In Nagios XI through 5.8.5, a read-only Nagios user (due to an incorrect permission check) is able to schedule downtime for any host/services. This allows an attacker to permanently disable all monitoring checks.
The Bulk Modifications functionality in Nagios XI versions prior to 5.8.5 is vulnerable to SQL injection. Exploitation requires the malicious actor to be authenticated to the vulnerable system, but once authenticated they would be able to execute arbitrary sql queries.
Nagios Enterprises NagiosXI <= 5.8.4 contains a Server-Side Request Forgery (SSRF) vulnerability in schedulereport.php. Any authenticated user can create scheduled reports containing PDF screenshots of any view in the NagiosXI application. Due to lack of input sanitisation, the target page can be replaced with an SSRF payload to access internal resources or disclose local system files.