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Issue metadata

Status: Fixed
Closed: Nov 2016

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Microsoft Edge: Type Confusion in FillFromPrototypes

Project Member Reported by, Sep 14 2016

Issue description

JavascriptArray::FillFromPrototypes is a method that is used by several Javascript functions available in the browser to set the native elements of an array to the values provide by its prototype. This function calls JavascriptArray::ForEachOwnMissingArrayIndexOfObject with the prototype of the object as a parameter, and if the prototype of the object is an array, it assumes that it is a Var array. While arrays are generally converted to var arrays if they are set as an object's prototype, if an object's prototype is a Proxy object, it can return a parent prototype that is a native int array. This can lead to type confusing, allowing an integer to be treated as an absolute pointer, when JavascriptArray::FillFromPrototypes is called. A minimal PoC is as follows, and a full PoC is attached.

var a = new Array(0x11111111, 0x22222222, 0x33333333, 0x44444444, 0x12121212, 0x23232323, 0x12345670, 0x7777);

var handler = {
    getPrototypeOf: function(target, name){
	return a;

var p = new Proxy([], handler);
var b = [{}, [], "natalie"];

b.__proto__ = p;
b.length = 4;;
 // b[2] is type confused

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.

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Project Member

Comment 1 by, Nov 17 2016

Labels: -Restrict-View-Commit CVE-2016-7201
Status: Fixed (was: New)
Fixed in November update

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