The Chrome Object Wrapper (COW) and System Only Wrapper (SOW) implementations in Mozilla Firefox before 19.0, Firefox ESR 17.x before 17.0.3, Thunderbird before 17.0.3, Thunderbird ESR 17.x before 17.0.3, and SeaMonkey before 2.16 do not prevent modifications to a prototype, which allows remote attackers to obtain sensitive information from chrome objects or possibly execute arbitrary JavaScript code with chrome privileges via a crafted web site.