The JetAppointment plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'friendlyTime' parameter in all versions up to, and including, 2.5.2.1 due to insufficient input sanitization and output escaping. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. The injected payload is stored in the wp_jet_appointments_meta table via the unauthenticated jet_engine_form_booking_submit endpoint and executes in the administrator's browser when the appointment details popup is opened in the WordPress admin panel.