The Events Manager – Calendar, Bookings, Tickets, and more! plugin for WordPress is vulnerable to Reflected Cross-Site Scripting via the 'header_format' parameter in all versions up to, and including, 7.4.0.1 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that execute if they can successfully trick a user into performing an action such as clicking on a link. The shortcode entry point sanitizes 'header_format' via wp_kses(), but the unauthenticated 'search_events_grouped' AJAX action bypasses this sanitization entirely, leaving the parameter unsanitized before it is echoed into the HTML body in output_grouped().