This indicates an attack attempt to exploit a Remote Code Execution vulnerability in Nagios XI.
The vulnerability is due to an error when the vulnerable software handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
Nagios XI 5.5.6
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.