The Easy Digital Downloads (EDD) Free Downloads 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.