The RSS Feed Widget WordPress plugin before 3.0.0 does not validate and escape some of its shortcode attributes before outputting them back in a page/post where the shortcode is embed, which could allow users with the contributor role and above to perform Stored Cross-Site Scripting attacks.
https://wpscan.com/vulnerability/f87af54e-3e58-4c29-8a30-e7d52234c9d4/