Nagios Core 4.4.2 has XSS via the alert summary reports of plugin results, as demonstrated by a SCRIPT element delivered by a modified check_load plugin to NRPE.
Nagios XI 5.5.6 allows remote authenticated attackers to reset and regenerate the API key of more privileged users. The attacker can then use the new API key to execute API calls at elevated privileges.
A privilege escalation vulnerability was found in nagios 4.2.x that occurs in daemon-init.in when creating necessary files and insecurely changing the ownership afterwards. It's possible for the local attacker to create symbolic links before the files are to be created and possibly escalating the privileges with the ownership change.
qh_help in Nagios Core version 4.4.1 and earlier is prone to a NULL pointer dereference vulnerability, which allows attacker to cause a local denial-of-service condition by sending a crafted payload to the listening UNIX socket.