SFTPGo vulnerable to recovery codes abuse

Impact

SFTPGo WebAdmin and WebClient support login using TOTP (Time-based One Time Passwords) as a seconday authentication factor. Because TOTPs are often configured on mobile devices that can be lost, stolen or damaged, SFTPGo also supports recovery codes. These are a set of one time use codes that can be used instead of the TOTP.

In SFTPGo versions from v2.2.0 to v2.3.3 recovery codes can be generated before enabling two-factor authentication.
An attacker who knows the user's password could potentially generate some recovery codes and then bypass two-factor authentication after it is enabled on the account at a later time.

Patches

Fixed in v2.3.4.
Recovery codes can now only be generated after enabling two-factor authentication and are deleted after disabling it.

Workarounds

Regenerate recovery codes after enabling two-factor authentication.

References

https://github.com/drakkan/sftpgo/issues/965