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.
secunia.com/advisories/29708
www.mwrinfosecurity.com/publications/mwri_watchguard-firebox-pptp-vpn-user-enumeration-advisory_2008-04-04.pdf
www.osvdb.org/44218
www.securityfocus.com/bid/28619
www.securitytracker.com/id?1019796
www.vupen.com/english/advisories/2008/1152/references
exchange.xforce.ibmcloud.com/vulnerabilities/41683