The Feeds for YouTube plugin for WordPress is vulnerable to Stored Cross-Site Scripting via 'youtube-feed' shortcode in versions up to, and including, 2.1 due to insufficient input sanitization and output escaping on user supplied attributes. This makes it possible for authenticated attackers with contributor-level and above permissions to inject arbitrary web scripts in pages that will execute whenever a user accesses an injected page.
https://plugins.trac.wordpress.org/changeset/2966017/feeds-for-youtube#file564
https://plugins.trac.wordpress.org/browser/feeds-for-youtube/tags/2.1/templates/feed.php#L33