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.
https://packetstormsecurity.com/files/142177/watchguardfbxtm-xxeinject.txt
http://watchguardsupport.force.com/publicKB?type=KBSecurityIssues&SFDCID=kA62A0000000KlGSAU