PHP-Fusion 9.03.50 panels.php is vulnerable to cross-site scripting (XSS) via the 'panel_content' POST parameter. The application fails to properly sanitize user input before rendering it in the browser, allowing attackers to inject arbitrary JavaScript. This can be exploited by submitting crafted input to the 'panel_content' field in panels.php, resulting in execution of malicious scripts in the context of the affected site.