There is a stored XSS in the 404-to-301 WP plugin < 2.3.1. Unauthenticated users can visit a specially crafted URL and the redirect path will be logged to the database. The redirection source is stored unescaped in the database, thus it is served as-is and evaluated in the browsers of logged-in admins when they check the redirection logs on http://wordpress/wp-admin/admin.php?page=i4t3-logs. Affected versions are <2.3.1.
Proof of Concept
A similar requests must be sent to the vulnerable server. Make sure to request a page serving a 404, ie by requesting a post with an unexisting post ID.
GET /?p=99999999999999999929"><script>alert(document.cookie)</script> HTTP/1.1
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36
Accept-Encoding: gzip, deflate, sdch