In Nagios XI through 5.8.5, in the schedule report function, an authenticated attacker is able to inject HTML tags that lead to the reformatting/editing of emails from an official email address.
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.