The HZ-GB-2312 character-set implementation in Mozilla Firefox before 17.0, Firefox ESR 10.x before 10.0.11, Thunderbird before 17.0, Thunderbird ESR 10.x before 10.0.11, and SeaMonkey before 2.14 does not properly handle a ~ (tilde) character in proximity to a chunk delimiter, which allows remote attackers to conduct cross-site scripting (XSS) attacks via a crafted document.