This indicates an attack attempt to exploit a Code Injection Vulnerability in Nagios XI.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when handling a crafted request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted request.
Nagios XI version 5.6.9
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.