Web Authorization Protocol J. Lombardo Internet-Draft AWS Intended status: Informational A. Babeanu Expires: 25 November 2025 IndyKite 24 May 2025 OAuth 2.0 step-up authorization challenge proto draft-lombardo-oauth-step-up-authz-challenge-proto-00 Abstract It is not uncommon for resource servers to require additional information like details of delegation authorization, or assurance proof of the delegation of authorization mechanism used according to the characteristics of a request. This document introduces a mechanism that resource servers can use to signal to a client that the data and metadata associated with the access token of the current request does not meet its authorization requirements and, further, how to meet them. This document also codifies a taxonomy to guide the client into starting a new request towards the authorization server in order to get issued, if applicable, a new set of tokens matching the requirements. About This Document This note is to be removed before publishing as an RFC. The latest revision of this draft can be found at https://identitymonk.github.io/draft-lombardo-oauth-step-up-authz- challenge/draft-lombardo-oauth-step-up-authz-challenge-proto.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-lombardo-oauth-step-up-authz- challenge-proto/. Discussion of this document takes place on the Web Authorization Protocol mailing list (mailto:oauth@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/oauth/. Subscribe at https://www.ietf.org/mailman/listinfo/oauth/. Source for this draft and an issue tracker can be found at https://github.com/identitymonk/draft-lombardo-oauth-step-up-authz- challenge. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Lombardo & Babeanu Expires 25 November 2025 [Page 1] Internet-Draft STEPZ May 2025 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." This Internet-Draft will expire on 25 November 2025. Copyright Notice Copyright (c) 2025 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4 3. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 4 4. Authorization Requirements Challenge . . . . . . . . . . . . 7 4.1. HTTP Error Status Code . . . . . . . . . . . . . . . . . 7 4.2. Error Codes . . . . . . . . . . . . . . . . . . . . . . . 7 4.3. WWW-Authenticate Header Error Code And Associated Payload . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.3.1. Case Of insufficient_delegated_authorization Error . 8 4.3.2. Case Of new_authorization_needed Error . . . . . . . 9 5. Auhtorization Request . . . . . . . . . . . . . . . . . . . . 10 5.1. Case Of insufficient_delegated_authorization Error . . . 10 5.1.1. Case Of new_authorization_needed Error . . . . . . . 10 6. Authorization Response . . . . . . . . . . . . . . . . . . . 12 7. Information conveyed via the Access Token . . . . . . . . . . 12 8. Authorization Server Metadata . . . . . . . . . . . . . . . . 12 9. Deployment Considerations . . . . . . . . . . . . . . . . . . 12 10. Security Considerations . . . . . . . . . . . . . . . . . . . 12 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 Lombardo & Babeanu Expires 25 November 2025 [Page 2] Internet-Draft STEPZ May 2025 12.1. Normative References . . . . . . . . . . . . . . . . . . 13 12.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Use Cases . . . . . . . . . . . . . . . . . . . . . 15 A.1. LLM Agent accessing a service via an LLM Tool on behalf of a user . . . . . . . . . . . . . . . . . . . . . . . . . 16 A.1.1. Preconditions . . . . . . . . . . . . . . . . . . . . 17 A.1.2. LLM Agent receives a response fromn the LLM . . . . . 17 A.1.3. LLM Tool receives the request . . . . . . . . . . . . 17 A.1.4. LLM Tool obtains a set of token from Authorization Server protecting the API . . . . . . . . . . . . . . 19 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 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]. Lombardo & Babeanu Expires 25 November 2025 [Page 3] Internet-Draft STEPZ May 2025 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. Lombardo & Babeanu Expires 25 November 2025 [Page 4] Internet-Draft STEPZ May 2025 +----------+ +--------------+ | | | | | |-----------(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. Lombardo & Babeanu Expires 25 November 2025 [Page 5] Internet-Draft STEPZ May 2025 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 Lombardo & Babeanu Expires 25 November 2025 [Page 6] Internet-Draft STEPZ May 2025 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 4.1. HTTP Error Status Code When a request compliant with this specification does not meet the authorization state requirements, the resource server responds using the 403 HTTP status code with the Bearer authentication scheme's error parameter (from [RFC6750]) . 4.2. Error Codes This specification introduces two new error code values for this challenge and other OAuth authentication schemes, as defined in [RFC9449] and in [RFC9470]: insufficient_delegated_authorization: The authorization mechansisms used for the issuance of the access token presented with the request does not meet the authorization state requirements of the protected resource. It is up to the client to decide what to perform next. new_authorization_needed: The authorization mechansisms used for the Lombardo & Babeanu Expires 25 November 2025 [Page 7] Internet-Draft STEPZ May 2025 issuance of the access token presented with the request does not meet the authorization state requirements of the protected resource. The client is guided on why new authorization mechanisms and details SHOULD be used through a new authorization request to the authorization server. Note: the logic through which the resource server determines that the current request does not meet the authorization state requirements of the protected resource, and associated functionality (such as expressing, deploying and publishing such requirements), is out of scope for this document. 4.3. WWW-Authenticate Header Error Code And Associated Payload Each error code can return different types of HTTP payload to guide the client into initiating the next Authorization Request. 4.3.1. Case Of insufficient_delegated_authorization Error If the error code insufficient_delegated_authorization is used, the HTTP payload MUST be formatted as an AuthZEN [D-OpenID-AuthZEN] response for a decision that is false and MUST include as part of the response context: error_msg: _REQUIRED_ - a string representing a human readable justification of the resource provider access control leading to a deny of the request. details: _REQUIRED_ - a valid [RFC8259] JSON structure The details JSON structure MUST include only one of the following element: expected_claims: _OPTIONAL_ - a list of space-delimited, case- sensitive strings representing the claim names that needs to be provided. expected_values: _OPTIONAL_ - a valid JSON [RFC8259] structure that will indicate the claim names that needs to be provided as JSON keys and the values that could be considered valid as a JSON array of values. pdp_message: _OPTIONAL_ - a valid JSON [RFC8259] structure representing the error details in a format of the resource provider or of the policy decision point that is the authority for the resource provider. Lombardo & Babeanu Expires 25 November 2025 [Page 8] Internet-Draft STEPZ May 2025 The following is a non-normative example of a WWW-Authenticate header with the error code insufficient_delegated_authorization and an ssociated payload: ```http HTTP/1.1 403 Forbidden WWW-Authenticate: Bearer error="insufficient_user_authorization", error_description="A different authorization level is required" { "decision": false, "context": { "error_msg": "The user must belongs to a project to access the resource", "details": { "expected_values": { "project" : ["phoenix", "eagle"] } } } } ``` This specification does not provide any guidance on which format is preferred when providing a pdp_message and leave this to the decision of the resource provide implementers. The authors still recommend that implements profile their format to ease the interoperbility between resource providers, clients, and potentially authorization servers. 4.3.2. Case Of new_authorization_needed Error If the error code new_authorization_needed is used, the HTTP payload MUST be formatted a valid JSON [RFC8259] structure wich will include: method: _REQUIRED_ - The value of this claim MUST be an absolute URI that can be registered with IANA. It SHOULD support present, future or custom values. If IANA registered URIs are used, then their meaning and semantics should be respected and used as defined in the registry. Parties using this custom claim values need to agree upon the semantics of the values used, which may be context specific. This specification recognizes primarily the value urn:ietf:params:oauth:grant-ext:rar defined by Rich Authorization Request [RFC9396] and urn:ietf:params:oauth:grant- ext:par defined by Pushed Authorization Request [RFC9126]. The structure MUST then include one of the following elements: authorization_details: _OPTIONAL_ - in JSON notation, an array of objects as defined in section 2 of Rich Authorization Request [RFC9396] jar: _OPTIONAL_ - a JSON Web Token (JWT) [RFC7519] whose JWT Claims Set holds the JSON-encoded OAuth 2.0 authorization request parameters as defined in section 2.1 of JWT-Secured Authorization Request [RFC9101] reference: _OPTIONAL_ - an absolute URI that references the set of Lombardo & Babeanu Expires 25 November 2025 [Page 9] Internet-Draft STEPZ May 2025 parameters comprising an OAuth 2.0 authorization request in a form of a Request Object as defined in section 2.2 of JWT-Secured Authorization Request [RFC9101] 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. 5.1. Case Of insufficient_delegated_authorization Error In this case, the client MUST determine by itself what is the best course of action for the next action. It SHALL at least provide feedback to the user of the situation but it CAN decide to trigger any OAuth2 grant flow by following the RFC that applied to it. 5.1.1. Case Of new_authorization_needed Error In this case, the client MUST follow the requirements fixed by the Resource Provider. The following situations CAN occur: +============================+=====================+=========+=========+=============+ |method |authorization_details|jar |reference|Expectation | | | | | |of the client| +============================+=====================+=========+=========+=============+ |urn:ietf:params:oauth:grant-|Set as specified |empty |empty |Starts an | |ext:rar | | | |Authorization| | | | | |Request as | | | | | |defined by | | | | | |RAR with the | | | | | |authorization| | | | | |details | +----------------------------+---------------------+---------+---------+-------------+ |urn:ietf:params:oauth:grant-|empty |Set as |empty |Starts an | |ext:rar | |specified| |Authorization| | | | | |Request as | | | | | |defined in | | | | | |RAR with the | | | | | |Request | | | | | |Object in JWT| | | | | |format | +----------------------------+---------------------+---------+---------+-------------+ |urn:ietf:params:oauth:grant-|empty |empty |Set as |Starts an | |ext:rar | | |specified|Authorization| | | | | |Request as | | | | | |defined in | | | | | |RAR with the | Lombardo & Babeanu Expires 25 November 2025 [Page 10] Internet-Draft STEPZ May 2025 | | | | |Request | | | | | |Object URI | | | | | |(it would | | | | | |mean the | | | | | |Resource | | | | | |Provider will| | | | | |have send the| | | | | |Request | | | | | |Object to AS | | | | | |directly) | +----------------------------+---------------------+---------+---------+-------------+ |urn:ietf:params:oauth:grant-|Set as specified |empty |empty |Starts an | |ext:par | | | |Authorization| | | | | |Request as | | | | | |defined by | | | | | |PAR with the | | | | | |authorization| | | | | |details | +----------------------------+---------------------+---------+---------+-------------+ |urn:ietf:params:oauth:grant-|empty |Set as |empty |Starts an | |ext:par | |specified| |Authorization| | | | | |Request as | | | | | |defined in | | | | | |PAR with the | | | | | |Request | | | | | |Object in JWT| | | | | |format | +----------------------------+---------------------+---------+---------+-------------+ |urn:ietf:params:oauth:grant-|empty |empty |Set as |Starts an | |ext:par | | |specified|Authorization| | | | | |Request as | | | | | |defined in | | | | | |PAR with the | | | | | |Request | | | | | |Object URI | | | | | |(it would | | | | | |mean the | | | | | |Resource | | | | | |Provider will| | | | | |have send the| | | | | |Request | | | | | |Object to AS | | | | | |directly) | +----------------------------+---------------------+---------+---------+-------------+ Table 1 Lombardo & Babeanu Expires 25 November 2025 [Page 11] Internet-Draft STEPZ May 2025 This specification recognizes that other method URI might be defined in the future. The associated specifications SHALL defined the expected behavior for such new methods. 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], Lombardo & Babeanu Expires 25 November 2025 [Page 12] Internet-Draft STEPZ May 2025 * 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 12.1. Normative References [D-OpenID-AuthZEN] GAzitt, O., Ed., Brossard, D., Ed., and A. Tulshibagwale, Ed., "Authorization API", 2025, . Lombardo & Babeanu Expires 25 November 2025 [Page 13] Internet-Draft STEPZ May 2025 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework", RFC 6749, DOI 10.17487/RFC6749, October 2012, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data Interchange Format", STD 90, RFC 8259, DOI 10.17487/RFC8259, December 2017, . [RFC9068] Bertocci, V., "JSON Web Token (JWT) Profile for OAuth 2.0 Access Tokens", RFC 9068, DOI 10.17487/RFC9068, October 2021, . [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, . [RFC9126] Lodderstedt, T., Campbell, B., Sakimura, N., Tonge, D., and F. Skokan, "OAuth 2.0 Pushed Authorization Requests", RFC 9126, DOI 10.17487/RFC9126, September 2021, . [RFC9396] Lodderstedt, T., Richer, J., and B. Campbell, "OAuth 2.0 Rich Authorization Requests", RFC 9396, DOI 10.17487/RFC9396, May 2023, . 12.2. Informative References Lombardo & Babeanu Expires 25 November 2025 [Page 14] Internet-Draft STEPZ May 2025 [I-D.lombardo-oauth-client-extension-claims] Lombardo, J. and A. Babeanu, "OAuth 2.0 client extension claims", Work in Progress, Internet-Draft, draft-lombardo- oauth-client-extension-claims-00, 11 April 2025, . [RFC7662] Richer, J., Ed., "OAuth 2.0 Token Introspection", RFC 7662, DOI 10.17487/RFC7662, October 2015, . [RFC8414] Jones, M., Sakimura, N., and J. Bradley, "OAuth 2.0 Authorization Server Metadata", RFC 8414, DOI 10.17487/RFC8414, June 2018, . [RFC9449] Fett, D., Campbell, B., Bradley, J., Lodderstedt, T., Jones, M., and D. Waite, "OAuth 2.0 Demonstrating Proof of Possession (DPoP)", RFC 9449, DOI 10.17487/RFC9449, September 2023, . [RFC9470] Bertocci, V. and B. Campbell, "OAuth 2.0 Step Up Authentication Challenge Protocol", RFC 9470, DOI 10.17487/RFC9470, September 2023, . [RFC9728] Jones, M.B., Hunt, P., and A. Parecki, "OAuth 2.0 Protected Resource Metadata", RFC 9728, DOI 10.17487/RFC9728, April 2025, . [SP.800-162] Hu, V., Ed., Ferraiolo, D., Ed., Kuhn, R., Ed., Schnitzer, A., Ed., Sandlin, K., Ed., Miller, R., Ed., and K. Scarfone, Ed., "Guide to Attribute Based Access Control (ABAC) Definition and Considerations", 2014, . [XACML] Godik, S., Ed. and T. M. (Ed.), Ed., "eXtensible Access Control Markup Language (XACML) Version 1.1", 2006, . Appendix A. Use Cases Lombardo & Babeanu Expires 25 November 2025 [Page 15] Internet-Draft STEPZ May 2025 A.1. LLM Agent accessing a service via an LLM Tool on behalf of a user LLM agents, including those based on large language models (LLMs), are designed to manage user context, memory, and interaction state across multi-turn conversations. To perform complex tasks, these agents often integrate with external systems such as SaaS applications, internal services, or enterprise data sources. When accessing these systems, the agent operates on behalf of the end user, and its actions are constrained by the user’s identity, role, and permissions as defined by the enterprise. This ensures that all data access and operations are properly scoped and compliant with organizational access controls. When using an LLM Tool, the LLM Agent is consumming an external API which has its own access control logic and policies on what conditions should be met for the access to the resources and the generation of the enriched information that the AI Agent can send return to the user who prompted it, with potential support of the LLM. Some access control conditions might require some authorization details as defined into, whithout being limited to, the examples of Rich Authorization Request [RFC9396]. A non normative example of such interaction would be at the functional level: +----------+ | | +--------------+ | |---(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 --| | | | +--------------+ +----------+ Lombardo & Babeanu Expires 25 November 2025 [Page 16] Internet-Draft STEPZ May 2025 _Figure 2: Abstract AI Agent Use Case Flow_ A.1.1. Preconditions * The LLM Agent has a registered OAuth 2.0 Client (com.example.llm- agent) with the Enterprise IdP (idp.example.com) * The LLM Tool has a registered OAuth 2.0 Client (4960880b83dc9) with the Enterprise IdP (idp.example.com) * The LLM Tool has a registered OAuth 2.0 Client (eb1e27d2df8b7) with External Service IdP (authorization-server.saas.net) * The External Service APIs is protected by the Trust Domain controlled by the External Service IdP (authorization- server.saas.net) * User already authenticated at the Enterprise IdP (idp.example.com) and delegated its authorization to the LLM Agent * The LLM Agent is in possession of an Identity Token, an Access Token, and a Refresh Token issued by the Enterprise IdP (idp.example.com) * We assume that the Access Token is valid for the duration of this example and possess the appropriate scopes and claims to be authorized to call the LLM Tool A.1.2. LLM Agent receives a response fromn the LLM LLM Agent receives a directive to use the LLM Tool with a specific payload and it calls the external LLM Tool provided by an Enterprise internal IT with a valid access token. ```http POST /Pay Host: tool.example.com Authorization: Bearer ejyfewfewfwefwefewf.e.fwefwe.fw.e.fwef to=DE02100100109307118603& amount=123.50 ``` A.1.3. LLM Tool receives the request LLM tool tries to call the service with the External Service APIs with the Access Token received using the JWT Bearer Authentication scheme (5) and is issued an authentication challenge. ```http HTTP/1.1 403 Forbidden WWW-Authenticate: Bearer error="new_authorization_needed", error_description="A new authorization request is needed" Lombardo & Babeanu Expires 25 November 2025 [Page 17] Internet-Draft STEPZ May 2025 { "decision": false, "context": { "error_msg": "The user must be authorized to initiate payment for Merchant A ", "details": { "method": "urn:ietf:params:oauth:grant-ext:rar", "authorization_details": [ { "type": "payment_initiation", "actions": [ "initiate", "status", "cancel" ], "locations": [ "https://example.com/payments" ], "instructedAmount": { "currency": "EUR", "amount": "123.50" }, "creditorName": "Merchant A", "creditorAccount": { "iban": "DE02100100109307118603" }, "remittanceInformationUnstructured": "Ref Number Merchant" } ] } } } ``` > Note: How agents discover available tools is out of scope of this specification LLM Agent fetches the external tool resource's OAuth 2.0 Protected Resource Metadata per [RFC9728] to dynamically discover an authorization server that can issue an access token for the resource. ```http GET /.well-known/oauth-protected-resource Host: api.saas.net Accept: application/json HTTP/1.1 200 Ok Content-Type: application/json { "resource": "https://api.saas.net/", "authorization_servers": [ "https://authorization-server.saas.net" ], "bearer_methods_supported": [ "header", "body" ], "scopes_supported": [ "agent.tools.read", "agent.tools.write" ], "resource_documentation": "https://idp.saas.net/tools/ resource_documentation.html" } ``` LLM Agent discovers the Authorization Server configuration per [RFC8414]. ```http GET /.well-known/oauth-authorization-server Host: authorization-server.saas.net Accept: application/json HTTP/1.1 200 Ok Content-Type: application/json { "issuer": "https://authorization-server.saas.net", "authorization_endpoint": "https://authorization- server.saas.net/oauth2/authorize", "token_endpoint": "https://authorization-server.saas.net/oauth2/token", "jwks_uri": "https://authorization-server.saas.net/oauth2/keys", "registration_endpoint": "authorization-server.saas.net/oauth2/ register", "scopes_supported": [ "agent.read", "agent.write" ], "response_types_supported": [ "code" ], "grant_types_supported": [ "authorization_code", "refresh_token" ] } ``` LLM Agent has learned all necessary endpoints and supported capabilites to obtain an access token for the external tool. Lombardo & Babeanu Expires 25 November 2025 [Page 18] Internet-Draft STEPZ May 2025 A.1.4. LLM Tool obtains a set of token from Authorization Server protecting the API The LLM tool redirects the LLM Agent for an authorization request: http GET /oauth2/authorize?response_type=code &client_id=eb1e27d2df8b7 &state=af0ifjsldkj &redirect_uri=https%3A%2F%2Ftool.example.com%2Fcb &code_challenge_method=S256 &code_challenge=K2- ltc83acc4h0c9w6ESC_rEMTJ3bwc-uCHaoeK1t8U &authorization_details=%5B%7B%22type%22:%20%22payment_initiation %22,%22actions%22:%20%5B%22initiate%22,%22status%22,%22cancel%22 %5D,%22locations%22:%20%5B%22https://example.com/payments%22%5D, %22instructedAmount%22:%20%7B%22currency%22:%20%22EUR%22,%22amount %22:%20%22123.50%22%7D,%22creditorName%22:%20%22Merchant%20A%22, %22creditorAccount%22:%20%7B%22iban%22:%20%22DE02100100109307118603 %22%7D,%22remittanceInformationUnstructured%22:%20%22Ref%20Number %20Merchant%22%7D%5D Host: authorization-server.saas.net We don't describe the wy the user is authenticated as it follows Rich Authorization Request [RFC9396] The LLM Tool will receive an Authorization Code that it will be able to exchange for a set of JWTs issued by the Authorization Server protecting the API. The LLM Tool can then make a new request to the External Service APIs (5). If it can meet the APIs Access Control requirement, the flow will follow with a response (6). Acknowledgments TODO acknowledge. Authors' Addresses Jean-François Lombardo AWS Canada Email: jeffsec@amazon.com Alexandre Babeanu IndyKite Canada Email: alex.babeanu@indykite.com Lombardo & Babeanu Expires 25 November 2025 [Page 19]