The XBL.__proto__.toString implementation in Mozilla Firefox before 18.0, Firefox ESR 10.x before 10.0.12 and 17.x before 17.0.2, Thunderbird before 17.0.2, Thunderbird ESR 10.x before 10.0.12 and 17.x before 17.0.2, and SeaMonkey before 2.15 makes it easier for remote attackers to bypass the ASLR protection mechanism by calling the toString function of an XBL object.