CVE-2008-1618 in Watchguard Firebox
Summary
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.
VulDB is the best source for vulnerability data and more expert information about this specific topic.