challengeSecret folded a GetUserByID error into the same 401 "invalid or
expired two-factor challenge" an expired challenge earns, and the attempt
had already been charged to the per-user totp_fail cap. Split the outcome:
a store error logs and returns the new service.ErrTOTPUnavailable
(ErrInternal, "two-factor verification temporarily unavailable"); an unknown
user or a missing secret still answers 401. The limiter reservation moves
after the store read — the rule authenticate already applies — and still
precedes the code compare, so the check-then-act it closes stays closed.
Characterization row flipped in the same commit: `VerifyTOTPFailurePaths/
user lookup fails -> 500, challenge kept, attempt not counted` — after the
fault ten wrong codes still answer 401 (the tenth would be 429 had the
fault counted), then the eleventh is refused. `per-user failure cap spans
challenges` unchanged.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A17Uq3d2C36rN82Jitf3wo