This indicates an attack attempt against a Command Injection vulnerability in Nagios XI.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application when parsing HTTP requests. A remote attacker may be able to exploit this to execute arbitrary command within the context of the application via a crafted HTTP request.
Nagios XI Network Monitor's component 'Graph Explorer'
System Compromise: Remote attackers can execute arbitrary code on vulnerable systems.
Currently we are unaware of any vendor supplied patch for this issue.