The XrayWrapper implementation 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 does not properly restrict use of DefaultValue for method calls, which allows remote attackers to execute arbitrary JavaScript code with chrome privileges via a crafted web site that triggers use of a user-defined (1) toString or (2) valueOf method.