WatchGuard Fireware v11.12.1 and earlier mishandles requests referring to an XML External Entity (XXE), in the XML-RPC agent. This causes the Firebox wgagent process to crash. This process crash ends all authenticated sessions to the Firebox, including management connections, and prevents new authenticated sessions until the process has recovered. The Firebox may also experience an overall degradation in performance while the wgagent process recovers. An attacker could continuously send XML-RPC requests that contain references to external entities to perform a limited Denial of Service (DoS) attack against an affected Firebox.