The JSONP endpoint in includes/api/ApiFormatJson.php in MediaWiki before 1.19.18, 1.20.x through 1.22.x before 1.22.9, and 1.23.x before 1.23.2 accepts certain long callback values and does not restrict the initial bytes of a JSONP response, which allows remote attackers to conduct cross-site request forgery (CSRF) attacks, and obtain sensitive information, via a crafted OBJECT element with SWF content consistent with a restricted character set.