vchkpw program in vpopmail before version 4.8 does not properly cleanse an untrusted format string used in a call to syslog, which allows remote attackers to cause a denial of service via a USER or PASS command that contains arbitrary formatting directives.
http://www.vpopmail.cx/vpopmail-ChangeLog
http://www.securityfocus.com/templates/archive.pike?list=1&msg=395BD2A8.5D3396A7%40secureaustin.com