CVE-2001-1068
qpopper 4.01 with PAM based authentication on Red Hat systems generates different error messages when an invalid username is provided instead of a valid name, which allows remote attackers to determine valid usernames on the system.
Date published : 2002-02-02
http://www.securityfocus.com/bid/3242
http://archives.neohapsis.com/archives/bugtraq/2001-08/0363.html