The WP Booking Calendar plugin for WordPress is vulnerable to Stored Cross-Site Scripting via admin settings in all versions up to, and including, 10.6 due to insufficient input sanitization and output escaping. This makes it possible for authenticated attackers, with administrator-level permissions and above, to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page. This only affects multi-site installations and installations where unfiltered_html has been disabled. In addition, site administrators have the option to grant lower-level users with access to manage the plugin's settings which may extend this vulnerability to those users.