The registration throttle called record_failure on every attempt, successful
ones included. Five legitimate signups from one shared or NAT address locked the
sixth real user out for up to 600s, doubling from there — while an attacker
sidestepped the limiter entirely through B-54. Failure backoff is the wrong
instrument here: nothing about creating an account is a failed guess at a
secret, so the only people it reliably punished were the honest ones.
RollingQuota says what was actually meant: 5 accounts per IP per hour, in a
rolling window. The caller over it waits exactly until the oldest of the five
ages out — an accurate Retry-After, and waiting never makes the next wait
longer. It is recorded only once an account exists, so attempts that create
nothing (a taken username, a validation error) leave the quota untouched, and
checked before the Argon2 hash, so an IP out of quota costs nothing to refuse.
Bounded like the failure limiter (B-56): the keys are caller-chosen, so the dict
gets both a sweep and a hard cap, evicting keys with room left in their quota
before full ones.
Also fixes the inline comment that cited B-31 (the resubscribe finding) where it
meant B-33.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
davideandClaude Opus 5
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