The navigator.sendBeacon implementation in Mozilla Firefox before 37.0, Firefox ESR 31.x before 31.6, and Thunderbird before 31.6 processes HTTP 30x status codes for redirects after a preflight request has occurred, which allows remote attackers to bypass intended CORS access-control checks and conduct cross-site request forgery (CSRF) attacks via a crafted web site, a similar issue to CVE-2014-8638.