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fix(auth): confidential clients may sign in without PKCE, as OpenID certification needs - #521
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Every authorization request had to carry an S256 challenge, so a confidential client that sends none, as OpenID certification's Basic and logout plans do, failed at its first authorization. Following RFC 9700 §2.1.1, a confidential client may now leave PKCE out: it authenticates with its secret at the token endpoint. Public clients must still send a challenge, a challenge sent is always enforced, and a code issued without one can't be redeemed with a code_verifier, so a request can't be downgraded. Closes #514
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Auth required S256 PKCE from every client. A confidential client that sends none, as the OpenID conformance suite's Basic and logout plans do, failed at its first authorization, so 37 of their tests could check nothing behind it.
Now a confidential client may leave PKCE out, as RFC 9700 §2.1.1 allows: it authenticates with its secret when it redeems the code. Nothing gets weaker for the rest:
code_verifier, so an attacker can't strip PKCE from a request (downgrade).pkce_challengebecomes nullable on pending requests and codes (migration 0048). The OIDC reference, the app guides and the security docs say who needs PKCE.Closes #514
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Written by Claude Opus 5.5 in Claude Code (via T3 Code)