The PPTP VPN service in Watchguard Firebox before 10, when performing the MS-CHAPv2 authentication handshake, generates different error codes depending on whether the username is valid or invalid, which allows remote attackers to enumerate valid usernames.
https://exchange.xforce.ibmcloud.com/vulnerabilities/41683
http://www.vupen.com/english/advisories/2008/1152/references
http://www.securitytracker.com/id?1019796