WatchGuard Fireware allows user enumeration, e.g., in the Firebox XML-RPC login handler

WatchGuard Fireware allows user enumeration, e.g., in the Firebox XML-RPC login handler. A login request that contains a blank password sent to the XML-RPC agent in Fireware v11.12.1 and earlier returns different responses for valid and invalid usernames. An attacker could exploit this vulnerability to enumerate valid usernames on an affected Firebox.