# Which Problems Are Solved
TOTP codes could be reused, violating [RFC 6238, section
5.2](https://datatracker.ietf.org/doc/html/rfc6238#section-5.2).
# How the Problems Are Solved
Transmitted codes are now recorded in each success event. As the codes
amount to secrets, they are not stored verbatim but as a randomly salted
HMAC-SHA256 (`crypto.HMACValue`). Each value gets a fresh random salt,
so an equal code always produces a different stored value and two
entries can never be recognized as being derived from the same code.
This is deliberately not a replacement for proper secret storage: with
the salt stored alongside the hash and a 6 digit payload, anyone with
event access can brute-force the code in milliseconds. That is
acceptable here because a code is worthless once its window has passed.
The `HMACValue` doc comment states the same.
TOTP codes are low-entropy (6 digits), pseudo random codes. Therefore it
is possible for the same code to be generated more than once over the
lifetime of a user. In order to prevent the (unlikely) case where users
login with large time intervalls and a generated code may be equal to
the one last used a long time ago, we only check for codes used within
the recent period window.
A simpler implementation would be simply storing the latest consumed
step, but the TOTP library does not return this value. Because of the
allowed skew there are always 3 steps within the validation window, so
we can't calculate the current step based on time alone. The retained
history spans one period more than the validation window: validity is
decided by the app server's clock, while the stored events carry the
database clock, and the extra period absorbs the drift between them.
**Note:** a reused code pushes a distinct
`user.human.mfa.otp.check.reused` event, not a failed check event. A
replay therefore stays observable through the Events API without
counting towards the lockout policy. Otherwise anyone who captured a
code could replay it to lock the user out.
# Additional Changes
- Read only the latest events in recheck query
# Additional Context
- Closes https://github.com/zitadel/zitadel/issues/6452
---------
Co-authored-by: Livio Spring <[email protected]>
Co-authored-by: Copilot Autofix powered by AI <[email protected]>
Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: muhlemmer <[email protected]>
