Cross-site scripting (XSS) vulnerability in thumb.php in MediaWiki before 1.23.10, 1.24.x before 1.24.3, and 1.25.x before 1.25.2 allows remote attackers to inject arbitrary web script or HTML via the rel404 parameter, which is not properly handled in an error page.
Above was an interesting XSS affecting all of Wikipedia and MediaWiki software. It was found during manual code review. Not much to be said about it other than failure to validate and / or sanitize input. Great response by the MediaWiki development and Wikipedia Security team!