Recovery codes abuse in SFTPGo

SFTPGo is configurable SFTP server with optional HTTP/S, FTP/S and WebDAV support. SFTPGo WebAdmin and WebClient support login using TOTP (Time-based One Time Passwords) as a secondary 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 version 2.2.0 to 2.3.3 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. This issue has been fixed in version 2.3.4. Recovery codes can now only be generated after enabling two-factor authentication and are deleted after disabling it.