The Chrome Object Wrapper (COW) implementation in Mozilla Firefox before 21.0, Firefox ESR 17.x before 17.0.6, Thunderbird before 17.0.6, and Thunderbird ESR 17.x before 17.0.6 does not prevent acquisition of chrome privileges during calls to content level constructors, which allows remote attackers to bypass certain read-only restrictions and conduct cross-site scripting (XSS) attacks via a crafted web site.