The Easy Digital Downloads (EDD) htaccess Editor extension for WordPress as used with EDD 1.8.x before 1.8.7 1.9.x before 1.9.10 2.0.x before 2.0.5 2.1.x before 2.1.11 2.2.x before 2.2.9 and 2.3.x before 2.3.7 has XSS because add_query_arg is misused.