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Bobby Ho
209d1ca498
The token endpoint accepted any non-empty `code_verifier`, so a one-character verifier was enough to authenticate. RFC 7636 §4.1 requires 43 to 128 characters from the unreserved set. That fix plus the related gaps review surfaced in the same path: - Enforce the length and charset floor on the verifier before the S256 comparison runs. - Validate the challenge at the authorize endpoint too. It was only checked for non-emptiness, so a malformed challenge was stored and then failed late at token exchange, blaming the wrong parameter. - A malformed verifier now returns `invalid_request` (RFC 6749 §5.2); a well-formed but wrong one still returns `invalid_grant` (RFC 7636 §4.6). Both looked identical before, so a client had no way to tell a syntax error from a hash mismatch and would retry the same bad verifier forever. - Revoke the authorization code when a PKCE check fails. Without that, a leaked code could be replayed with unlimited verifier guesses for its remaining lifetime, and RFC 6749 §10.5 requires codes to be single use. - Fix verifier generation in `scripts/oauth2/*.sh` and the docs example. They deleted reserved base64 characters instead of translating them to the URL-safe alphabet, so most runs produced verifiers under the new floor. Also carries #28041, which merged into this branch: public clients may register bare custom schemes such as `vscode://` again, with `mailto`, `tel`, and `sms` rejected. Split out of #27873 (public OAuth2 client support). PKCE is already mandatory for every client, so this stands on its own. <details> <summary>Manual verification</summary> Ran against a local dev server on this branch, using a session token and a throwaway app from `scripts/oauth2/setup-test-app.sh`. 1. Happy path unchanged: HTTP 200, verifier length 43. 2. `code_verifier=short`, and a 43-character verifier ending in `!`: both HTTP 400 `invalid_request`, so charset is enforced and not just length. 3. `code_challenge=tooshort` at authorize: HTTP 400 `invalid_request`, no code issued. An empty challenge still hits the older "required and cannot be empty" message. 4. Well-formed but wrong verifier: HTTP 400 `invalid_grant`, distinct from the cases above. 5. Retrying that same code with the correct verifier: HTTP 400, code already revoked by the failed check. 6. `generate-pkce.sh` produces a 43-character verifier (20 out of 20 runs); the docs example produces 128. 7. `scripts/oauth2/test-mcp-oauth2.sh` passes end to end. The two bearer-token failures in its output are a pre-existing script bug (`09c50559f3`, July 2025) that reuses a resource-scoped token against the real API, not a regression here. </details>