Nagios Fusion versions prior to 4.2.0 contain a stored cross-site scripting (XSS) vulnerability when adding or configuring Email Settings. Unsanitized user input can be stored and later rendered in the administrative UI, causing JavaScript to execute in the browser of any user who views the affected page. An attacker who can add or modify SMTP/email settings or manipulate the sendmail configuration fields could persist a malicious payload that executes in the context of other users' browsers.