The gPluginHandler.handleEvent function in the plugin handler in Mozilla Firefox before 18.0, Firefox ESR 17.x before 17.0.2, Thunderbird before 17.0.2, Thunderbird ESR 17.x before 17.0.2, and SeaMonkey before 2.15 does not properly enforce the Same Origin Policy, which allows remote attackers to conduct clickjacking attacks via crafted JavaScript code that listens for a mutation event.