The Chrome Object Wrapper (COW) implementation in Mozilla Firefox before 16.0, Firefox ESR 10.x before 10.0.8, Thunderbird before 16.0, Thunderbird ESR 10.x before 10.0.8, and SeaMonkey before 2.13 does not prevent access to properties of a prototype for a standard class, which allows remote attackers to execute arbitrary JavaScript code with chrome privileges via a crafted web site.