JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens
draft-ietf-oauth-access-token-jwt-09

Document Type Active Internet-Draft (oauth WG)
Last updated 2020-09-18
Replaces draft-bertocci-oauth-access-token-jwt
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OAuth Working Group                                          V. Bertocci
Internet-Draft                                                     Auth0
Intended status: Standards Track                      September 18, 2020
Expires: March 22, 2021

        JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens
                  draft-ietf-oauth-access-token-jwt-09

Abstract

   This specification defines a profile for issuing OAuth 2.0 access
   tokens in JSON web token (JWT) format.  Authorization servers and
   resource servers from different vendors can leverage this profile to
   issue and consume access tokens in interoperable manner.

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 https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   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 March 22, 2021.

Copyright Notice

   Copyright (c) 2020 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
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   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  JWT Access Token Header and Data Structure  . . . . . . . . .   4
     2.1.  Header  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Data Structure  . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Authentication Information Claims . . . . . . . . . .   5
       2.2.2.  Identity Claims . . . . . . . . . . . . . . . . . . .   5
       2.2.3.  Authorization Claims  . . . . . . . . . . . . . . . .   6
         2.2.3.1.  Claims for Authorization Outside of Delegation
                   Scenarios . . . . . . . . . . . . . . . . . . . .   6
   3.  Requesting a JWT Access Token . . . . . . . . . . . . . . . .   6
   4.  Validating JWT Access Tokens  . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Media Type Registration . . . . . . . . . . . . . . . . .  11
       7.1.1.  Registry Content  . . . . . . . . . . . . . . . . . .  11
     7.2.  Claims Registration . . . . . . . . . . . . . . . . . . .  12
       7.2.1.  Registry Contents . . . . . . . . . . . . . . . . . .  13
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  15
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The original OAuth 2.0 Authorization Framework [RFC6749]
   specification does not mandate any specific format for access tokens.
   While that remains perfectly appropriate for many important
   scenarios, in-market use has shown that many commercial OAuth 2.0
   implementations elected to issue access tokens using a format that
   can be parsed and validated by resource servers directly, without
   further authorization server involvement.  The approach is
   particularly common in topologies where the authorization server and
   resource server are not co-located, are not run by the same entity,
   or are otherwise separated by some boundary.  At the time of writing,
   many commercial implementations leverage the JSON Web Tokens (JWT)
   [RFC7519] format.

   Many vendor specific JWT access tokens share the same functional
   layout, using JWT claims to convey the information needed to support
   a common set of use cases: token validation, transporting
   authorization information in forms of scopes and entitlements,

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   carrying identity information about the subject, and so on.  The
   differences are mostly confined to the claim names and syntax used to
   represent the same entities, suggesting that interoperability could
   be easily achieved by standardizing on a common set of claims and
   validation rules.

   The assumption that access tokens are associated to specific
   information doesn't appear only in commercial implementations.
   Various specifications in the OAuth 2.0 family (such as resource
   indicators [RFC8707], OAuth 2.0 bearer token usage [RFC6750] and
   others) postulate the presence in access tokens of scoping
   mechanisms, such as an audience.  The family of specifications
   associated to introspection also indirectly suggest a fundamental set
   of information access tokens are expected to carry or at least be
   associated with.

   This specification aims to provide a standardized and interoperable
   profile as an alternative to the proprietary JWT access token layouts
   going forward.  Besides defining a common set of mandatory and
   optional claims, the profile provides clear indications on how
   authorization request parameters determine the content of the issued
   JWT access token, how an authorization server can publish metadata
   relevant to the JWT access tokens it issues, and how a resource
   server should validate incoming JWT access tokens.

   Finally, this specification provides security and privacy
   considerations meant to prevent common mistakes and anti patterns
   that are likely to occur in naive use of the JWT format to represent
   access tokens.

1.1.  Requirements Notation and 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 BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   JWT access token  An OAuth 2.0 access token encoded in JWT format and
      complying with the requirements described in this specification.

   This specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", "authorization response", "token endpoint",
   "grant type", "access token request", "access token response", and
   "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

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2.  JWT Access Token Header and Data Structure

2.1.  Header

   Although JWT access tokens can use any signing algorithm, use of
   asymmetric cryptography is RECOMMENDED as it simplifies the process
   of acquiring validation information for resource servers (see
   Section 4).  JWT access tokens MUST NOT use "none" as the signing
   algorithm.  See Section 4 for more details.

   This specification registers the "application/at+jwt" media type,
   which can be used to indicate that the content is a JWT access token.
   JWT access tokens MUST include this media type in the "typ" header
   parameter to explicitly declare that the JWT represents an access
   token complying with this profile.  Per the definition of "typ" in
   Section 4.1.9 of [RFC7515], it is RECOMMENDED that the "application/"
   prefix be omitted.  Therefore, the "typ" value used SHOULD be
   "at+jwt".  See the security considerations section for details on the
   importance of preventing OpenID Connect ID Tokens from being accepted
   as access tokens by resource servers implementing this profile.

2.2.  Data Structure

   The following claims are used in the JWT access token data structure.

   iss  REQUIRED - as defined in Section 4.1.1 of [RFC7519].

   exp  REQUIRED - as defined in Section 4.1.4 of [RFC7519].

   aud  REQUIRED - as defined in Section 4.1.3 of [RFC7519].  See
      Section 3 for indications on how an authorization server should
      determine the value of "aud" depending on the request.

   sub  REQUIRED - as defined in Section 4.1.2 of [RFC7519].  In case of
      access tokens obtained through grants where a resource owner is
      involved, such as the authorization code grant, the value of "sub"
      SHOULD correspond to the subject identifier of the resource owner.
      In case of access tokens obtained through grants where no resource
      owner is involved, such as the client credentials grant, the value
      of "sub" SHOULD correspond to an identifier the authorization
      server uses to indicate the client application.See Section 5 for
      more details on this scenario.  Also, see Section 6 for a
      discussion about how different choices in assigning "sub" values
      can impact privacy.

   client_id  REQUIRED - as defined in Section 4.3 of [RFC8693].

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   iat  REQUIRED - as defined in Section 4.1.6 of [RFC7519].  This claim
      identifies the time at which the JWT access token was issued.

   jti  REQUIRED - as defined in Section 4.1.7 of [RFC7519].

2.2.1.  Authentication Information Claims

   The claims listed in this section MAY be issued in the context of
   authorization grants involving the resource owner, and reflect in the
   access token the types and strength of authentication that the
   authentication server enforced prior to returning the authorization
   response to the client.  Their values are fixed, and remain the same
   across all access tokens that derive from a given authorization
   response, whether the access token was obtained directly in the
   response (e.g., via the implicit flow) or after one or more token
   exchanges (e.g., obtaining a fresh access token using a refresh
   token, or exchanging one access token for another via [RFC8693]).

   auth_time  OPTIONAL - as defined in Section 2 of [OpenID.Core].

   acr, amr  OPTIONAL - as defined in Section 2 of [OpenID.Core].

2.2.2.  Identity Claims

   In the context of authorization grants involving the resource owner,
   commercial authorization servers will often include resource owner
   attributes directly in access tokens, so that resource servers can
   consume them directly for authorization or other purposes without any
   further round trips to introspection ( [RFC7662]) or userinfo (
   [OpenID.Core]) endpoints.  This is particularly common in scenarios
   where the client and the resource server belong to the same entity
   and are part of the same solution, as is the case for first party
   clients invoking their own backend API.

   This profile does not introduce any mechanism for a client to
   directly request the presence of specific claims in JWT access
   tokens, as the authorization server can determine what additional
   claims are required by a particular resource server by taking in
   consideration the client_id of the client, the "scope" and the
   "resource" parameters included in the request.

   Any additional attributes whose semantics are well described by the
   attribute's description found in Section 5.1 of [OpenID.Core] SHOULD
   be codified in JWT access tokens via the corresponding claim names in
   that section of the OpenID Connect specification.  The same holds for
   attributes defined in [RFC7662] and other identity related
   specifications registering claims in the JSON Web Token (JWT) IANA
   registry introduced in [RFC7519].

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   Authorization servers MAY return arbitrary attributes not defined in
   any existing specification, as long as the corresponding claim names
   are collision resistant or the access tokens are meant to be used
   only within a private subsystem.  Please refer to Sections 4.2 and
   4.3 of [RFC7519] for details.

   Authorization servers including resource owner attributes in JWT
   access tokens should exercise care and verify that all privacy
   requirements are met, as discussed in Section 6.

2.2.3.  Authorization Claims

   If an authorization request includes a scope parameter, the
   corresponding issued JWT access token SHOULD include a "scope" claim
   as defined in Section 4.2 of [RFC8693].

   All the individual scope strings in the "scope" claim MUST have
   meaning for the resources indicated in the "aud" claim.  See
   Section 5 for more considerations about the relationship between
   scope strings and resources indicated by the "aud" claim.

2.2.3.1.  Claims for Authorization Outside of Delegation Scenarios

   Many authorization servers embed in the access tokens they issue
   authorization attributes that go beyond the delegated scenarios
   described by [RFC7519].  Typical examples include resource owner
   memberships in roles and groups that are relevant to the resource
   being accessed, entitlements assigned to the resource owner for the
   targeted resource that the authorization server knows about, and so
   on.

   An authorization server wanting to include such attributes in a JWT
   access token SHOULD use as claim types the attributes described by
   the SCIM Core specification ( [RFC7643]), Section 4.1.2 and in
   particular roles, groups and entitlements.  This profile does not
   provide a specific vocabulary for those entities.  Section 7 of this
   document does provide entries for registering the roles, groups and
   entitlements attributes from [RFC7643] as claim types to be used in
   this profile.

3.  Requesting a JWT Access Token

   An authorization server can issue a JWT access token in response to
   any authorization grant defined by [RFC6749] and subsequent
   extensions meant to result in an access token.

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   If the request includes a "resource" parameter (as defined in
   [RFC8707]), the resulting JWT access token "aud" claim SHOULD have
   the same value as the "resource" parameter in the request.

   Example request below:

   GET /as/authorization.oauth2?response_type=code
           &client_id=s6BhdRkqt3&
           state=laeb
           &scope=openid%20profile%20reademail
           &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb
           &resource=https%3A%2F%2Frs.example.com%2F HTTP/1.1
        Host: authorization-server.example.com

    Figure 1: Authorization Request with Resource and Scope Parameters

   Once redeemed, the code obtained from the request above will result
   in a JWT access token in the form shown below:

   {"typ":"at+JWT","alg":"RS256","kid":"RjEwOwOA"}
   {
     "iss": "https://authorization-server.example.com/",
     "sub": " 5ba552d67",
     "aud":   "https://rs.example.com/",
     "exp": 1544645174,
     "client_id": "s6BhdRkqt3_",
     "scope": "openid profile reademail"
   }

                       Figure 2: A JWT Access Token

   The authorization server MUST NOT issue a JWT access token if the
   authorization granted by the token would be ambiguous.  See Section 5
   for more details about common cases that might lead to ambiguity and
   strategies an authorization server can enact to prevent them.

   If the request does not include a "resource" parameter, the
   authorization server MUST use in the "aud" claim a default resource
   indicator.  If a "scope" parameter is present in the request, the
   authorization server SHOULD use it to infer the value of the default
   resource indicator to be used in the "aud" claim.  The mechanism
   through which scopes are associated to default resource indicator
   values is outside the scope of this specification.  If the values in
   the "scope" parameter refer to different default resource indicator

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   values, the authorization server SHOULD reject the request with
   "invalid_scope" as described in Section 4.1.2.1 of [RFC6749].

4.  Validating JWT Access Tokens

   For the purpose of facilitating validation data retrieval, it is
   RECOMMENDED that authorization servers sign JWT access tokens with an
   asymmetric algorithm.

   Authorization servers SHOULD use OAuth 2.0 Authorization Server
   Metadata [RFC8414] to advertise to resource servers their signing
   keys via "jwks_uri" and what "iss" claim value to expect via the
   issuer metadata value.  Alternatively, authorization servers
   implementing OpenID Connect MAY use the OpenID Connect discovery
   document for the same purpose.  If an authorization server supports
   both OAuth 2.0 Authorization Server Metadata and OpenID Connect
   discovery, the values provided MUST be consistent across the two
   publication methods.

   An authorization server MAY elect to use different keys to sign
   OpenID Connect ID Tokens and JWT access tokens.  This specification
   does not provide a mechanism for identifying a specific key as the
   one used to sign JWT access tokens.  An authorization server can sign
   JWT access tokens with any of the keys advertised via AS metadata or
   OpenID Connect discovery.  See Section 5 for further guidance on
   security implications.

   Resource servers receiving a JWT access token MUST validate it in the
   following manner.

   o  The resource server MUST verify that the typ header value is
      "at+jwt" or "application/at+jwt" and reject tokens carrying any
      other value.

   o  If the JWT access token is encrypted, decrypt it using the keys
      and algorithms that the resource server specified during
      registration.  If encryption was negotiated with the authorization
      server at registration time and the incoming JWT access token is
      not encrypted, the resource server SHOULD reject it.

   o  The Issuer Identifier for the authorization server (which is
      typically obtained during discovery) MUST exactly match the value
      of the "iss" claim.

   o  The resource server MUST validate that the "aud" claim contains a
      resource indicator value corresponding to an identifier the
      resource server expects for itself.  The JWT access token MUST be

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      rejected if "aud" does not contain a resource indicator of the
      current resource server as a valid audience.

   o  The resource server MUST validate the signature of all incoming
      JWT access tokens according to [RFC7515] using the algorithm
      specified in the JWT alg Header Parameter.  The resource server
      MUST reject any JWT in which the value of "alg" is "none".  The
      resource server MUST use the keys provided by the authorization
      server.

   o  The current time MUST be before the time represented by the "exp"
      claim.

   The resource server MUST handle errors as described in Section 3.1 of
   [RFC6750].  In particular, in case of any failure in the validation
   checks listed above the authorization server response MUST include
   the error code "invalid_token".

   If the JWT access token includes authorization claims as described in
   Section 2.2.3, the resource server SHOULD use them in combination
   with any other contextual information available to determine whether
   the current call should be authorized or rejected.  Details about how
   a resource server performs those checks is beyond the scope of this
   profile specification.

5.  Security Considerations

   The JWT access token data layout described here is very similar to
   the one of the id_token as defined by [OpenID.Core].  The explicit
   typing required in this profile, in line with the recommendations in
   [RFC8725] helps the resource server to distinguish between JWT access
   tokens and OpenID Connect ID Tokens.

   Authorization servers should prevent scenarios where clients can
   affect the value of the "sub" claim in ways that could confuse
   resource servers.  For example, if the authorization server elects to
   use the client_id as the "sub" value for access tokens issued client
   credentials grant, the authorization server should prevent clients to
   register an arbitrary client_id value, as this would allow malicious
   clients to select the sub of a high privilege resource owner and
   confuse any authorization logic on the resource server relying on the
   "sub" value.  For more details please refer to Section 4.13 of
   [OAuth2.Security.BestPractices].

   To preventing cross-JWT confusion, authorization servers MUST use a
   distinct identifier as "aud" claim value to uniquely identify access
   tokens issued by the same issuer for distinct resources.  For more

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   details on cross-JWT confusion please refer to Section 2.8 of
   [RFC8725].

   Authorization servers should use particular care when handling
   requests that might lead to ambiguous authorization grants.  For
   example: if a request includes multiple resource indicators, the
   authorization server should ensure that each scope string included in
   the resulting JWT access token, if any, can be unambiguously
   correlated to a specific resource among the ones listed in the "aud"
   claim.  The details on how to recognize and mitigate this and other
   ambiguous situations is highly scenario-dependent, hence out of scope
   for this profile.

   Authorization servers should not rely on the use of different keys
   for signing OpenID Connect ID Tokens and JWT tokens as a method to
   safeguard against the consequences of leaking specific keys.  Given
   that resource servers have no way of knowing what key should be used
   to validate JWT access tokens in particular, they have to accept
   signatures performed with any of the keys published in AS metadata or
   OpenID Connect discovery: consequently, an attacker just needs to
   compromise any key among the ones published to be able to generate
   and sign JWTs that will be accepted as valid by the resource server.

6.  Privacy Considerations

   As JWT access tokens carry information by value, it now becomes
   possible for clients and potentially even end users to directly peek
   inside the token claims collection.

   The client MUST NOT inspect the content of the access token: the
   authorization server and the resource server might decide to change
   token format at any time (for example by switching from this profile
   to opaque tokens) hence any logic in the client relying on the
   ability to read the access token content would break without
   recourse.  The OAuth 2.0 framework assumes that access tokens are
   treated as opaque by clients.  Administrators of authorization
   servers should also take into account that the content of an access
   token is visible to the client.  Whenever client access to the access
   token content presents privacy issues for a given scenario, the
   authorization server should take explicit steps to prevent it.

   In scenarios in which JWT access tokens are accessible to the end
   user, it should be evaluated whether the information can be accessed
   without privacy violations (for example, if an end user would simply
   access his or her own personal information) or if steps must be taken
   to enforce confidentiality.

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   Possible measures to prevent leakage of information to clients and
   end users include: encrypting the access token, encrypting the
   sensitive claims, omitting the sensitive claims or not using this
   profile, falling back on opaque access tokens.

   In every scenario, the content of the JWT access token will
   eventually be accessible to the resource server.  It's important to
   evaluate whether the resource server gained the proper entitlement to
   have access to any content received in form of claims, for example
   through user consent in some form, policies and agreements with the
   organization running the authorization servers, and so on.

   This profile mandates the presence of the "sub" claim in every JWT
   access token, making it possible for resource servers to rely on that
   information for correlating incoming requests with data stored
   locally for the authenticated principal.  Although the ability to
   correlate requests might be required by design in many scenarios,
   there are scenarios where the authorization server might want to
   prevent correlation.  The "sub" claim should be populated by the
   authorization servers according to a privacy impact assessment.  For
   instance, if a solution requires preventing tracking principal
   activities across multiple resource servers, the authorization server
   should ensure that JWT access tokens meant for different resource
   servers have distinct "sub" values that cannot be correlated in the
   event of resource servers collusion.  Similarly, if a solution
   requires preventing a resource server from correlating the
   principal's activity within the resource itself, the authorization
   server should assign different "sub" values for every JWT access
   token issued.  In turn, the client should obtain a new JWT access
   token for every call to the resource server, to ensure that the
   resource server receives different "sub" and "jti" values at every
   call, thus preventing correlation between distinct requests.

7.  IANA Considerations

7.1.  Media Type Registration

7.1.1.  Registry Content

   This section registers the "application/at+jwt" media type [RFC2046]
   in the "Media Types" registry [IANA.MediaTypes] in the manner
   described in [RFC6838], which can be used to indicate that the
   content is an access token encoded in JWT format.

   o  Type name: application

   o  Subtype name: at+jwt

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   o  Required parameters: N/A

   o  Optional parameters: N/A

   o  Encoding considerations: binary; JWT values are encoded as a
      series of base64url-encoded values (with trailing '=' characters
      removed), some of which may be the empty string, separated by
      period ('.') characters.

   o  Security considerations: See the Security Considerations
      Section of [[TODO: update once there's a RFC number for the JWT AT
      profile]]

   o  Interoperability considerations: N/A

   o  Published specification: [[TODO: update once there's a RFC number
      for the JWT AT profile]]

   o  Applications that use this media type: Applications that access
      resource servers using OAuth 2.0 access tokens encoded in JWT
      format

   o  Fragment identifier considerations: N/A

   o  Additional information: Magic number(s): N/A File extension(s): N/
      A Macintosh file type code(s): N/A

   o  Person and email address to contact for further information:
      Vittorio Bertocci, vittorio@auth0.com

   o  Intended usage: COMMON

   o  Restrictions on usage: none

   o  Author: Vittorio Bertocci, vittorio@auth0.com

   o  Change controller: IESG

   o  Provisional registration?  No

7.2.  Claims Registration

   Section 2.2.3.1 of this specification refers to the attributes
   "roles", "groups", "entitlements" defined in [RFC7643] to express
   authorization information in JWT access tokens.  This section
   registers those attributes as claims in the JSON Web Token (JWT) IANA
   registry introduced in [RFC7519].

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7.2.1.  Registry Contents

   o  Claim Name: "roles"

   o  Claim Description: Roles

   o  Change Controller: IESG

   o  Specification Document(s): Section 4.1.2 of [RFC7643] and
      Section 2.2.2.1 of [[this specification]]

   o  Claim Name: "groups"

   o  Claim Description: Groups

   o  Change Controller: IESG

   o  Specification Document(s): Section 4.1.2 of [RFC7643] and
      Section 2.2.2.1 of [[this specification]]

   o  Claim Name: "entitlements"

   o  Claim Description: Entitlements

   o  Change Controller: IESG

   o  Specification Document(s): Section 4.1.2 of [RFC7643] and
      Section 2.2.2.1 of [[this specification]]

8.  References

8.1.  Normative References

   [OpenID.Core]
              Sakimura, N., Bradley, J., Jones, M., Medeiros, B., and C.
              Mortimore, "OpenID Connect Core 1.0", November 2014.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7643]  Hunt, P., Ed., Grizzle, K., Wahlstroem, E., and C.
              Mortimore, "System for Cross-domain Identity Management:
              Core Schema", RFC 7643, DOI 10.17487/RFC7643, September
              2015, <https://www.rfc-editor.org/info/rfc7643>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8414]  Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0
              Authorization Server Metadata", RFC 8414,
              DOI 10.17487/RFC8414, June 2018,
              <https://www.rfc-editor.org/info/rfc8414>.

   [RFC8693]  Jones, M., Nadalin, A., Campbell, B., Ed., Bradley, J.,
              and C. Mortimore, "OAuth 2.0 Token Exchange", RFC 8693,
              DOI 10.17487/RFC8693, January 2020,
              <https://www.rfc-editor.org/info/rfc8693>.

   [RFC8707]  Campbell, B., Bradley, J., and H. Tschofenig, "Resource
              Indicators for OAuth 2.0", RFC 8707, DOI 10.17487/RFC8707,
              February 2020, <https://www.rfc-editor.org/info/rfc8707>.

   [RFC8725]  Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", BCP 225, RFC 8725,
              DOI 10.17487/RFC8725, February 2020,
              <https://www.rfc-editor.org/info/rfc8725>.

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8.2.  Informative References

   [OAuth2.Security.BestPractices]
              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", July 2019.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

Appendix A.  Acknowledgements

   The initial set of requirements informing this specification was
   extracted by numerous examples of access tokens issued in JWT format
   by production systems.  Thanks to Dominick Baier (IdentityServer),
   Brian Campbell (Ping Identity), Daniel Dobalian (Microsoft), Karl
   Guinness (Okta) for providing sample tokens issued by their products
   and services.  Brian Campbell and Filip Skokan provided early
   feedback that shaped the direction of the specification.  This
   profile was discussed at lenght during the OAuth Security Workshop
   2019, with several individuals contributing ideas and feedback.  The
   author would like to acknowledge the contributions of:

   John Bradley, Brian Campbell, Vladimir Dzhuvinov, Torsten
   Lodderstedt, Nat Sakimura, Hannes Tschofenig and everyone who
   actively participated in the unconference discussions.

   The following individuals contributed useful feedback and insights on
   the drafts, both on the IETF OAuth 2.0 WG DL and during the IIW28
   conference:

   Dale Olds, George Fletcher, David Waite, Michael Engan, Mike Jones,
   Hans Zandbelt, Vladimir Dzhuvinov, Martin Schanzenbach , Aaron
   Parecki, Annabelle Richard Backman, Dick Hardt, Denis Pinkas,
   Benjamin Kaduk, Dominick Baier, Mike Jones and everyone who actively
   participated in the IIW28 unconference discussions and the IETF OAuth
   2.0 WG DL discussions.

Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   draft-ietf-oauth-access-token-jwt-09

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   o  Removed unused reference to http://www.iana.org/assignments/oauth-
      parameters; moved the OAuth2 security BCP to the informative
      references section.
   o  Restructured opening paragraphs in Section 6 for clarity.

   draft-ietf-oauth-access-token-jwt-08

   o  Numerous edits for correcting typos, improving clarity and
      precision of language.
   o  Moved RFC7519 to the normative section; eliminated unused
      references RFC7644 and RFC3986.

   draft-ietf-oauth-access-token-jwt-07

   o  In Section 2.1, added language that forbids use of none as alg
      value, and references Section 4 where the same prohibition was
      already expressed from the RS perspective.
   o  In the sub definition in Section 2.2, added a sentence that
      clarifies what goes in the sub in the case of grants where a
      resource owner is involved.
   o  Updated acknowledgements.
   o  Updated Section 2.2.1 to clarify that acr, amr and auth_type can
      occur if the AT has been obtained by grants where the resource
      owner is involved.
   o  Updated Section 2.2.2 to clarify that identity claims can occur if
      the AT has been obtained by grants where the resource owner is
      involved.
   o  In Section 2.2.3.1 eliminated the claim that SCIM doesn't provide
      a vocabulary for the attributes listed there.
   o  In Section 5 added reference to 8725.
   o  In Section 4 added application/jwt+at as accepted typ value.
   o  Various typos and formatting issues fixed.

   draft-ietf-oauth-access-token-jwt-06

   o  In Section 2.2 and Section 6 added a discussion about how
      different sub values affect the privacy properties of a solution.
   o  In Section 2.2.3 and Section 3 eliminated language prohibiting JWT
      AT requests featuring multiple resources, substituting it with the
      prohibition for the AS to emit JWT ATs expressing ambiguous
      authorization grants.  In Section 5, added language warning
      against scope confusion and mentioned the existence of other
      ambiguous authorization grant.
   o  In Section 2.2 promoted claims iat and jti from RECOMMENDED to
      REQUIRED.
   o  In Section 2.1 eliminated temporary note on the lack of
      authenticated encryption methods specifications.
   o  Updated acknowledgements.

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   draft-ietf-oauth-access-token-jwt-05

   o  Varios typos, grammar issues and improper abbreviations fixed.
   o  Reworded the definition of at+jwt in Section 2.1.
   o  In Section 2.2, clarified that iat refers to the issuance time of
      the JWT itself.
   o  In Section 2.2.2, added a reference to public/private claims
      definitions (Sections 4.2, 4.3) of [RFC7519].
   o  In Section 3, removed the paragrah stating that every JWT AT MUST
      have an aud, as it is already defined in Section 2.2.
   o  Reworded description of the JWT AT adoption landscape in
      Section 1.
   o  Simplified the individual descriptions of the claims list in
      Section 2.2.1.
   o  Updated Section 4 and Section 5 to clarify that the AS can use any
      of the published keys to sign JWT access tokens, and that the AS
      should not rely on use of different signing keys per token type as
      a security mechanism.
   o  In Section 2.2 promoted claims iat and jti from OPTIONAL to
      RECOMMENDED
   o  In Section 4, switched the validation steps list type from numbers
      to bullets.
   o  In Section 4, eliminated the auth_time instructions from the
      validation steps list.
   o  In Section 2.2.2, added a reference to the JWT claims registry as
      source of claims for JWT ATs
   o  In Section 4, clarified that failures in JWT AT validation checks
      will result in invalid_token.

   draft-ietf-oauth-access-token-jwt-04

   o  Eliminated reference to resource aliases list from the aud claim
      description in Section 2.
   o  Eliminated references to resource aliases list from the aud
      validation guidance in Section 4.
   o  Introduced a new subsection Section 2.2.1, moved the definitions
      of auth_time, acr and amr there and incorporated the language
      proposed by Annabelle and Brian on the WG mailing list.
   o  In section Section 3 softened (from MUST to SHOULD) the
      requirement that ties the resource identifier in the request to
      the value in the aud claim of the issued access token.
   o  Updated acknowledgements.
   o  In the section Section 3, the example request now has
      response_type=code.
   o  Updated text in the Privacy Consideration section to clarify what
      protection steps the text refers to.

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   o  Updated the typ header discussion in Section 2.1 to clarify that
      it helps preventing resources from accepting OpenID Connect ID
      Tokens as JWT access tokens.
   o  Updated refrences to token exchange, resource indicators and JWT
      best practices to reflect their RFC status (8693,8707,8725).

   draft-ietf-oauth-access-token-jwt-03

   o  Varios typos fixed.
   o  In the security considerations section, relaxed the claim that the
      typ header value "at+jwt" will prevent RS from misinterpreting JWT
      ATs as idtokens.
   o  In the "Requesting JWT Access Tokens" section, added
      "invalid_target" as a possible error returned for the multiple
      resources request case.
   o  In the Validating JWT Access Tokens" section, disallowed JWTs with
      "alg":"none"
   o  in the IANA registration entries for the SCIM claim types,
      complemented the reference to the SCIM spec with a reference to
      this spec so that the eventual registration entries have better
      context.
   o  Updated acknowledgements.
   o  In the section Section 3, the example request now has
      response_type=code.
   o  Updated text in the Privacy Consideration section to clarify what
      protection steps the text refers to.

   draft-ietf-oauth-access-token-jwt-02

   o  In 2.2.1, opened the sources of identity attributes to any
      identity related specification.
   o  In 2.2.2, relaxed from MUST to SHOULD the requirement that
      requests including a scope always result in access tkens
      containing a corresponding scope claim.
   o  In the security considerations setting, added a requirement for
      the authorization server to assing unique identifiers for
      different resources- to prevent cross JWT confusion.
   o  Added IANA registration for the authorization attributes borrowed
      from SCIM CORE

   draft-ietf-oauth-access-token-jwt-01

   o  Added note on authenticated encryption.
   o  Added a mention to the 1st party clients scenarios in the identity
      claims section.
   o  Changed the definition reference for the iss, exp, aud, sub, iat
      claims from OpenID.Core to RFC7519.

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   o  Added a mention of the client_id==sub case in the security
      considerations section, added a reference to draft-ietf-oauth-
      security-topics-13.  Added a reference to the security
      considerations from the sub claim definition section.
   o  Specified invalid_request as the error code the authorization
      server should return in case of multiple resources in the access
      token request.
   o  Specified invalid_scope as the error code the authorization server
      should return in case it isn;t possible to determine to which
      resource the requested scopes refers to.
   o  In the identity claims section, added a reference to introspection
      as possible source of claim types and added language explicitly
      stating that the AS can add arbitrary attributes as long as they
      are collision resistant or private.
   o  Updated language for the auth_time claim to include the case in
      which the AS reauthenticates the user mid-session (e.g. during
      step up auth).
   o  Removed note about adding a mechanism for extablishing whether the
      token was obtained on behalf or the resource owner or of the
      client itself (client credentials grant).
   o  Removed note about adding a mechanism for indicating whether the
      authorization server sent the resource owner to authenticate with
      a federated identity provider, and the identity of that federated
      provider.
   o  Removed the note in the security consideration sections about
      discussing the purpose of aud, iss, exp validation (redundant).
   o  In the authorization claims section, stated intent to register
      roles, groups and entitlements as claim types in IANA
   o  Clarified in the privacy considerations that clients should not
      inspect access tokens.
   o  Expanded the privacy considerations with more explicit guidance
      about privacy preserving approaches.
   o  Added IANA registry content for the at+JWT MIME type.
   o  Updated acknowledgements.

   draft-ietf-oauth-access-token-jwt-00

   o  Initial draft to define a JWTt profile for OAuth 2.0 access
      tokens.

Author's Address

   Vittorio Bertocci
   Auth0

   Email: vittorio@auth0.com

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