The Related Posts plugin before 1.8.2 for WordPress has XSS via addqueryarg() and removequeryarg().
Cross-site Scripting (XSS) - Stored in GitHub repository barrykooij/related-posts-for-wp prior to 2.1.3.
The Related Posts for WordPress plugin through 2.0.4 does not sanitise its headingtext and CSS settings, allowing high privilege users (admin) to set XSS payloads in them, leading to Stored Cross-Site Scripting issues.