This indicates an attack attempt to exploit a Command Injection Vulnerability in Nagios Enterprises Nagios XI.
The vulnerability is due to insufficient input validation of the requests submitted to the watchguard.inc.php. A remote authenticated attacker can exploit this vulnerability by sending a crafted request to the server. Successful exploitation could result in arbitrary command execution with privileges of the web server on the target system.
Nagios Enterprises Nagios XI prior to 5.8.5
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.