this parameter, particularly if it is able to detect the token type automatically. Values for this field are defined in the "OAuth Token Type Hints" registry defined in OAuth Token Revocation [RFC7009]. The introspection endpoint MAY accept other OPTIONAL parameters to provide further context to the query. For instance, an authorization server may desire to know the IP address of the client accessing the protected resource to determine if the correct client is likely to be presenting the token. The definition of this or any other parameters are outside the scope of this specification, to be defined by service documentation or extensions to this specification. If the authorization server is unable to determine the state of the token
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   without additional information, it SHOULD return an introspection
   response indicating the token is not active as described in
   Section 2.2.

   To prevent token scanning attacks, the endpoint MUST also require
   some form of authorization to access this endpoint, such as client
   authentication as described in OAuth 2.0 [RFC6749] or a separate
   OAuth 2.0 access token such as the bearer token described in OAuth
   2.0 Bearer Token Usage [RFC6750].  The methods of managing and
   validating these authentication credentials are out of scope of this
   specification.

   For example, the following shows a protected resource calling the
   token introspection endpoint to query about an OAuth 2.0 bearer
   token.  The protected resource is using a separate OAuth 2.0 bearer
   token to authorize this call.

   The following is a non-normative example request:

     POST /introspect HTTP/1.1
     Host: server.example.com
     Accept: application/json
     Content-Type: application/x-www-form-urlencoded
     Authorization: Bearer 23410913-abewfq.123483

     token=2YotnFZFEjr1zCsicMWpAA


   In this example, the protected resource uses a client identifier and
   client secret to authenticate itself to the introspection endpoint.
   The protected resource also sends a token type hint indicating that
   it is inquiring about an access token.

   The following is a non-normative example request:

     POST /introspect HTTP/1.1
     Host: server.example.com
     Accept: application/json
     Content-Type: application/x-www-form-urlencoded
     Authorization: Basic czZCaGRSa3F0MzpnWDFmQmF0M2JW

     token=mF_9.B5f-4.1JqM&token_type_hint=access_token
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2.2. Introspection Response

The server responds with a JSON object [RFC7159] in "application/ json" format with the following top-level members. active REQUIRED. Boolean indicator of whether or not the presented token is currently active. The specifics of a token's "active" state will vary depending on the implementation of the authorization server and the information it keeps about its tokens, but a "true" value return for the "active" property will generally indicate that a given token has been issued by this authorization server, has not been revoked by the resource owner, and is within its given time window of validity (e.g., after its issuance time and before its expiration time). See Section 4 for information on implementation of such checks. scope OPTIONAL. A JSON string containing a space-separated list of scopes associated with this token, in the format described in Section 3.3 of OAuth 2.0 [RFC6749]. client_id OPTIONAL. Client identifier for the OAuth 2.0 client that requested this token. username OPTIONAL. Human-readable identifier for the resource owner who authorized this token. token_type OPTIONAL. Type of the token as defined in Section 5.1 of OAuth 2.0 [RFC6749]. exp OPTIONAL. Integer timestamp, measured in the number of seconds since January 1 1970 UTC, indicating when this token will expire, as defined in JWT [RFC7519]. iat OPTIONAL. Integer timestamp, measured in the number of seconds since January 1 1970 UTC, indicating when this token was originally issued, as defined in JWT [RFC7519]. nbf OPTIONAL. Integer timestamp, measured in the number of seconds since January 1 1970 UTC, indicating when this token is not to be used before, as defined in JWT [RFC7519].
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   sub
      OPTIONAL.  Subject of the token, as defined in JWT [RFC7519].
      Usually a machine-readable identifier of the resource owner who
      authorized this token.

   aud
      OPTIONAL.  Service-specific string identifier or list of string
      identifiers representing the intended audience for this token, as
      defined in JWT [RFC7519].

   iss
      OPTIONAL.  String representing the issuer of this token, as
      defined in JWT [RFC7519].

   jti
      OPTIONAL.  String identifier for the token, as defined in JWT
      [RFC7519].

   Specific implementations MAY extend this structure with their own
   service-specific response names as top-level members of this JSON
   object.  Response names intended to be used across domains MUST be
   registered in the "OAuth Token Introspection Response" registry
   defined in Section 3.1.

   The authorization server MAY respond differently to different
   protected resources making the same request.  For instance, an
   authorization server MAY limit which scopes from a given token are
   returned for each protected resource to prevent a protected resource
   from learning more about the larger network than is necessary for its
   operation.

   The response MAY be cached by the protected resource to improve
   performance and reduce load on the introspection endpoint, but at the
   cost of liveness of the information used by the protected resource to
   make authorization decisions.  See Section 4 for more information
   regarding the trade off when the response is cached.
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   For example, the following response contains a set of information
   about an active token:

   The following is a non-normative example response:

     HTTP/1.1 200 OK
     Content-Type: application/json

     {
      "active": true,
      "client_id": "l238j323ds-23ij4",
      "username": "jdoe",
      "scope": "read write dolphin",
      "sub": "Z5O3upPC88QrAjx00dis",
      "aud": "https://protected.example.net/resource",
      "iss": "https://server.example.com/",
      "exp": 1419356238,
      "iat": 1419350238,
      "extension_field": "twenty-seven"
     }

   If the introspection call is properly authorized but the token is not
   active, does not exist on this server, or the protected resource is
   not allowed to introspect this particular token, then the
   authorization server MUST return an introspection response with the
   "active" field set to "false".  Note that to avoid disclosing too
   much of the authorization server's state to a third party, the
   authorization server SHOULD NOT include any additional information
   about an inactive token, including why the token is inactive.

   The following is a non-normative example response for a token that
   has been revoked or is otherwise invalid:

     HTTP/1.1 200 OK
     Content-Type: application/json

     {
      "active": false
     }

2.3. Error Response

If the protected resource uses OAuth 2.0 client credentials to authenticate to the introspection endpoint and its credentials are invalid, the authorization server responds with an HTTP 401 (Unauthorized) as described in Section 5.2 of OAuth 2.0 [RFC6749].
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   If the protected resource uses an OAuth 2.0 bearer token to authorize
   its call to the introspection endpoint and the token used for
   authorization does not contain sufficient privileges or is otherwise
   invalid for this request, the authorization server responds with an
   HTTP 401 code as described in Section 3 of OAuth 2.0 Bearer Token
   Usage [RFC6750].

   Note that a properly formed and authorized query for an inactive or
   otherwise invalid token (or a token the protected resource is not
   allowed to know about) is not considered an error response by this
   specification.  In these cases, the authorization server MUST instead
   respond with an introspection response with the "active" field set to
   "false" as described in Section 2.2.

3. IANA Considerations

3.1. OAuth Token Introspection Response Registry

This specification establishes the "OAuth Token Introspection Response" registry. OAuth registration client metadata names and descriptions are registered by Specification Required [RFC5226] after a two-week review period on the oauth-ext-review@ietf.org mailing list, on the advice of one or more Designated Experts. However, to allow for the allocation of names prior to publication, the Designated Expert(s) may approve registration once they are satisfied that such a specification will be published. Registration requests sent to the mailing list for review should use an appropriate subject (e.g., "Request to register OAuth Token Introspection Response name: example"). Within the review period, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful. IANA must only accept registry updates from the Designated Expert(s) and should direct all requests for registration to the review mailing list.
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3.1.1. Registration Template

Name: The name requested (e.g., "example"). This name is case sensitive. Names that match other registered names in a case insensitive manner SHOULD NOT be accepted. Names that match claims registered in the "JSON Web Token Claims" registry established by [RFC7519] SHOULD have comparable definitions and semantics. Description: Brief description of the metadata value (e.g., "Example description"). Change controller: For Standards Track RFCs, state "IESG". For other documents, give the name of the responsible party. Other details (e.g., postal address, email address, home page URI) may also be included. Specification document(s): Reference to the document(s) that specify the token endpoint authorization method, preferably including a URI that can be used to retrieve a copy of the document(s). An indication of the relevant sections may also be included but is not required.

3.1.2. Initial Registry Contents

The initial contents of the "OAuth Token Introspection Response" registry are as follows: o Name: "active" o Description: Token active status o Change Controller: IESG o Specification Document(s): Section 2.2 of RFC 7662 (this document). o Name: "username" o Description: User identifier of the resource owner o Change Controller: IESG o Specification Document(s): Section 2.2 of RFC 7662 (this document). o Name: "client_id" o Description: Client identifier of the client o Change Controller: IESG o Specification Document(s): Section 2.2 of RFC 7662 (this document).
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   o  Name: "scope"
   o  Description: Authorized scopes of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "token_type"
   o  Description: Type of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "exp"
   o  Description: Expiration timestamp of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "iat"
   o  Description: Issuance timestamp of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "nbf"
   o  Description: Timestamp before which the token is not valid
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "sub"
   o  Description: Subject of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "aud"
   o  Description: Audience of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

   o  Name: "iss"
   o  Description: Issuer of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).
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   o  Name: "jti"
   o  Description: Unique identifier of the token
   o  Change Controller: IESG
   o  Specification Document(s): Section 2.2 of RFC 7662 (this
      document).

4. Security Considerations

Since there are many different and valid ways to implement an OAuth 2.0 system, there are consequently many ways for an authorization server to determine whether or not a token is currently "active". However, since resource servers using token introspection rely on the authorization server to determine the state of a token, the authorization server MUST perform all applicable checks against a token's state. For instance, these tests include the following: o If the token can expire, the authorization server MUST determine whether or not the token has expired. o If the token can be issued before it is able to be used, the authorization server MUST determine whether or not a token's valid period has started yet. o If the token can be revoked after it was issued, the authorization server MUST determine whether or not such a revocation has taken place. o If the token has been signed, the authorization server MUST validate the signature. o If the token can be used only at certain resource servers, the authorization server MUST determine whether or not the token can be used at the resource server making the introspection call. If an authorization server fails to perform any applicable check, the resource server could make an erroneous security decision based on that response. Note that not all of these checks will be applicable to all OAuth 2.0 deployments and it is up to the authorization server to determine which of these checks (and any other checks) apply. If left unprotected and un-throttled, the introspection endpoint could present a means for an attacker to poll a series of possible token values, fishing for a valid token. To prevent this, the authorization server MUST require authentication of protected resources that need to access the introspection endpoint and SHOULD require protected resources to be specifically authorized to call the introspection endpoint. The specifics of such authentication credentials are out of scope of this specification, but commonly these credentials could take the form of any valid client authentication mechanism used with the token endpoint, an OAuth 2.0 access token, or other HTTP authorization or authentication mechanism. A single piece of software acting as both a client and a
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   protected resource MAY reuse the same credentials between the token
   endpoint and the introspection endpoint, though doing so potentially
   conflates the activities of the client and protected resource
   portions of the software and the authorization server MAY require
   separate credentials for each mode.

   Since the introspection endpoint takes in OAuth 2.0 tokens as
   parameters and responds with information used to make authorization
   decisions, the server MUST support Transport Layer Security (TLS) 1.2
   [RFC5246] and MAY support additional transport-layer mechanisms
   meeting its security requirements.  When using TLS, the client or
   protected resource MUST perform a TLS/SSL server certificate check,
   as specified in [RFC6125].  Implementation security considerations
   can be found in Recommendations for Secure Use of TLS and DTLS
   [BCP195].

   To prevent the values of access tokens from leaking into server-side
   logs via query parameters, an authorization server offering token
   introspection MAY disallow the use of HTTP GET on the introspection
   endpoint and instead require the HTTP POST method to be used at the
   introspection endpoint.

   To avoid disclosing the internal state of the authorization server,
   an introspection response for an inactive token SHOULD NOT contain
   any additional claims beyond the required "active" claim (with its
   value set to "false").

   Since a protected resource MAY cache the response of the
   introspection endpoint, designers of an OAuth 2.0 system using this
   protocol MUST consider the performance and security trade-offs
   inherent in caching security information such as this.  A less
   aggressive cache with a short timeout will provide the protected
   resource with more up-to-date information (due to it needing to query
   the introspection endpoint more often) at the cost of increased
   network traffic and load on the introspection endpoint.  A more
   aggressive cache with a longer duration will minimize network traffic
   and load on the introspection endpoint, but at the risk of stale
   information about the token.  For example, the token may be revoked
   while the protected resource is relying on the value of the cached
   response to make authorization decisions.  This creates a window
   during which a revoked token could be used at the protected resource.
   Consequently, an acceptable cache validity duration needs to be
   carefully considered given the concerns and sensitivities of the
   protected resource being accessed and the likelihood of a token being
   revoked or invalidated in the interim period.  Highly sensitive
   environments can opt to disable caching entirely on the protected
   resource to eliminate the risk of stale cached information entirely,
   again at the cost of increased network traffic and server load.  If
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   the response contains the "exp" parameter (expiration), the response
   MUST NOT be cached beyond the time indicated therein.

   An authorization server offering token introspection must be able to
   understand the token values being presented to it during this call.
   The exact means by which this happens is an implementation detail and
   is outside the scope of this specification.  For unstructured tokens,
   this could take the form of a simple server-side database query
   against a data store containing the context information for the
   token.  For structured tokens, this could take the form of the server
   parsing the token, validating its signature or other protection
   mechanisms, and returning the information contained in the token back
   to the protected resource (allowing the protected resource to be
   unaware of the token's contents, much like the client).  Note that
   for tokens carrying encrypted information that is needed during the
   introspection process, the authorization server must be able to
   decrypt and validate the token to access this information.  Also note
   that in cases where the authorization server stores no information
   about the token and has no means of accessing information about the
   token by parsing the token itself, it cannot likely offer an
   introspection service.

5. Privacy Considerations

The introspection response may contain privacy-sensitive information such as user identifiers for resource owners. When this is the case, measures MUST be taken to prevent disclosure of this information to unintended parties. One method is to transmit user identifiers as opaque service-specific strings, potentially returning different identifiers to each protected resource. If the protected resource sends additional information about the client's request to the authorization server (such as the client's IP address) using an extension of this specification, such information could have additional privacy considerations that the extension should detail. However, the nature and implications of such extensions are outside the scope of this specification. Omitting privacy-sensitive information from an introspection response is the simplest way of minimizing privacy issues.
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6. References

6.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, August 2008, <http://www.rfc-editor.org/info/rfc5246>. [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, DOI 10.17487/RFC6125, March 2011, <http://www.rfc-editor.org/info/rfc6125>. [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, <http://www.rfc-editor.org/info/rfc6749>. [RFC6750] Jones, M. and D. Hardt, "The OAuth 2.0 Authorization Framework: Bearer Token Usage", RFC 6750, DOI 10.17487/RFC6750, October 2012, <http://www.rfc-editor.org/info/rfc6750>. [RFC7009] Lodderstedt, T., Ed., Dronia, S., and M. Scurtescu, "OAuth 2.0 Token Revocation", RFC 7009, DOI 10.17487/RFC7009, August 2013, <http://www.rfc-editor.org/info/rfc7009>. [RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March 2014, <http://www.rfc-editor.org/info/rfc7159>. [RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/info/rfc7231>.
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   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <http://www.rfc-editor.org/info/rfc7519>.

   [W3C.REC-html5-20141028]
              Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., 0'Connor, E., and S. Pfeiffer, "HTML5", World
              Wide Web Consortium Recommendation
              REC-html5-20141028, October 2014,
              <http://www.w3.org/TR/2014/REC-html5-20141028>.

6.2. Informative References

[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, May 2015, <http://www.rfc-editor.org/info/bcp195>.
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Appendix A. Use with Proof-of-Possession Tokens

With bearer tokens such as those defined by OAuth 2.0 Bearer Token Usage [RFC6750], the protected resource will have in its possession the entire secret portion of the token for submission to the introspection service. However, for proof-of-possession style tokens, the protected resource will have only a token identifier used during the request, along with the cryptographic signature on the request. To validate the signature on the request, the protected resource could be able to submit the token identifier to the authorization server's introspection endpoint to obtain the necessary key information needed for that token. The details of this usage are outside the scope of this specification and will be defined in an extension to this specification in concert with the definition of proof-of-possession tokens.

Acknowledgements

Thanks to the OAuth Working Group and the User Managed Access Working Group for feedback and review of this document, and to the various implementors of both the client and server components of this specification. In particular, the author would like to thank Amanda Anganes, John Bradley, Thomas Broyer, Brian Campbell, George Fletcher, Paul Freemantle, Thomas Hardjono, Eve Maler, Josh Mandel, Steve Moore, Mike Schwartz, Prabath Siriwardena, Sarah Squire, and Hannes Tschofennig.

Author's Address

Justin Richer (editor) Email: ietf@justin.richer.org