This indicates an attack attempt to exploit a Privilege Elevation vulnerability in Nagios XI.
The vulnerability is due to the application's failure to properly sanitize user input before using it in an API key regeneration. As a result, a remote attacker can send a crafted request to execute API calls at elevated privileges.
Nagios XI version 5.5.6
Privilege Escalation: Remote attackers can leverage their privileges on vulnerable systems.
Apply the most recent upgrade or patch from the vendor.