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OAuth 2.0 Step Up Authentication Challenge Protocol
draft-ietf-oauth-step-up-authn-challenge-17

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9470.
Authors Vittorio Bertocci , Brian Campbell
Last updated 2023-09-08 (Latest revision 2023-06-26)
Replaces draft-bertocci-oauth-step-up-authn-challenge
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Stream WG state Submitted to IESG for Publication
Document shepherd Rifaat Shekh-Yusef
Shepherd write-up Show Last changed 2023-01-20
IESG IESG state Became RFC 9470 (Proposed Standard)
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(None)
Consensus boilerplate Yes
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Responsible AD Roman Danyliw
Send notices to rifaat.s.ietf@gmail.com
IANA IANA review state Version Changed - Review Needed
IANA action state RFC-Ed-Ack
IANA expert review state Expert Reviews OK
draft-ietf-oauth-step-up-authn-challenge-17
Web Authorization Protocol                                   V. Bertocci
Internet-Draft                                                Auth0/Okta
Intended status: Standards Track                             B. Campbell
Expires: 28 December 2023                                  Ping Identity
                                                            26 June 2023

          OAuth 2.0 Step-up Authentication Challenge Protocol
              draft-ietf-oauth-step-up-authn-challenge-17

Abstract

   It is not uncommon for resource servers to require different
   authentication strengths or recentness according to the
   characteristics of a request.  This document introduces a mechanism
   for a resource server to signal to a client that the authentication
   event associated with the access token of the current request does
   not meet its authentication requirements and specify how to meet
   them.  This document also codifies a mechanism for a client to
   request that an authorization server achieve a specific
   authentication strength or recentness when processing an
   authorization request.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Web Authorization
   Protocol Working Group mailing list (oauth@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/oauth/.

   Source for this draft can be found at https://github.com/oauth-wg/
   oauth-step-up-authn-challenge.

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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 28 December 2023.

Copyright Notice

   Copyright (c) 2023 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 (https://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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions and Terminology . . . . . . . . . . . . . . .   3
   2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Authentication Requirements Challenge . . . . . . . . . . . .   6
   4.  Authorization Request . . . . . . . . . . . . . . . . . . . .   8
   5.  Authorization Response  . . . . . . . . . . . . . . . . . . .   8
   6.  Authentication Information Conveyed via Access Token  . . . .   9
     6.1.  JWT Access Tokens . . . . . . . . . . . . . . . . . . . .   9
     6.2.  OAuth 2.0 Token Introspection . . . . . . . . . . . . . .  10
   7.  Authorization Server Metadata . . . . . . . . . . . . . . . .  11
   8.  Deployment Considerations . . . . . . . . . . . . . . . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  OAuth Extensions Error Registration  . . . . . . . . . .  13
     10.2.  OAuth Token Introspection Response Registration  . . . .  13
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  13
   12. Informative References  . . . . . . . . . . . . . . . . . . .  14
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction

   In simple API authorization scenarios, an authorization server will
   determine what authentication technique to use to handle a given
   request on the basis of aspects such as the scopes requested, the
   resource, the identity of the client and other characteristics known
   at provisioning time.  Although the approach is viable in many
   situations, it falls short in several important circumstances.
   Consider, for instance, an eCommerce API requiring different

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   authentication strengths depending on whether the item being
   purchased exceeds a certain threshold, dynamically estimated by the
   API itself using a logic that is opaque to the authorization server.
   An API might also determine that a more recent user authentication is
   required based on its own risk evaluation of the API request.

   This document extends the error codes collection defined by [RFC6750]
   with a new value, insufficient_user_authentication, which can be used
   by resource servers to signal to the client that the authentication
   event associated with the access token presented with the request
   does not meet the authentication requirements of the resource server.
   This document also introduces acr_values and max_age parameters for
   the Bearer authentication scheme challenge defined by [RFC6750],
   which the resource server can use to explicitly communicate to the
   client the required authentication strength or recentness.

   The client can use that information to reach back to the
   authorization server with an authorization request specifying the
   authentication requirements indicated by protected resource, by
   including the acr_values or max_age authorization request parameters
   as defined in [OIDC].

   Those extensions will make it possible to implement interoperable
   step up authentication with minimal work from resource servers,
   clients and authorization servers.

1.1.  Conventions and Terminology

   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.

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

2.  Protocol Overview

   The following is an end-to-end sequence of a typical step-up
   authentication scenario implemented according to this specification.
   The scenario assumes that, before the sequence described below takes
   place, the client already obtained an access token for the protected
   resource.

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  +----------+                                          +--------------+
  |          |                                          |              |
  |          |-----------(1) request ------------------>|              |
  |          |                                          |              |
  |          |<---------(2) challenge ------------------|   Resource   |
  |          |                                          |    Server    |
  |  Client  |                                          |              |
  |          |-----------(5) request ------------------>|              |
  |          |                                          |              |
  |          |<-----(6) protected resource -------------|              |
  |          |                                          +--------------+
  |          |
  |          |
  |          |  +-------+                              +---------------+
  |          |->|       |                              |               |
  |          |  |       |--(3) authorization request-->|               |
  |          |  | User  |                              |               |
  |          |  | Agent |<-----------[...]------------>| Authorization |
  |          |  |       |                              |     Server    |
  |          |<-|       |                              |               |
  |          |  +-------+                              |               |
  |          |                                         |               |
  |          |<-------- (4) access token --------------|               |
  |          |                                         |               |
  +----------+                                         +---------------+

                     Figure 1: Abstract protocol flow

   1.  The client requests a protected resource, presenting an access
       token.

   2.  The resource server determines that the circumstances in which
       the presented access token was obtained offer insufficient
       authentication strength and/or recentness, hence it denies the
       request and returns a challenge describing (using a combination
       of acr_values and max_age) what authentication requirements must
       be met for the resource server to authorize a request.

   3.  The client directs the user agent to the authorization server
       with an authorization request that includes the acr_values and/or
       max_age indicated by the resource server in the previous step.

   4.  After whatever sequence required by the grant of choice plays
       out, which will include the necessary steps to authenticate the
       user in accordance with the acr_values and/or max_age values of
       the authorization request, the authorization server returns a new
       access token to the client.  The access token contains or
       references information about the authentication event.

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   5.  The client repeats the request from step 1, presenting the newly
       obtained access token.

   6.  The resource server finds that the user authentication performed
       during the acquisition of the new access token complies with its
       requirements, and returns the representation of the requested
       protected resource.

   The validation operations mentioned in step 2 and 6 imply that the
   resource server has a way of evaluating the authentication that
   occurred during the process by which the access token was obtained.
   In the context of this document, the assessment by the resource
   server of the specific authentication method used to obtain a token
   for the requested resource is called an "authentication level."  This
   document will describe how the resource server can perform this
   assessment of an "authentication level" when the access token is a
   JWT Access token [RFC9068] or is validated via introspection
   [RFC7662].  Other methods of determining the authentication level by
   which the access token was obtained are possible, per agreement by
   the authorization server and the protected resource, but are beyond
   the scope of this specification.  Given an authentication level of a
   token, the resource server determines whether it meets the security
   criteria for the requested resource.

   "Authentication level" and "step-up" are metaphors in this
   specification.  These metaphors do not suggest that there is an
   absolute hierarchy of authentication methods expressed in
   interoperable fashion.  The notion of a level emerges from the fact
   that the resource server may only want to accept certain
   authentication methods.  When presented with a token derived from a
   particular authentication method (i.e., a given authentication level)
   that it does not want to accept (i.e., below the threshold or level
   it will accept), the resource server seeks to "step-up" (i.e.,
   renegotiate) from the current authentication level to one that it may
   accept.  The "step-up" metaphor is intended to convey a shift from
   the original authentication level to one that is acceptable to the
   resource server.

   Although the case in which the new access token supersedes old tokens
   by virtue of a higher authentication level is common, in line with
   the intuition the term "step-up authentication" suggests, it is
   important to keep in mind that this might not necessarily hold true
   in the general case.  For example: a resource server might require
   for a particular request a higher authentication level and a shorter
   validity, resulting in a token suitable for one-off calls but leading
   to frequent prompts, hence a suboptimal user experience, if reused
   for routine operations.  In those scenarios, the client would be
   better served by keeping both the old tokens, associated with a lower

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   authentication level, and the new one - selecting the appropriate
   token for each API call.  This is not a new requirement for clients,
   as incremental consent and least privilege principles will require
   similar heuristics for managing access tokens associated to different
   scopes and permission levels.  This document does not recommend any
   specific token caching strategy, as that will be dependent on the
   characteristics of every particular scenario and remains application-
   dependent as in the core OAuth cases.  Also recall that OAuth 2.0
   [RFC6749] assumes access tokens are treated as opaque by clients.
   The token format might be unreadable to the client or might change at
   any time to become unreadable.  So, during the course of any token
   caching strategy, a client must not attempt to inspect the content of
   the access token to determine the associated authentication
   information or other details (see Section 6 of [RFC9068] for a more
   detailed discussion).

3.  Authentication Requirements Challenge

   This specification introduces a new error code value for the error
   parameter of the challenge of the Bearer authentication scheme from
   [RFC6750] and other OAuth authentication schemes, such as
   [I-D.ietf-oauth-dpop], which use the same error parameter:

   insufficient_user_authentication  The authentication event associated
      with the access token presented with the request does not meet the
      authentication requirements of the protected resource.

   Note: the logic through which the resource server determines that the
   current request does not meet the authentication requirements of the
   protected resource, and associated functionality (such as expressing,
   deploying and publishing such requirements) is out of scope for this
   document.

   Furthermore, this specification defines the following WWW-
   Authenticate auth-param values for those OAuth authentication schemes
   to convey the authentication requirements back to the client.

   acr_values  A space-separated string listing the authentication
      context class reference values, in order of preference, one of
      which the protected resource requires for the authentication event
      associated with the access token.  The authentication context, as
      defined in section 1.2 of [OIDC] conveys information about how
      authentication takes place (e.g., what authentication method(s) or
      assurance level to meet).

   max_age  Indicates the allowable elapsed time in seconds since the

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      last active authentication event associated with the access token.
      An active authentication event entails a user interacting with the
      authorization server in response to an authentication prompt.
      Note that while the auth-param value can be conveyed as a token or
      quoted-string (see section 11.2 of [RFC9110]), it has to represent
      a non-negative integer.

   Figure 2 below is an example of a Bearer authentication scheme
   challenge with the WWW-Authenticate header using the
   insufficient_user_authentication error code value to inform the
   client that the access token presented is not sufficient to gain
   access to the protected resource, and the acr_values parameter to let
   the client know that the expected authentication level corresponds to
   the authentication context class reference identified by myACR.

   Note that while this specification only defines usage of the above
   auth-params with the insufficient_user_authentication error code, it
   does not preclude future specifications or profiles from defining
   their usage with other error codes.

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Bearer error="insufficient_user_authentication",
     error_description="A different authentication level is required",
     acr_values="myACR"

   Figure 2: Authentication Requirements Challenge indicating acr_values

   The following example in Figure 3 shows a challenge informing the
   client that last active authentication event associated with the
   presented access token is too old and a more recent authentication is
   needed.

   HTTP/1.1 401 Unauthorized
   WWW-Authenticate: Bearer error="insufficient_user_authentication",
     error_description="More recent authentication is required",
     max_age="5"

     Figure 3: Authentication Requirements Challenge indicating max_age

   The auth-params max_age and acr_values MAY both occur in the same
   challenge if the resource server needs to express requirements both
   about recency and authentication levels.  If the resource server
   determines that the request is also lacking the scopes required by
   the requested resource, it MAY include the scope attribute with the
   scope necessary to access the protected resource, as described in
   section 3.1 of [RFC6750].

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4.  Authorization Request

   A client receiving a challenge from the resource server carrying the
   error code insufficient_user_authentication SHOULD parse the WWW-
   Authenticate header for acr_values and max_age and use them, if
   present, in constructing an authorization request, which is then
   conveyed to the authorization server's authorization endpoint via the
   user agent in order to obtain a new access token complying with the
   corresponding requirements.  Both acr_values and max_age
   authorization request parameters are OPTIONAL parameters defined in
   Section 3.1.2.1. of [OIDC].  This document does not introduce any
   changes in the authorization server behavior defined in [OIDC] for
   processing those parameters, hence any authorization server
   implementing OpenID Connect will be able to participate in the flow
   described here with little or no changes.  See Section 5 for more
   details.

   The example authorization request URI below, which might be used
   after receiving the challenge in Figure 2, indicates to the
   authorization server that the client would like the authentication to
   occur according to the authentication context class reference
   identified by myACR.

   https://as.example.net/authorize?client_id=s6BhdRkqt3
   &response_type=code&scope=purchase&acr_values=myACR

           Figure 4: Authorization Request indicating acr_values

   After the challenge in Figure 3, a client might direct the user agent
   to the following example authorization request URI where the max_age
   parameter indicates to the authorization server that the user
   authentication event needs to have occurred no more than five seconds
   prior.

   https://as.example.net/authorize?client_id=s6BhdRkqt3
   &response_type=code&scope=purchase&max_age=5

             Figure 5: Authorization Request indicating max_age

5.  Authorization Response

   Section 5.5.1.1 of [OIDC] establishes that an authorization server
   receiving a request containing the acr_values parameter MAY attempt
   to authenticate the user in a manner that satisfies the requested
   Authentication Context Class Reference, and include the corresponding
   value in the acr claim in the resulting ID Token.  The same section
   also establishes that in case the desired authentication level cannot
   be met, the authorization server SHOULD include in the acr claim a

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   value reflecting the authentication level of the current session (if
   any).  Furthermore, Section 3.1.2.1 [OIDC] states that if a request
   includes the max_age parameter, the authorization server MUST include
   the auth_time claim in the issued ID Token.  An authorization server
   complying with this specification will react to the presence of the
   acr_values and max_age parameters by including acr and auth_time in
   the access token (see Section 6 for details).  Although [OIDC] leaves
   the authorization server free to decide how to handle the inclusion
   of acr in the ID Token when requested via acr_values, when it comes
   to access tokens in this specification, the authorization server
   SHOULD consider the requested acr value as necessary for successfully
   fulfilling the request.  That is, the requested acr value is included
   in the access token if the authentication operation successfully met
   its requirements, or that the authorization request fails in all
   other cases, returning unmet_authentication_requirements as defined
   in [OIDCUAR].  The recommended behavior will help prevent clients
   getting stuck in a loop where the authorization server keeps
   returning tokens that the resource server already identified as not
   meeting its requirements hence known to be rejected as well.

6.  Authentication Information Conveyed via Access Token

   To evaluate whether an access token meets the protected resource's
   requirements, the resource server needs a way of accessing
   information about the authentication event by which that access token
   was obtained.  This specification provides guidance on how to convey
   that information in conjunction with two common access token
   validation methods: the one described in [RFC9068], where the access
   token is encoded in JWT format and verified via a set of validation
   rules, and the one described in [RFC7662], where the token is
   validated and decoded by sending it to an introspection endpoint.
   Authorization servers and resource servers MAY elect to use other
   encoding and validation methods, however those are out of scope for
   this document.

6.1.  JWT Access Tokens

   When access tokens are represented as JSON Web Tokens (JWT)
   [RFC7519], the auth_time and acr claims (per Section 2.2.1 of
   [RFC9068]) are used to convey the time and context of the user
   authentication event that the authentication server performed during
   the course of obtaining the access token.  It is useful to bear in
   mind that the values of those two parameters are established at user
   authentication time and will not change in the event of access token
   renewals.  See the aforementioned Section 2.2.1 of [RFC9068] for
   details.  The following is a conceptual example showing the decoded
   content of such a JWT access token.

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   Header:

   {"typ":"at+JWT","alg":"ES256","kid":"LTacESbw"}

   Claims:

   {
    "iss": "https://as.example.net",
    "sub": "someone@example.net",
    "aud": "https://rs.example.com",
    "exp": 1646343000,
    "iat": 1646340200,
    "jti" : "e1j3V_bKic8-LAEB_lccD0G",
    "client_id": "s6BhdRkqt3",
    "scope": "purchase",
    "auth_time": 1646340198,
    "acr": "myACR"
   }

                                  Figure 6

6.2.  OAuth 2.0 Token Introspection

   OAuth 2.0 Token Introspection [RFC7662] defines a method for a
   protected resource to query an authorization server about the active
   state of an access token as well as to determine metainformation
   about the token.  The following two top-level introspection response
   members are defined to convey information about the user
   authentication event that the authentication server performed during
   the course of obtaining the access token.

   acr  Authentication Context Class Reference.  String specifying an
      Authentication Context Class Reference value that identifies the
      Authentication Context Class that the user authentication
      performed satisfied.

   auth_time  Time when the user authentication occurred.  A JSON
      numeric value representing the number of seconds from
      1970-01-01T00:00:00Z UTC until the time of date/time of the
      authentication event.

   The following example shows an introspection response with
   information about the user authentication event by which the access
   token was obtained.

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

   {
     "active": true,
     "client_id": "s6BhdRkqt3",
     "scope": "purchase",
     "sub": "someone@example.net",
     "aud": "https://rs.example.com",
     "iss": "https://as.example.net",
     "exp": 1639528912,
     "iat": 1618354090,
     "auth_time": 1646340198,
     "acr": "myACR"
   }

                                  Figure 7

7.  Authorization Server Metadata

   Authorization Servers can advertise their support of this
   specification by including in their metadata document (as defined in
   [RFC8414]) the value acr_values_supported as defined in section 3 of
   [OIDCDISC].  The presence of acr_values_supported in the
   authorization server metadata document signals that the authorization
   server will understand and honor the acr_values and max_age
   parameters in incoming authorization requests.

8.  Deployment Considerations

   This specification facilitates the communication of requirements from
   a resource server to a client, which in turn can enable a smooth
   step-up authentication experience.  However, it is important to
   realize that the user experience achievable in every specific
   deployment is a function of the policies each resource server and
   authorization server pairs establish.  Imposing constraints on those
   policies is out of scope for this specification, hence it is
   perfectly possible for resource servers and authorization servers to
   impose requirements that are impossible for users to comply with, or
   leading to an undesirable user experience outcome.  The
   authentication prompts presented by the authorization server as a
   result of the method of propagating authentication requirements
   described here might require the user to perform some specific
   actions such as using multiple devices, having access to devices
   complying with specific security requirements, and so on.  Those
   extra requirements, concerning more about how to comply with a
   particular requirement rather than indicating the identifier of the
   requirement itself, are out of scope for this specification.

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9.  Security Considerations

   This specification adds to previously defined OAuth mechanisms.
   Their respective Security Considerations apply - OAuth 2.0 [RFC6749],
   JWT access tokens [RFC9068], Bearer WWW-Authentication [RFC6750],
   token introspection [RFC7662], and authorization server metadata
   [RFC8414].

   This document MUST NOT be used to position OAuth as an authentication
   protocol.  For the purposes of this specification, the way in which a
   user authenticated with the authorization server to obtain an access
   token is salient information, as a resource server might decide
   whether to grant access on the basis of how that authentication
   operation was performed.  Nonetheless, this specification does not
   attempt to define the mechanics by which authentication takes place,
   relying on a separate authentication layer to take care of the
   details.  In line with other specifications of the OAuth family, this
   document assumes the existence of a session without going into the
   details of how it is established or maintained, what protocols are
   used to implement that layer (e.g., OpenID Connect), and so forth.
   Depending on the policies adopted by the resource server, the
   acr_values parameter introduced in Section 3 might unintentionally
   disclose information about the authenticated user, the resource
   itself, the authorization server, and any other context-specific data
   that an attacker might use to gain knowledge about their target.  For
   example, a resource server requesting an acr value corresponding to a
   high level of assurance for some users but not others might identify
   possible high privilege users to target with spearhead phishing
   attacks.  Implementers should use care in determining what to
   disclose in the challenge and in what circumstances.  The logic
   examining the incoming access token to determine whether a challenge
   should be returned can execute either before or after the
   conventional token validation logic, be it based on JWT token
   validation, introspection, or any other method.  The resource server
   MAY return a challenge without verifying the client presented a valid
   token.  However, this approach will leak the required properties of
   an authorization token to an actor who has not proven they can obtain
   a token for this resource server.

   As this specification provides a mechanism for the resource server to
   trigger user interaction, it's important for the authorization server
   and clients to consider that a malicious resource server might abuse
   of that feature.

10.  IANA Considerations

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10.1.  OAuth Extensions Error Registration

   This specification requests registration of the following error value
   in the "OAuth Extensions Error" registry [IANA.OAuth.Params]
   established by [RFC6749].

   *  Name: insufficient_user_authentication

   *  Usage Location: resource access error response

   *  Protocol Extension: OAuth 2.0 Step-up Authentication Challenge
      Protocol

   *  Change controller: IETF

   *  Specification document(s): Section 3 of [[ this specification ]]

10.2.  OAuth Token Introspection Response Registration

   This specification requests registration of the following values in
   the "OAuth Token Introspection Response" registry [IANA.OAuth.Params]
   established by [RFC7662].

   Authentication Context Class Reference:

   *  Name: acr

   *  Description: Authentication Context Class Reference

   *  Change Controller: IETF

   *  Specification Document(s): Section 6.2 of [[ this specification ]]

   Authentication Time:

   *  Name: auth_time

   *  Description: Time when the user authentication occurred

   *  Change Controller: IETF

   *  Specification Document(s): Section 6.2 of [[ this specification ]]

11.  Normative References

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   [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>.

   [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>.

   [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>.

   [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>.

12.  Informative References

   [I-D.ietf-oauth-dpop]
              Fett, D., Campbell, B., Bradley, J., Lodderstedt, T.,
              Jones, M. B., and D. Waite, "OAuth 2.0 Demonstrating
              Proof-of-Possession at the Application Layer (DPoP)", Work
              in Progress, Internet-Draft, draft-ietf-oauth-dpop-16, 13
              April 2023, <https://datatracker.ietf.org/doc/html/draft-
              ietf-oauth-dpop-16>.

   [IANA.OAuth.Params]
              IANA, "OAuth Parameters",
              <https://www.iana.org/assignments/oauth-parameters>.

   [OIDC]     Sakimura, N., Bradley, J., Jones, M., de Medeiros, B., and
              C. Mortimore, "OpenID Connect Core 1.0 incorporating
              errata set 1", 8 November 2014,
              <https://openid.net/specs/openid-connect-core-1_0.html>.

   [OIDCDISC] Sakimura, N., Bradley, J., Jones, M., and E. Jay, "OpenID
              Connect Core 1.0 incorporating errata set 1", 8 November
              2014, <https://openid.net/specs/openid-connect-discovery-
              1_0.html>.

   [OIDCUAR]  Lodderstedt, T., "OpenID Connect Core Error Code
              unmet_authentication_requirements", 8 May 2019,
              <https://openid.net/specs/openid-connect-unmet-
              authentication-requirements-1_0.html>.

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   [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>.

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

   [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>.

   [RFC9068]  Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0
              Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October
              2021, <https://www.rfc-editor.org/info/rfc9068>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

Appendix A.  Acknowledgements

   I wanted to thank the Academy, the viewers at home, the shampoo
   manufacturers, etc..

   This specification was developed within the OAuth Working Group under
   the chairpersonship of Rifaat Shekh-Yusef and Hannes Tschofenig with
   Paul Wouters, and Roman Danyliw serving as Security Area Directors.
   Additionally, the following individuals contributed ideas, feedback,
   corrections, and wording that helped shape this specification: Caleb
   Baker, Ivan Kanakarakis, Pieter Kasselman, Aaron Parecki, Denis
   Pinkas, Dima Postnikov, and Filip Skokan.

   Some early discussion of the motivations and concepts that
   precipitated the initial version of this document occurred at the
   2021 OAuth Security Workshop.  The authors thank the organizers of
   the workshop (Guido Schmitz, Steinar Noem, and Daniel Fett) for
   hosting an event that is conducive to collaboration and community
   input.

Appendix B.  Document History

   [[ To be removed from the final specification ]]

   -17

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   *  Fix mistake in -16 update

   -16

   *  AD suggested editorial updates

   -15

   *  Editorial updates from IESG review/ballot

   -14

   *  Updates from Httpdir telechat review

   -13

   *  Make IETF the Change Controller for all registration requests per
      IANA suggestion

   *  More updates from Genart review

   *  Updates from Artart review

   *  Updates from Secdir review

   -12

   *  Updates from Genart Last Call review

   -11

   *  Updates in the Protocol Overview section clarifying the nature of
      "authentication levels" and caching strategies, addressing AD
      review comments

   -10

   *  Fix two references where the section numbers got lost presumably
      due to tooling issues

   -09

   *  Updates addressing AD review comments

   -07/-08

   *  Editorial updates addressing Shepherd Review comments

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   -06

   *  Update examples/figures to be clear that the authorization request
      is sent by the client via directing the user agent (not directly
      from client to AS)

   -05

   *  Forgotten Acknowledgements

   *  Minor updates to the updates in -04

   -04

   *  Editorial updates/notes from WGLC feedback

   -03

   *  Clarified that acr_values and max_age can co-occur in the
      challenge when necessary

   *  fleshed out deployment and security considerations

   *  fleshed out IANA considerations

   *  Attempt to clarify that while acr_values can request more than one
      value, only one of them is used and ends up in the token

   -02

   *  Fix typos introduced in -01

   *  Begin to fill out the Acknowledgements

   -01

   *  Added AS Metadata section with pointer to acr_values_supported

   *  Mention that it's not necessarily the case that a new 'stepped-up'
      token always supersedes older tokens

   *  Add examples with max_age

   -00 (Working Group Draft)

   *  Initial WG revision (content unchanged from draft-bertocci-oauth-
      step-up-authn-challenge-01)

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   -01 draft-bertocci-oauth-step-up-authn-challenge

   *  Fixed example

   *  Clarified/noted that scope can also be in the WWW-Authenticate/401

   -00 draft-bertocci-oauth-step-up-authn-challenge

   *  Initial Individual Draft (with all the authority thereby bestowed
      https://datatracker.ietf.org/doc/html/draft-abr-twitter-reply
      (https://datatracker.ietf.org/doc/html/draft-abr-twitter-reply)).

Authors' Addresses

   Vittorio Bertocci
   Auth0/Okta
   Email: vittorio@auth0.com

   Brian Campbell
   Ping Identity
   Email: bcampbell@pingidentity.com

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