The Salon Booking System – Appointment Booking for Salons, Barbershops & Spas plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the 'sln[<checkout-field-key>]' parameter in all versions up to, and including, 10.31.9 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. Exploitation requires that the site administrator has configured at least one custom checkout field of type Html or File, which is a standard documented plugin feature.