Mozilla Firefox 4.x through 11.0, Thunderbird 5.0 through 11.0, and SeaMonkey before 2.9 do not properly construct the Origin and Sec-WebSocket-Origin HTTP headers, which might allow remote attackers to bypass an IPv6 literal ACL via a cross-site (1) XMLHttpRequest or (2) WebSocket operation involving a nonstandard port number and an IPv6 address that contains certain zero fields.