This indicates an attack attempt to exploit a reflected Cross-site Scripting vulnerability in Nagios XI.
The vulnerability is due to insufficient validation of requests to api_tool.php. A remote attacker may be able to exploit this to execute arbitrary script code within the context of an affected system.
Nagios XI version 5.5.6
System Compromise: Remote attackers can execute arbitrary script code within the context of the affected site.
Apply the most recent upgrade or patch from the vendor.