The Easy Digital Downloads (EDD) Recommended Products 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.