OAuth Working Group                                          V. Bertocci
Internet-Draft                                                     Auth0
Intended status: Standards Track                          April 22, 2019
Expires: October 24, 2019

        JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens


   This specification defines a profile for issuing OAuth2 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.

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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  . . . . . . . . .   3
     2.1.  Header  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Data Structure  . . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Identity Claims . . . . . . . . . . . . . . . . . . .   5
       2.2.2.  Authorization Claims  . . . . . . . . . . . . . . . .   5  Claims for Authorization Outside of Delegation
                   Scenarios . . . . . . . . . . . . . . . . . . . .   5
   3.  Requesting a JWT Access Token . . . . . . . . . . . . . . . .   6
   4.  Validating JWT Access Tokens  . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  12
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  12

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 scenario,
   in-market use has shown that many commercial OAuth2 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 ran by the same entity, or are otherwise
   separated by some boundary.  All of the known commercial
   implementations known at this time leverage the JSON Web Tokens(JWT)
   [RFC7519] format.

   Most vendor specific JWT access tokens share the same functional
   layout, including information in forms of claims meant to support the
   same scenarios: token validation, transporting authorization
   information in forms of scopes and entitlements, 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.

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   The assumption that access tokens are associated to specific
   information doesn't appear only in commercial implementations.
   Various specifications in the OAuth2 family (such as resource
   indicators [ResourceIndicators], 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 tokens
   layouts going forward.  Besides defining a common set of mandatory
   and optional claims, the profile provides clear indications on how
   authorization requests 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",
   "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].

2.  JWT Access Token Header and Data Structure

   JWT access tokens are regular JWT tokens complying with the
   requirements described in this section.

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2.1.  Header

   Although JWT access tokens can use any signing algorithm, use of
   asymmetric algorithms is RECOMMENDED as it simplifies the process of
   acquiring validation information for resource servers (see
   Section 4).

   The typ header parameter for a JWT access token MUST be at+jwt.  See
   the security considerations section for details on the importance of
   preventing JWT access tokens to be interpreted as id_tokens.

2.2.  Data Structure

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

   iss  REQUIRED - as defined in section 2 of [OpenID.Core].

   exp  REQUIRED - as defined in section 2 of [OpenID.Core].

   aud  REQUIRED - as defined in section 2 of [OpenID.Core].  See
      Section 3 for indications on how an authorization server should
      determine the value of aud depending on the request.  [Note: some
      vendors seem to rely on resource aliases.  If we believe this to
      be a valuable feature, here's some proposed language: The aud
      claim MAY include a list of individual resource indicators if they
      are all aliases referring to the same requested resource known by
      the authorization server. ]

   sub  REQUIRED - as defined in section 2 of [OpenID.Core]. . 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 (such as the client_id).

   client_id  REQUIRED - as defined in section 4.3 of [TokenExchange].

   iat  OPTIONAL - as defined in section 2 of [OpenID.Core].

   auth_time  OPTIONAL - as defined in section 2 of [OpenID.Core].
      Important: as this claim represents the time at which the end user
      authenticated, its value will remain the same for all the JWT
      access tokens issued within that session.  For example: all the
      JWT access tokens obtained with a given refresh token will all
      have the same value of auth_time, corresponding to the instant in
      which the user first authenticated to obtain the refresh token.

   jti  OPTIONAL - as defined in section 4.1.7 of [RFC7519].

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   acr, amr  OPTIONAL - as defined in section 2 of [OpenID.Core].  The
      same considerations presented for auth_time apply to acr and amr:
      those values reflect the authentication context and method used
      when the end user originally authenticated, and will remain
      unchanged for the JWT access tokens issued within the context of
      that session.

2.2.1.  Identity Claims

   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 roudtrips to introspection ([RFC7662]) or userinfo
   ([OpenID.Core]) endpoints.

   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 semantic is well described by the
   attributes 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.

   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.

2.2.2.  Authorization Claims

   If an authorization request includes a scope parameter, the
   corresponding issued JWT access token MUST include a scope claim as
   defined in section 4.2 of [TokenExchange].

   All the individual scopes strings in the scope claim MUST have
   meaning for the resource indicated in the aud claim.  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

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   targeted resource that the authorization server knows about, and so

   An authorization server wanting to include such attributes in a JWT
   access token SHOULD use as claim types the attributes described by
   section 4.1.2 of SCIM Core ([RFC7643]) and in particular roles,
   groups and entitlements.  As in their original definition in
   [RFC7643] , this profile does not provide a specific vocabulary for
   those entities.

   [[note 1 some commercial authorization server include claims
   indicating whether the client authenticated with the authorization
   server as a confidential client, for the purpose of determining
   whether the client_id can be used as a reliable indicator of the
   identity of the caller (and take thatinto account for authorization
   decisions).  Discussions at OSW2019 on how to achieve this were
   inconclusive hence this was punted for further discussion]]

   [[note 2 some commercial authorization server include claims
   indicating whether the resource owner authenticated with a federated
   identity provider rather than directly with the authorization server.
   During discussions at OSW2019 there were lukewarm reactions.  One
   proposed line of investigation was to examine what
   https://tools.ietf.org/html/draft-ietf-secevent-token-13#page-10 does
   with the sub structure and see whether some mechanisms can be
   applicable here; however for this early draft no further
   investigation was made and no info is provided beyond this note]]

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.

   Every JWT access token MUST include an aud claim (see Section 2.2).

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

   Example request below:

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   GET /as/authorization.oauth2?response_type=token
           &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:

     "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

   If it receives a request for an access token containing more than one
   resource parameter, an authorization server issuing JWT access tokens
   MUST reject the request and fail with [[TODO: select appropriate
   error code]].  See Section 2.2 and Section 5 for more details on how
   this measure ensures there's no confusion on to what resource the
   access token granted scopes apply.

   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
   values, the authorization server SHOULD reject the request with
   [[TODO: select appropriate error code]].

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.

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   Authorization servers SHOULD implement OAuth 2.0 Authorization Server
   Metadata [RFC8414] to advertise to resource servers its 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 AS metadata
   and Openid discovery, the values provided MUST be consistent across
   the two publication methods.

   An authorization server MAY elect to use different keys to sign
   id_tokens and JWT access tokens.

   When invoked as described in OAuth2 bearer token usage, resource
   servers receiving a JWT access token MUST validate it in the
   following manner.

   1.  The resource server MUST verify that the typ header value is
       at+jwt and reject tokens carrying any other value.

   2.  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

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

   4.  The resource server MUST validate that the aud claim contains the
       resource indicator value corresponding to the identifier the
       resource server expects for itself.  The aud claim MAY contain an
       array with more than one element.  The JWT access token MUST be
       rejected if aud does not list the resource indicator of the
       current resource server as a valid audience, or if it contains
       additional audiences that are not known aliases of the resource
       indicator of the current resource server.

   5.  The resource server MUST validate the signature of all incoming
       JWT access token according to [RFC7515] using the algorithm
       specified in the JWT alg Header Parameter.  The resource server
       MUST use the keys provided by the authorization server.

   6.  The current time MUST be before the time represented by the exp

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   7.  If the auth_time claim is present, the resource server SHOULD
       check the auth_time value and request re-authentication if it
       determines too much time has elapsed since the last resource
       owner authentication.

   [[Note: I would like to express the requirement that the resource
   server should not ignore authorization information when present in
   the JWT access token.  I don't know if this belongs here or
   elsewhere.  Here's some possible language: If the JWT access token
   includes authorization claims as described in the authorization
   claims section, 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].  Without the
   explicit typing required in this profile, in line with the
   recommendations in [JWT.BestPractices] there would be the risk of
   attackers using JWT access tokens in lieu of id_tokens.

   This profile explicitly forbids the use of multi value aud claim when
   the individual values refer to different resources, as that would
   introduce confusion about what scopes apply to which resource-
   possibly opening up avenues for elevation of delegated privileges
   attacks.  Alternative techniques to prevent scope confusion include
   "scope stuffing", imposing to every individual scope string to
   include a reference to the resource they are meant to be applied to,
   but its application is problematic (scope opacity violations, size
   inflation, more error conditions become possible when the combination
   of requested scopes and resource indicators is invalid) and the
   observed frequency of the scenario doesn't warrant complicating the
   more common cases.

   [[todo: expand on Audience, issuer and expiration validation checks
   serve the usual purposes.  What else?]]

6.  Privacy Considerations

   As JWT access tokens carry information by value, it now becomes
   possible for requestors and receivers to directly peek inside the
   token claims collection.

   In scenarios in which JWT access tokens are accessible to the end
   user, it should be evaluated whether the information can be accessed

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   without privacy violations (for example, if an end user would simply
   access his or her own personal information) or if the token should be

   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.

7.  IANA Considerations

   [[TODO: MIME type registration for at+jwt ]]

8.  References

8.1.  Normative References

              IANA, "OAuth Parameters",

              Sheffer, Y., Hardt, D., and M. Jones, "JSON Web Token Best
              Current Practices", November 2018.

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

              Campbell, B., Bradley, J., and H. Tschofenig, "OAuth 2.0
              Token Exchange", November 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,

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   [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>.

   [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,

              Nadalin, A., Bradley, J., Jones, M., Campbell, B., and C.
              Mortimore, "OAuth 2.0 Token Exchange", October 2018.

8.2.  Informative References

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [RFC7644]  Hunt, P., Ed., Grizzle, K., Ansari, M., Wahlstroem, E.,
              and C. Mortimore, "System for Cross-domain Identity
              Management: Protocol", RFC 7644, DOI 10.17487/RFC7644,
              September 2015, <https://www.rfc-editor.org/info/rfc7644>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,

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 Bauer (IdentityServer),
   Brian Campbell (PingIdentity), Daniel Dobalian (Microsoft), Karl

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   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.

Appendix B.  Document History

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


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

Author's Address

   Vittorio Bertocci

   Email: vittorio@auth0.com

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