Versions prior to 1.4.2.1 of the ArGoSoft FTP server will disclose whether or not a supplied username is valid or not. A login name supplied with the USER command will not be accepted unless it is valid.
You can download this advisory from the following link: https://packetstormsecurity.com/files/download/35574/argosoft_advisory.txt
Source: https://packetstormsecurity.com/files/35574/argosoft_advisory.txt.html