v0.1.20 — fix(oidc): reject token requests when codeChallengeMethod is not S256

{"content":"# Release v0.1.20: PKCE Downgrade Path Closed\n\nFebruary 26, 2026\n\n## What changed\n\n- 🔒 PKCE security fix: The OIDC token endpoint now strictly requires S256 code challenge method. Requests with null or non-S256 methods are rejected with invalid_grant, eliminating a downgrade path where the insecure plain PKCE method could be silently accepted.\n- 🧹 Removed dead code path in verifyPkce() that supported the deprecated plain method.\n- ✅ Added 20 test cases covering the vulnerability, the fix, and valid S256 flows.\n\n## Why it matters\n\nPKCE (Proof Key for Code Exchange) prevents authorization code interception attacks. The plain method provides no security benefit — the code verifier equals the challenge, making it useless as a proof. This fix ensures that any authorization code with a corrupted or missing challenge method cannot be exchanged for tokens, closing a potential bypass.\n\n## Technical notes\n\nNo breaking changes. All clients using S256 PKCE (the only supported method) are unaffected. Authorization codes stored with null or non-S256 challenge methods will be rejected at token exchange time.","date":"February 26, 2026","title":"Release v0.1.20: PKCE Downgrade Path Closed"}