The Easy Digital Downloads (EDD) Content Restriction 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.