This indicates an attack attempt to exploit a Buffer Overflow Vulnerability in WatchGuard Firebox and XTM appliances.
The vulnerability is due to improper validation of the length of user-supplied data in HTTP requests. A remote attacker can exploit the vulnerability by sending a malicious request to the target server. Successful exploitation could result in arbitrary code execution on the target system.
Fireware OS before 12.7.2_U2, 12.x before 12.1.3_U8, and 12.2.x through 12.5.x before 12.5.9_U2
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.