The System Only Wrapper (SOW) and Chrome Object Wrapper (COW) implementations in Mozilla Firefox before 22.0, Firefox ESR 17.x before 17.0.7, Thunderbird before 17.0.7, and Thunderbird ESR 17.x before 17.0.7 do not properly restrict XBL user-defined functions, which allows remote attackers to execute arbitrary JavaScript code with chrome privileges, or conduct cross-site scripting (XSS) attacks, via a crafted web site.