The Chrome Object Wrapper (COW) implementation in Mozilla Firefox before 18.0, Firefox ESR 17.x before 17.0.2, Thunderbird before 17.0.2, Thunderbird ESR 17.x before 17.0.2, and SeaMonkey before 2.15 does not prevent modifications to the prototype of an object, which allows remote attackers to execute arbitrary JavaScript code with chrome privileges by referencing Object.prototype.__proto__ in a crafted HTML document.