4.1. X.509 Certificate to Access Token Profile

In [draft-ietf-wimse-arch], Workloads may be issued Identity Credentials in the form of X.509 Certificates [RFC5280], for Workload-to-Workload communication over mututal TLS (mTLS). Workload Agents must request the X.509 Certificate Credentials by undergoing Attestation against both the local Host Operating System and Hardware, and a remote Server with access to a Certificate Authority (CA). If the Server confirms sufficient evidence has been presented for Attestation, the Workload is issued X.509 Certificates identifying it. The identity is conveyed in a URI Subject Aleternative Name (SAN) within the X.509 Certificate.¶

Authorization servers issue OAuth 2.0 Access Tokens to client applications. If the resource owner has granted sufficient privileges to a protected resource, the issued access token can be used to access protected resources on resource servers.¶

The Workloads possessing X.509 Certificate Identity Credentials may operate in an environment that is isolated from the security domain of a protected resource. In the case where the protected resource is protected by an external OAuth 2.0 Authorization Server, X.509 Certificate-to-Access Token exchange may be configured. The Trust across the isolated security domains must first be established. Relying parties must describe, via secure configurations, a mapping that crosses the security domains from the X.509 certificate authority(ies) to the OAuth 2.0 authorization server. The specification for authenticating the relying party and for the format of the configurations are out of scope of this specification. The following configurations MAY be registered by a relying party at the OAuth 2.0 authorization server:¶

1. A set of one or more Trust Anchors MUST be configured for the relying party at the OAuth 2.0 authorization server, representing authoritative entities with a public key and associated data, as defined in [RFC6024]. These configurations MUST be represented as X.509 CA certificates in either DER or PEM format.¶

2. A set of intermediate entities with public key and associated data, expressed as X.509 CA certificates in either DER or PEM format, MAY be configured for the relying party at the OAuth 2.0 authorization server. The intermediate CA certificates are for the purposes of certificate chain path building in scenarios where clients cannot or may not provide these intermediate certificates during mTLS handshakes.¶

3. A mapping MUST describe the certificate attribute(s) used to select and or construct the subject claim in the OAuth 2.0 access tokens. Possible X.509 certificate properties include the following:¶

   • subject's common name (CN)¶

   • first subject alternative names (SAN) DNS Name entry¶

   • first subject alternative names (SAN) URI entry¶

4. A set of conditions MAY be defined to constrain the client certificates that SHALL be accepted. An example might be a constraint that the certificate's first SAN URI entry must start with spiffe://example.com/foo, or that the certificate's first SAN DNS Name entry must end with .example.com.¶

5. A mapping MAY describe additional certificate attributes that may be encoded in the issued OAuth 2.0 access tokens to be interpreted as part of access policy decisions. These MAY include any of the following properties¶

   • hex-encoded serial number¶

   • subject DN common name¶

   • subject DN organization name¶

   • subject DN subject organization unit¶

   • issuer DN common name¶

   • issuer DN organization name¶

   • issuer DN organization unit¶

   • the first subject alternative name (SAN) of type DNS name¶

   • the first subject alternative name (SAN) of type URI¶

Compatible OAuth 2.0 Authorization Servers supporting this token exchange profile MUST support mTLS. After the relying party has registered an X.509 Certificate federation profile with the OAuth 2.0 authorization server, in order to obtain access tokens, Workloads MUST present their X.509 Certificates during mTLS handshakes to establish a connection to the OAuth 2.0 Authorization Server. Workloads MUST then send a request to token endpoint, in the manner described in [RFC8693]. The access token request is sent with the following properties:¶

• grant_type: REQUIRED. The value urn:ietf:params:oauth:grant-type:token-exchange indicates that a token exchange is being performed.¶

• resource: OPTIONAL. A URI that indicates the target service or resource where the client intends to use the requested security token.¶

• audience: REQUIRED for this Profile. A URI or other unique identifier for the relying party, assigned by the OAuth 2.0 Authorization Server.¶

• scope: OPTIONAL. A list of space-delimited, case-sensitive strings, as defined in Section 3.3 of [RFC6749], that allow the client to specify the desired scope of the requested security token in the context of the service or resource where the token will be used.¶

• requested_token_type: MUST be urn:ietf:params:oauth:token-type:access_token for this token exchange profile.¶

• subject_token: REQUIRED. Either the fixed string mtls_client_certificate or an X.509 Certificate Chain value. Whether conveyed by-reference to mTLS using mtls_client_certificate or as an X.509 Certificate Chain value, the Authorization Server MUST validate that the X.509 Certificate chain can be validated by a path to a previously-configured Trust Anchor certificate for the relying party, either directly or through one of the previously-configured Intermediate CA path-building certificates.¶

  • The value mtls_client_certificate instructs the Authorization Server to obtain the subject token from the client's mTLS Certificate message, as defined in Section 2 of [RFC8446].¶

  • Any value other than mtls_client_certificate MUST be parsed as an X.509 Certificate Chain for this token exchange profile. The X.509 Certificate chain MUST be formatted as described in the "x5c" (X.509 Certificate Chain) Header Parameter in Section 4.1.6 of [RFC7515]. The leaf X.509 Certificate used in the subject_token MUST match the leaf X.509 Certificate used during mTLS.* subject_token_type: MUST be urn:ietf:params:oauth:token-type:mtls for this token exchange profile.¶

The request MUST ONLY be accepted if the X.509 Certificate used during mTLS chain to a previously-configured Trust Anchor via a certificate path that may include previously-configured intermediate CA certificates. The previously-configured subject claim selector MUST select a non-blank string from the certificate. The previously-configured conditions MUST accept the X.509 Certificate.¶

The response document contains the following properties (per [RFC8693]):¶

• access_token: REQUIRED. The security token issued by the authorization server in response to the token exchange request.¶

• issued_token_type: REQUIRED. Must be urn:ietf:params:oauth:token-type:access_token for this Profile.¶

• token_type: REQUIRED. Must be bearer for this Profile.¶

• expires_in: RECOMMENDED. The validity lifetime, in seconds, of the token issued by the authorization server.¶

• scope: OPTIONAL if the scope of the issued security token is identical to the scope requested by the client; otherwise, it is REQUIRED.¶

• refresh_token: MUST NOT be returned for this Profile.¶

The X.509 Certificate-to-Access Token Exchange Profile MUST NOT relax the validity constraint of the input security context. The returned Access Token MUST NOT have a not before claim that preceeds the notBefore constraint of the X.509 Certificate used. The returned Access Token MUST NOT have an expiration time claim that exceeds the notAfter constraint of the X.509 Certificate used.¶

To mitigate the impact of Access Token theft, it is RECOMMENDED that the returned Access Token be sender-constrained. The Authorization Server MAY bind the Access Token to the X.509 Certificate that was used to obtain it, in the manner described in Section 3 of [RFC8705]. Based on authentication policy, Resource Servers MAY enforce that an Access Token bound to an X.509 Certificate CAN NOT be used to access any protected resources, unless the same X.509 Certificate was used during the mTLS handshake to the Resource Server.¶

8.1. Appendix 1 - Non-normative token exchange examples