OAuth 2.0 step-up authorization challenge proto

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Table of Contents

1. Introduction

In simple API authorization scenarios, an authorization server will determine what claims to embed in the tokkens to issue on the basis of aspects such as the scopes requested, the resource, the identity of the client, and other characteristics known a provisioning time. Although that approach is viable in many situations, it falls short in several important circumstances. Consider, for instance, a FAPI 2.0 or SMART on FHIR regulated API requiring peculiar client authentication mechanism to be enforced or transaction specific details to be present in the token depending on whether the resource being accessed need to meet some rules estimated by the API itself, or a policy decision point it relies on, using a logic that is opaque to the authorization server.¶

This document extends the collection of error codes defined by [RFC6750] and by [RFC9470] with a new error codes, insufficient_delegated_authorization and new_authorization_needed, which can be used by resource servers to signal to clients that the authorization delegation represented by the access token presented with the request does not meet the authorization requirements of the resource server. This document also introduces associated payload definitions. The resource server can use these payloads to explicitly communicate to the client the required authorization details required.¶

The client can use that information to reach back to the authorization server with through a new authorization request that specifies the additional authorization details required to be described in the tokens to be issued. This document does not describe new methods to perform the new authorization request but will rely on OAuth 2.0 Rich Auhtorization Request [RFC9396], OAuth 2.0 Pushed Authorization Request [RFC9126], or OAuth 2.0 JWT-Secured Authorization Request [RFC9101].¶

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

2. Conventions and Definitions

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 [RFC6749].¶

3. Protocol Overview

The following is an end-to-end sequence of a typical step up authorization 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.¶

+----------+                                          +--------------+
|          |                                          |              |
|          |-----------(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 authorization details, wrong grant flow, or inadequate client authentication mechanism; hence, it denies the request and returns a challenge describing (using a combination of error code and payload details) what authorization details requirements must be met for the resource server to allow a request.¶

3. The client directs the user agent to the authorization server with an authorization request that includes the authorization details indicated by the resource server in the previous step.¶

4. Whatever sequence required by the grant of choice plays out; this will include the necessary steps to authenticate the client in accordance with the playoad, to use the required grant flow type, or to validate that the authorization details are met. Then, the authorization server returns a new access token to the client. The new access token contains or references information about the elements required, including but not limited to what [I-D.lombardo-oauth-client-extension-claims] defines.¶

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

6. The resource server finds that the authorization details, grant flow, or client authentication mechanism used 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 steps 2 and 6 imply that the resource server has a way of evaluating, on top of the requirements of authentication as defined in [RFC9470], the authorization requirements 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 authorization mechanisms used to obtain a token for the requested resource is called an "authorization state".¶

This document will not describe how the resource server can perform this assessment of the authorization state whatever the access token is a JSON Web Token (JWT) [RFC9068] or is validated via introspection [RFC7662] and whatever the resource provider is performing this assessment natively or by offloading the assessment to a policy decision point as defined in [XACML] and NIST's ABAC [SP.800-162] Still, this document describes how the resource provider can signal to the client any insatisfying authorization is expected to be obtained from the authorization server if applicable.¶

The terms "authorization state" and "step up" are metaphors in this specification. These metaphors do not suggest that there is an absolute hierarchy of authorization states expressed in interoperable fashion. The notion of a state emerges from the fact that the resource server may only want to accept certain authorization mechanisms. When presented with a token derived from particuliar authorization mechanisms (i.e., a given authorization state) that it does not want to accept (i.e., below the threshold it will accept), the resource server seeks to step up (i.e., renegotiate) from the current authorization state to one that it may accept. The "step up" metaphor is intended to convey a shift from the original authorization state 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 authorization state is common, in line with the connotation of the term "step up authorization", it is important to keep in mind that this might not necessarily hold true in the general case. For example, for a particular request, a resource server might require a higher authorization state and a shorter validity, resulting in a token suitable for one-off calls but leading to frequent prompts: hence, offering a suboptimal user experience if the token is reused for routine operations. In such a scenario, the client would be better served by keeping both the old tokens, which are associated with a lower authorization state, 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 with different scopes and permission levels. This document does not recommend any specific token-caching strategy: that choice 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).¶

4. Authorization Requirements Challenge

5. Auhtorization Request

A client receiving a challenge as defined in section 4 of this specification from the resource MUST take the action appropriate to each use case.¶

6. Authorization Response

An authorization server complying with this specification will react to the presence of the presented parameters and react as defined by the associated specification being, without limitations to, the response described in [RFC6749].¶

7. Information conveyed via the Access Token

To evaluate whether an access token meets the protected resource's requirements, the resource server will enforce the conventional token-validation logic before analysing the content of the payload of the token as defined by [RFC7519], [RFC6749], and [RFC9068] as long as any specification that applies to IANA registered claims.¶

8. Authorization Server Metadata

TODO¶

9. Deployment Considerations

This specification facilitates the communication of requirements from a resource server to a client, which, in turn, can enable a more granular and appropriate Authorization Request at the Authorization Server using either an OAuth 2.0 [RFC6749] defined grant flow, a Rich Authorization Request [RFC9396], a Push Authorization Request [RFC9126], or a JWT-Secured Authorization Request [RFC9101]. 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 pair establishes. 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 subjects or clients to comply with or that lead to an undesirable user-experience outcome.¶

10. 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-Authenticate [RFC6750],¶

• token introspection [RFC7662],¶

• authorization server metadata [RFC8414],¶

• Rich Authorization Request [RFC9396],¶

• Push Authorization Request [RFC9126], and¶

• JWT-Secured Authorization Request [RFC9101].¶

This specification does not attempt to define the mechanics by which access control is made by the resource provider and how the result of such access control evaluation should be translated into one of the challenges defined in Section 4. Still, such payload might unintentionally disclose information about the subject, the resource, the action to be performed, as long as context-specific data such as but not limited to authorization details that an attacker might use to gain knowledge about their target. Implementers should use care in determining what to disclose in the challenge and in what circumstances.¶

For this specification, the resource provider MUST examine the incoming access token and enforce the conventional token-validation logic - be it based on JWT validation, introspection, or any other method - before determining whether or not a challenge should be returned.¶

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 that feature.¶

11. IANA Considerations

This document has no IANA actions.¶

12. References

Appendix A. Use Cases

+----------+
|          |                  +--------------+
|          |---(1) prompt --->|              |                                                                                      +--------------+
|          |                  |              |-----------------------------------(2) LLM request ---------------------------------->|              |
|          |                  |              |                                                                                      |              |
|          |                  |              |<-----------------------------------(3) Tool usage -----------------------------------|              |
|  User    |                  |              |                        +--------------+                                              |              |
|          |                  |              |---(4) tool request --->|              |                                              |              |
|          |                  |              |                        |              |                           +--------------+   |              |
|          |                  |              |                        |              |---(5) service request --->|              |   |              |
|          |                  |     LLM      |                        |   LLM Tool   |                           | Service APIs |   |      LLM     |
|          |                  |    Agent     |                        |              |<--(6) service response ---|              |   |              |
|          |                  |              |                        |              |                           +--------------+   |              |
|          |                  |              |<--(7) tool response ---|              |                                              |              |
|          |                  |              |                        +--------------+                                              |              |
|          |                  |              |                                                                                      |              |
|          |                  |              |-----------------------------------(8) LLM request ---------------------------------->|              |
|          |                  |              |                                                                                      |              |
|          |                  |              |<----------------------------------(9) LLM outcome -----------------------------------|              |
|          |                  |              |                                                                                      +--------------+
|          |<-(10) response --|              |
|          |                  +--------------+
+----------+

Acknowledgments

TODO acknowledge.¶

Authors' Addresses