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SPIRL

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Security Web Authorization Protocol dynamic client registration SPIFFE platform credentials An OAuth 2.0 client requires specific information to interact with an authorization server, including a client identifier registered with the authorization server. The OAuth 2.0 Dynamic Client Registration Protocol defines a mechansim for dynamic client registration that removes the need for manual registration. This provides a more scalable mechanism that can be used by clients that do not have a pre-existing relationship with an authorization server, or where manually configuring such relationships is prohibitive from a cost and scale perspective. Examples of deployments that benefit from dynamic client registration includes modern cloud architectures where microservices are created on demand to meet scale requirements or for use with emerging protocols like the Model Context Protocol (MCP) which requires clients to register with an authorization server even if they do not have a predefined relationship with the authorization server. Similar to modern cloud native workloads, MCP service integrations may be ephemeral in nature. The OAuth 2.0 Dynamic Client Registration Protocol defines a software statement that includes metadata about the client and is signed by a developer or trusted third party. This specification describes the use of specific third party credentials issued to workloads and applications as software statements. The specification describes two types of credentials that may be used as software statements. The first is Secure Production Identity Framework For Everyone (SPIFFE) credentials. The second is the use of JWT representation of a Verifiable Credentials About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Web Authorization Protocol Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The OAuth framework is a widely deployed authorization protocol standard that enables applications to obtain limited access to user resources. OAuth clients must be registered with the OAuth authorization server, which poses significant operational challenges in dynamically scaling environments. Manual registration and subsequent lifecycle management is common, but is costly, difficult to scale and hard to maintain throughout the lifecycle of a client. These challenges are exacerbated by the increasingly dynamic nature of modern cloud native workloads that are instantiated as needed. In addition, emerging protocols like Model Context Protocol (MCP) requires clients to register with an authorization server, but these clients may not always have a predefined relationship with the authorization server, and similar to modern cloud native workloads, may be ephemeral in nature. defines a mechansim for dynamic client registration that removes the need for manual registration. This provides a more scalable mechanism that can be used by clients that do not have a pre-existing relationship with an authorization server, or where manually configuring such relationships is prohibitive from a cost and scale perspective. It defines the concept of a software statement, which is a JSON Web Token (JWT) that asserts metadata values about the client that is signed by a developer or trusted third party. This specification describes the use of SPIFFE JWT-SVIDs and Verifiable Credentials as software statements that can be submitted as software statements when using OAuth 2.0 Dynamic Client Registration Protocol .

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 when, and only when, they appear in all capitals, as shown here.

Overview This specification describes the use of two credential types as software statements, namely Secure Production Identity Framework For Everyone (SPIFFE) credentials and Verifiable Credentials (VC).

SPIFFE Credentials as Software Statements The Secure Production Identity Framework For Everyone (SPIFFE) is a graduated Cloud Native Compute Foundation project designed to dynamically attest and verify workload identity (see . SPIFFE is commonly deployed to support large scale deployment of workloads in enterprise and cloud native compute environments. It ensures that every workload is attested and issued with X.509-SVID and JWT-SVID credentials. JWT-SVIDs contain a minimal set of claims, but may be extended to include additional claims, including those defined in the Dynamic Client Registration specification (see Section 2 of ). A SPIFFE issuer may attest a workload, add additional metadata based on the attestation information and issue a JWT-SVID. Workloads that are provisioned with JWT-SVIDs present these credentials as software statements when using Dynamic Client Registration. The registration endpoint is configured to trust the SPIFFE issuer and can verify the JWT-SVID, as well as retrieve additional claims that represents metadata attested to by the SPIFFE issuer. Upon succesful validation of the software statement, the registration endpoint completes the client registration. When SPIFFE JWT-SVIDs are used, the authorization server MUST NOT provision additional client secrets, as the workload can use its JWT-SVID or X.509 SVID to authenticate when initiating any OAuth flow. This removes the risk of additional secrets proliferation, reduces the overheads of securing and managing secrets and improves the overall risk posture.

Verifiable Credentials as Software Statements A Verifiable Credential (VC) is a tamper-evident digital assertion made by an issuer about a subject, which can be cryptographically verified by a third party. VCs follow an issuer–holder–verifier model where the issuer creates and signs the credential, the holder stores and presents it, and the verifier checks its authenticity and validity. VCs can be represented as a JSON Web Token (JWT) . A VC encoded as a JWT can serve as a software statement in dynamic client registration by including metadata as defined in Section 2 of about the client as additional claims in the VC. Workloads that are provisioned with VCs present these credentials as software statements when using Dynamic Client Registration. The registration endpoint is configured to trust the VC issuer and can verify the VC and use the additional metadata claims included by the issuer. Upon succesful validation of the software statement, the registration endpoint completes the client registration. The use of Verifiable Credentials for client authentication is currently undefined and it is up to the authorization server to make a risk based decision on which authentication methods to use.

SPIFFE and VC Deployment Models SPIFFE and VC address different deployment models. SPIFFE is more commonly deployed within large enterprise compute environments and provides high levels of assurance about a workload identity (in this case an OAuth client) along with a highly robust, low latency provisioning pipeline with built in credential lifecycle management to maximise availability. It is commonly deployed as an alternative to managing secrets. VCs provide an option for clients that are deployed outside an enterprise where SPIFFE is not deployed. These clients use VC issuance protocols to obtain VCs. The use of these protocols or any attestation that needs to be provided is out of scope for this specification.

Protocol Flow | Client | | Developer | with SPIFFE JWT-SVID or VC | Registration | | | as software statement | Endpoint | | | | | | |<-(D)- Client Information Response ---| | | | or Client Error Response +---------------+ +-----------+ ]]> Figure 1: Abstracted Dynamic Client Registration Flow with Software Statement The OAuth 2.0 Client Dynamic Registration flow illustrated in Figure 1 describes the interaction between the SPIFFE or VC issuers, the client and the client registration endpoint defined in . It includes the following steps:

• (A) The client registration endpoint establishes a trust relationship with the SPIFFE or VC Issuer. Once trust is established, the client registration endpoint can verify credentials issued by the SPIFFE or VC Issuer when presented as software statements.
• (B) The client is attested and credentialed by the SPIFFE Issuer , after which it assigns a SPIFFE ID , along with SPIFFE credentials (e.g. JWT-SVID and X.509-SVID ). Alternatively a VC issuer issues and provisions a VC. Issuers may include additional claims containing metadata (e.g the request URI).
• (C) The client calls the client registration endpoint with the client's desired registration metadata, and the software statement (either a SPIFFE JWT-SVID or a VC).
• (D) The client registration endpoint verifies the SPIFFE JWT-SVID or VC that was presented as a software statement. It extracts any additional claims that should be used as metadata before registering the client. It may return additional metadata, client identifiers and optionally credentials if a VC is used. Clients with SPIFFE credentials MUST use the SPIFFE credentials when authenticating as part of an OAuth flow.

Issuer and Client Registration Endpoint Trust Relationship

SPIFFE Issuer and Client Registration Endpoint Trust Relationship SPIFFE makes provision for multiple Trust Domains, which are represented in the workload identifier. Trust Domains offers additional segmentation within a SPIFFE deployment and each Trust Domain has its own keys for signing credentials. The OAuth authorization server may choose to trust one or more trust domains as defined in .

Verifiable Credentials and Client Registration Endpoint Trust Relationship To validate Verifiable Credentials (VCs) during dynamic client registration, the client registration endpoint MUST be configured to trust the VC issuer’s public keys or a certificate chain anchored in a trusted root. This trust may be established statically or through a dynamic trust discovery mechanism. The registration endpoint SHOULD periodically refresh or revalidate the issuer’s key material against a trusted source to ensure ongoing integrity. When a client presents a VC, the registration endpoint verifies its signature using the trusted key material. It MAY also perform additional validation steps, such as checking the issuer’s identity, validating the credential’s expiration, and evaluating its revocation status using mechanisms such as decentralized revocation registries or issuer-specific status endpoints.

Client Registration Endpoint Processing The client registration endpoint MUST process software statements (e.g., SPIFFE JWT-SVIDs or Verifiable Credentials) as follows: 1. Verify Authenticity and Integrity: Validate the software statement's signature using a trusted issuer key (e.g., SPIFFE Trust Bundle or VC issuer metadata). 2. Validate Credential Expiry: Check that the credential is not expired and is within its valid time window. 3. Evaluate Revocation Status (if applicable): Optionally verify revocation status via decentralized registries or issuer-specific status endpoints (for VCs). 4. Extract and Validate Metadata Claims: Parse the credential and validate required claims needed for OAuth client registration (e.g., client_name, redirect_uris, grant_types). 5. Register the Client: If all validations succeed, proceed with client registration using the verified metadata. 6. Return Response: Send an appropriate OAuth client registration response.

SPIFFE JWT-SVIDs Processing by Client Registration Endpoint When a client presents a SPIFFE JWT-SVID as a software statement, the client registration endpoint MUST:

1. Verify the Signature: Validate the JWT signature using the trusted key material from the SPIFFE Trust Bundle corresponding to the issuer’s trust domain.
2. Validate Standard Claims: Ensure that the JWT includes and satisfies the following claims:
   • iss: Must match the expected SPIFFE trust domain.
   • sub: Must be a valid SPIFFE ID URI identifying the client.
   • aud: Must include the registration endpoint’s audience value.
   • exp and iat: Must be present and within acceptable bounds.
3. Extract and Validate Metadata Claims: Parse and validate any additional claims used to supply OAuth client metadata (e.g., client_name, redirect_uris, scope).
4. Reject Invalid Credentials: Reject the registration if the JWT is expired, not correctly signed, or missing required claims.
5. Register the Client (if all checks succeed): Upon successful validation, register the client using the extracted metadata and return a client identifier. The server MUST NOT issue a client secret, as the SPIFFE credential is used for subsequent authentication.

VCs Processing by Client Registration Endpoint When a client presents a Verifiable Credential (VC) as a software statement, the client registration endpoint MUST: 1. Verify the Credential Signature: Validate the cryptographic signature of the VC using the issuer’s trusted public key or certificate chain. 2. Check Credential Validity: Ensure the VC has not expired and is not revoked, using appropriate status mechanisms such as decentralized revocation registries or issuer-hosted status endpoints. 3. Extract and Validate Metadata Claims: Extract required client metadata claims (e.g., client_name, redirect_uris, scope) embedded in the VC, and validate their presence and format according to the authorization server’s policy. 4. Reject Invalid Credentials: Reject the registration request if the VC fails any verification step, is malformed, or lacks required claims. Optionally, upon successful validation, the registration endpoint MAY provision additional credentials but SHOULD avoid provisioning traditional client secrets to minimize risk.

Security Considerations

Client Secrets Dynamic client registration is designed to increase the ease with which clients are registered in large scale deployments. The increased use of client secrets amplifies the risks of secret theft and information compromise. This risk is further amplified if those secrets are long lived or infrequently rotated. Clients that are provisioned with SPIFFE JWT-SVIDs or X.509-SVIDs, and use this specification MUST use them not only as software statements, but also when authenticating to the authorization server in subsequent OAuth flows. The use of VCs as client authentication mechansims in OAuth is undefine. Consequently, when using VCs as software statements, the authorization server MAY provision additional credentials, but SHOULD avoid provisioning client secrets to limit the risks of secret proliferation and the consequences of secret theft.

IANA Considerations This document has no IANA actions.

Normative References JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. This specification defines mechanisms for dynamically registering OAuth 2.0 clients with authorization servers. Registration requests send a set of desired client metadata values to the authorization server. The resulting registration responses return a client identifier to use at the authorization server and the client metadata values registered for the client. The client can then use this registration information to communicate with the authorization server using the OAuth 2.0 protocol. This specification also defines a set of common client metadata fields and values for clients to use during registration. The OAuth 2.0 authorization framework enables a third-party application to obtain limited access to an HTTP service, either on behalf of a resource owner by orchestrating an approval interaction between the resource owner and the HTTP service, or by allowing the third-party application to obtain access on its own behalf. This specification replaces and obsoletes the OAuth 1.0 protocol described in RFC 5849. [STANDARDS-TRACK] n.d. n.d. n.d. n.d. n.d. n.d. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments TODO acknowledge.

Document History [[ To be removed from the final specification ]] -latest * Editorial updates * Corrected markup to show all authors -00 * Initial draft submitted to Datatracker