Mozilla Firefox 3.5.x before 3.5.10 and 3.6.x before 3.6.4, and SeaMonkey before 2.0.5, does not properly handle situations in which both "Content-Disposition: attachment" and "Content-Type: multipart" are present in HTTP headers, which allows remote attackers to conduct cross-site scripting (XSS) attacks via an uploaded HTML document.