This indicates an attack attempt against a Command Execution 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 Enterprises Nagios XI 2012 R1.5b and prior
System Compromise: Remote attackers can gain control of vulnerable systems.
Currently we are unaware any vendor supplied patch for this issue.