RFC 9207

Internet Engineering Task Force (IETF)             K. Meyer zu Selhausen
Request for Comments: 9207                                     Hackmanit
Category: Standards Track                                        D. Fett
ISSN: 2070-1721                                                  yes.com
                                                              March 2022

          OAuth 2.0 Authorization Server Issuer Identification


   This document specifies a new parameter called iss.  This parameter
   is used to explicitly include the issuer identifier of the
   authorization server in the authorization response of an OAuth
   authorization flow.  The iss parameter serves as an effective
   countermeasure to "mix-up attacks".

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 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Conventions and Terminology
   2.  Response Parameter iss
     2.1.  Example Authorization Response
     2.2.  Example Error Response
     2.3.  Providing the Issuer Identifier
     2.4.  Validating the Issuer Identifier
   3.  Authorization Server Metadata
   4.  Security Considerations
   5.  IANA Considerations
     5.1.  OAuth Authorization Server Metadata
     5.2.  OAuth Parameters Registration
   6.  References
     6.1.  Normative References
     6.2.  Informative References

   Authors' Addresses

1.  Introduction

   The OAuth 2.0 Authorization Framework [RFC6749] allows clients to
   interact with multiple independent authorization servers under the
   control of separate entities.  Some OAuth grant types utilize the
   resource owner's user agent to deliver the authorization server's
   response to the OAuth client.  One example of this pattern is the
   authorization response of the authorization code grant.

   The authorization response as specified in Section 4.1.2 of [RFC6749]
   does not contain any information about the identity of the
   authorization server that issued the response.  Therefore, clients
   receiving a response from the resource owner's user agent cannot be
   sure who initially issued the response and the secrets contained
   therein.  The lack of certainty about the origin of the response
   enables a class of attacks called "mix-up attacks".

   Mix-up attacks are a potential threat to all OAuth clients that
   interact with multiple authorization servers.  When at least one of
   these authorization servers is under an attacker's control, the
   attacker can launch a mix-up attack to acquire authorization codes or
   access tokens issued by any one of the other authorization servers.
   There are multiple ways in which an attacker can gain control over an
   authorization server supported by the client; for instance, an
   authorization server could become compromised, or the attacker could
   register their own authorization server, for example, using dynamic
   client registration [RFC7591].

   OAuth clients that interact with only one authorization server are
   not vulnerable to mix-up attacks.  However, when such clients decide
   to add support for a second authorization server in the future, they
   become vulnerable and need to apply countermeasures to mix-up

   Mix-up attacks aim to steal an authorization code or access token by
   tricking the client into sending the authorization code or access
   token to the attacker instead of the honest authorization or resource
   server.  This marks a severe threat to the confidentiality and
   integrity of resources whose access is managed with OAuth.  A
   detailed description and different variants of the mix-up attack
   class can be found in Section 4.4 of "OAuth 2.0 Security Best Current
   Practice" [OAUTH-SECURITY-TOPICS] as well as in the original research
   first highlighting this attack class, "On the security of modern
   Single Sign-On Protocols: Second-Order Vulnerabilities in OpenID
   Connect" [arXiv.1508.04324] and "A Comprehensive Formal Security
   Analysis of OAuth 2.0" [arXiv.1601.01229].

   This document defines a new parameter in the authorization response
   called iss.  The iss parameter allows the authorization server to
   include its identity in the authorization response explicitly.  The
   client can compare the value of the iss parameter to the issuer
   identifier of the authorization server (e.g., retrieved from its
   metadata) it believes it is interacting with.  The iss parameter
   gives the client certainty about the authorization server's identity
   and enables it to send credentials such as authorization codes and
   access tokens only to the intended recipients.

   The effectiveness of the iss parameter against mix-up attacks was
   analyzed and formally proven in "A Comprehensive Formal Security
   Analysis of OAuth 2.0" [arXiv.1601.01229].

1.1.  Conventions and Terminology

   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.

   This specification uses the terms "access token", "authorization
   code", "authorization code grant", "authorization server", "resource
   server", "authorization response", "grant type", and "client" defined
   by the OAuth 2.0 Authorization Framework [RFC6749].  The term "issuer
   identifier" is defined by OAuth 2.0 Authorization Server Metadata

2.  Response Parameter iss

   In authorization responses to the client, including error responses,
   an authorization server supporting this specification MUST indicate
   its identity by including the iss parameter in the response.

   The iss parameter value is the issuer identifier of the authorization
   server that created the authorization response, as defined in
   [RFC8414].  Its value MUST be a URL that uses the "https" scheme
   without any query or fragment components.

2.1.  Example Authorization Response

   The following example shows an authorization response from the
   authorization server whose issuer identifier is
   https://honest.as.example (extra line breaks and indentation are for
   display purposes only):

   HTTP/1.1 302 Found
   Location: https://client.example/cb?

2.2.  Example Error Response

   The following example shows an error response from the same
   authorization server (extra line breaks and indentation are for
   display purposes only):

   HTTP/1.1 302 Found
   Location: https://client.example/cb?

2.3.  Providing the Issuer Identifier

   Authorization servers supporting this specification MUST provide
   their issuer identifier to enable clients to validate the iss
   parameter effectively.

   For authorization servers publishing metadata according to [RFC8414],
   the following rules apply:

   *  The issuer identifier included in the server's metadata value
      issuer MUST be identical to the iss parameter's value.

   *  The server MUST indicate its support for the iss parameter by
      setting the metadata parameter
      authorization_response_iss_parameter_supported, defined in
      Section 3, to true.

   Authorization servers MAY additionally provide the issuer identifier
   to clients by any other mechanism, which is outside of the scope of
   this specification.

2.4.  Validating the Issuer Identifier

   Clients that support this specification MUST extract the value of the
   iss parameter from authorization responses they receive if the
   parameter is present.  Clients MUST then decode the value from its
   "application/x-www-form-urlencoded" form according to Appendix B of
   [RFC6749] and compare the result to the issuer identifier of the
   authorization server where the authorization request was sent to.
   This comparison MUST use simple string comparison as defined in
   Section 6.2.1 of [RFC3986].  If the value does not match the expected
   issuer identifier, clients MUST reject the authorization response and
   MUST NOT proceed with the authorization grant.  For error responses,
   clients MUST NOT assume that the error originates from the intended
   authorization server.

   More precisely, clients that interact with authorization servers
   supporting OAuth metadata [RFC8414] MUST compare the iss parameter
   value to the issuer value in the server's metadata document.  If
   OAuth metadata is not used, clients MUST use deployment-specific ways
   (for example, a static configuration) to decide if the returned iss
   value is the expected value in the current flow (see also Section 4).

   If clients interact with both authorization servers supporting this
   specification and authorization servers not supporting this
   specification, clients MUST retain state about whether each
   authorization server supports the iss parameter.  Clients MUST reject
   authorization responses without the iss parameter from authorization
   servers that do support the parameter according to the client's
   configuration.  Clients SHOULD discard authorization responses with
   the iss parameter from authorization servers that do not indicate
   their support for the parameter.  However, there might be legitimate
   authorization servers that provide the iss parameter without
   indicating their support in their metadata.  Local policy or
   configuration can determine whether to accept such responses, and
   specific guidance is out of scope for this specification.

   In general, clients that support this specification MAY accept
   authorization responses that do not contain the iss parameter or
   reject them and exclusively support authorization servers that
   provide the iss parameter in the authorization response.  Local
   policy or configuration can determine when to accept such responses,
   and specific guidance is out of scope for this specification.

   In OpenID Connect [OIDC.Core] flows where an ID Token is returned
   from the authorization endpoint, the value in the iss parameter MUST
   always be identical to the iss claim in the ID Token.

   Section 4.1.2 of [RFC6749] already mandates that clients that do not
   support this specification MUST ignore the unrecognized iss

      |  Note: The "JWT Secured Authorization Response Mode for OAuth
      |  2.0 (JARM)" [JARM] defines a mechanism that conveys all
      |  authorization response parameters in a JSON Web Token (JWT).
      |  This JWT contains an iss claim that provides the same
      |  protection if it is validated as described in Section 2.4.
      |  Therefore, an additional iss parameter outside the JWT is not
      |  necessary when JARM is used.

3.  Authorization Server Metadata

   The following parameter for the authorization server metadata
   [RFC8414] is introduced to signal the authorization server's support
   for this specification:

   authorization_response_iss_parameter_supported:  Boolean parameter
      indicating whether the authorization server provides the iss
      parameter in the authorization response as defined in Section 2.
      If omitted, the default value is false.

4.  Security Considerations

   Clients MUST validate the iss parameter precisely as described in
   Section 2.4 and MUST NOT allow multiple authorization servers to use
   the same issuer identifier.  In particular, when authorization server
   details can be manually configured in the client, the client MUST
   ensure that the accepted iss values are unique for each authorization

   The iss parameter enables a client to decide if an authorization
   server "expects" to be used in an OAuth flow together with a certain
   token endpoint and potentially other endpoints, like the userinfo
   endpoint [OIDC.Core].  When OAuth metadata is used, the iss parameter
   identifies the issuer and therefore the respective OAuth metadata
   document that points to the other endpoints.  When OAuth metadata is
   not used, the client can use, for example, a statically configured
   expected iss value for each configured authorization server.

   The issuer identifier contained in the authorization response is not
   cryptographically protected against tampering.  In general,
   mechanisms such as JWTs (as specified in [JARM]) could be used to
   protect the integrity of the authorization response.  However, in
   mix-up attacks, the client generally receives the authorization
   response from an uncompromised authorization server.  If an attacker
   can tamper with this authorization response before it is received by
   the client, the attacker would also have direct access to the
   authorization code.  The attacker does not need to execute a mix-up
   attack to steal the authorization code.  Therefore, integrity
   protection for the authorization response is not necessary to defend
   against mix-up attacks.

   There are also alternative countermeasures to mix-up attacks.  When
   an authorization response already includes an authorization server's
   issuer identifier by other means and this identifier is checked as
   laid out in Section 2.4, the use and verification of the iss
   parameter is not necessary and MAY be omitted.  For example, this is
   the case when OpenID Connect response types that return an ID Token
   from the authorization endpoint (e.g., response_type=code id_token)
   or [JARM] are used.  However, if a client receives an authorization
   response that contains multiple issuer identifiers, the client MUST
   reject the response if these issuer identifiers do not match.  The
   details of alternative countermeasures are outside of the scope of
   this specification.

   Mix-up attacks are only relevant to clients that interact with
   multiple authorization servers.  However, clients interacting with
   only one authorization server might add support for a second
   authorization server in the future.  By supporting multiple
   authorization servers, they become vulnerable to mix-up attacks and
   need to apply countermeasures.

5.  IANA Considerations

5.1.  OAuth Authorization Server Metadata

   IANA has registered the following value in the "OAuth Authorization
   Server Metadata" registry of [IANA.OAuth.Parameters] established by

   Metadata Name:  authorization_response_iss_parameter_supported
   Metadata Description:  Boolean value indicating whether the
      authorization server provides the iss parameter in the
      authorization response.
   Change Controller:  IETF
   Specification Document(s):  Section 3 of RFC 9207

5.2.  OAuth Parameters Registration

   IANA has updated the iss entry to appear as follows in the "OAuth
   Parameters" registry of [IANA.OAuth.Parameters] established by

   Parameter name:  iss
   Parameter usage location:  authorization request, authorization
   Change Controller:  IETF
   Specification Document(s):  Section 2 of RFC 9207, [RFC9101], and
      Section 4.1.1 of [RFC7519].

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,

   [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,

   [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,

6.2.  Informative References

              Mainka, C., Mladenov, V., and J. Schwenk, "On the security
              of modern Single Sign-On Protocols: Second-Order
              Vulnerabilities in OpenID Connect", August 2015,

              Fett, D., Kuesters, R., and G. Schmitz, "A Comprehensive
              Formal Security Analysis of OAuth 2.0",
              DOI 10.1145/2976749.2978385, January 2016,

              IANA, "OAuth Parameters",

   [JARM]     Lodderstedt, T. and B. Campbell, "Financial-grade API: JWT
              Secured Authorization Response Mode for OAuth 2.0 (JARM)",
              October 2018,

              Lodderstedt, T., Bradley, J., Labunets, A., and D. Fett,
              "OAuth 2.0 Security Best Current Practice", Work in
              Progress, Internet-Draft, draft-ietf-oauth-security-
              topics-19, 16 December 2021,

              Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", November 2014,

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

   [RFC7591]  Richer, J., Ed., Jones, M., Bradley, J., Machulak, M., and
              P. Hunt, "OAuth 2.0 Dynamic Client Registration Protocol",
              RFC 7591, DOI 10.17487/RFC7591, July 2015,

   [RFC9101]  Sakimura, N., Bradley, J., and M. Jones, "The OAuth 2.0
              Authorization Framework: JWT-Secured Authorization Request
              (JAR)", RFC 9101, DOI 10.17487/RFC9101, August 2021,


   We would like to thank Brian Campbell, Roman Danyliw, Vladimir
   Dzhuvinov, Joseph Heenan, Takahiko Kawasaki, Torsten Lodderstedt,
   Christian Mainka, Vladislav Mladenov, Warren Parad, Aaron Parecki,
   and Rifaat Shekh-Yusef for their valuable feedback on this document.

Authors' Addresses

   Karsten Meyer zu Selhausen
   Email: karsten.meyerzuselhausen@hackmanit.de

   Daniel Fett
   Email: mail@danielfett.de