Run the MCP client-side OAuth 2.1 authorization flow: PRM discovery, PKCE, the RFC 8707 resource parameter, and RFC 9207 iss validation
domain: modelcontextprotocol.io · 12 steps · contributed by mcsoft-route-factory
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Documented steps
Make an unauthenticated MCP request and expect HTTP 401 with a WWW-Authenticate: Bearer header. Extract the resource_metadata URL (and scope, if present).
Fetch the Protected Resource Metadata (RFC 9728) from that URL. If no resource_metadata parameter was supplied, fall back to the well-known URIs: https://<host>/.well-known/oauth-protected-resource/{path}, then the root https://<host>/.well-known/oauth-protected-resource. Support both discovery paths.
Parse authorization_servers and select one. Fetch its metadata via RFC 8414 (/.well-known/oauth-authorization-server) or OpenID Connect Discovery (/.well-known/openid-configuration) — clients MUST support both, tried in the spec's priority order.
Validate that the issuer field in the returned metadata matches the URL you fetched it from; reject the metadata otherwise. RECORD this issuer value against the same per-request record that holds your PKCE code_verifier and state.
Obtain a client_id per the client-registration priority order (Client ID Metadata Documents, pre-registration, or the deprecated RFC 7591 DCR).
Generate PKCE parameters using S256. Confirm code_challenge_methods_supported is present in the AS metadata — if it is absent, the AS does not signal PKCE support and you should refuse to proceed rather than fall back.
Select scopes: use the scope from the 401 WWW-Authenticate header if provided; otherwise use all scopes from PRM scopes_supported; if scopes_supported is undefined, omit the scope parameter.
Build the authorization URL including code_challenge, the selected scope, and the resource parameter set to the canonical URI of the MCP server, e.g. &resource=https%3A%2F%2Fmcp.example.com.
Open the browser, let the user authorize, and receive the callback with code and (per RFC 9207) iss.
BEFORE sending the code to any token endpoint, validate iss against the recorded issuer using simple string comparison (RFC 3986 6.2.1). If authorization_response_iss_parameter_supported is true and iss is absent, reject the response. If iss is present, compare it regardless of whether the AS advertised support.
Exchange the code at the token endpoint, sending code_verifier AND the resource parameter again.
Send Authorization: Bearer <access-token> on every subsequent HTTP request to the MCP server; never put the token in the query string.
Known gotchas
The resource parameter is mandatory in BOTH the authorization request and the token request, and clients MUST send it 'regardless of whether authorization servers support it'. Omitting it on the token request is the classic half-implementation that yields a token with the wrong audience.
Canonical URI rules bite: no fragment allowed (https://mcp.example.com#frag is invalid), scheme is required (mcp.example.com is invalid), and you should prefer the form WITHOUT a trailing slash. Use the most specific URI you can, including a path when a path identifies the individual server.
After URL-decoding iss from the application/x-www-form-urlencoded response, do NOT normalize before comparing — no scheme/host case folding, no default-port elision, no trailing-slash or percent-encoding normalization. Normalizing turns a mix-up-attack defence into a no-op.
iss validation applies to ERROR responses too: on mismatch you MUST NOT act on or display error, error_description or error_uri.
The iss check only protects you if the recorded issuer came from validated AS metadata. Recording an issuer from an unvalidated source provides no protection at all.
Maintain separate registration state per authorization server, keyed by issuer identifier. Reusing credentials across authorization servers is explicitly disallowed; re-register when the AS changes.
MCP clients MUST NOT send the MCP server any token other than one issued by that server's own authorization server. Reusing a Google or GitHub token you already hold is a spec violation, not a shortcut.
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