Nagios XI versions prior to 5.11.3 are vulnerable to cross-site scripting (XSS) via the Graph Explorer component. Insufficient validation or escaping of user-supplied input may allow an attacker to inject and execute arbitrary script in the context of a victim's browser.
Nagios addresses this vulnerability as "The Graph Explorer component is vulnerable to a cross-site scripting attack" and "Fixed an XSS vulnerability in the Graph Explorer component."