[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)

Beginne mit der Verarbeitung der Authorization-Code-Weiterleitung des Clients und dem AS-Metadatenstatus, aus dem die authorization_endpoint-URL hervorgegangen ist. Überprüfe die Grundlage für Auth Code + PKCE in #319 und die Anforderungen in RFC 9207; als abgeschlossen gilt die Validierung des zurückgegebenen iss gegen den initiierenden Authorization Server, einschließlich Flows mit mehreren AS.