[Client][Auth] SEP-2468: Validate iss parameter in authorization response (RFC 9207)

Implements SEP-2468 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #338.

Spec summary

Requires MCP authorization responses to include an explicit iss (issuer) parameter per RFC 9207. Clients MUST validate iss against the AS that initiated the flow to mitigate authorization mix-up attacks in multi-IdP environments.

PHP SDK changes

• When handling the authorization-code redirect, the client must validate the returned iss matches the AS that produced the authorization_endpoint URL.
• Required for any multi-AS scenario; cross-cuts with #319 (Auth Code + PKCE) but filed separately because the validation logic crosses callback + AS-metadata state.
• Server-side: N/A — MCP servers are resource servers, not authorization servers; iss is emitted by the AS.

Related

• Umbrella #338
• Sub-task / depends on: #319 (Auth Code + PKCE foundation)

Commencez par la gestion de la redirection de l’authorization code du client et par l’état des métadonnées de l’AS qui a produit l’URL authorization_endpoint. Examinez les fondations de Auth Code + PKCE dans #319 et les exigences de RFC 9207 ; le travail est considéré comme terminé lorsque le iss renvoyé est validé par rapport au serveur d’autorisation à l’origine de la demande, y compris dans les flux avec plusieurs AS.