Microsoft Internet Explorer 11 - Garbage Collector Attribute Type Confusion (MS16-063)











CVE-2016-0199 / MS16-063: MSIE 11 garbage collector attribute type confusion
This information is available in an easier to read format on my blog at

With [MS16-063] Microsoft has patched [CVE-2016-0199]: a memory
corruption bug
in the garbage collector of the JavaScript engine used in Internet
Explorer 11.
By exploiting this vulnerability, a website can causes this garbage
to handle some data in memory as if it was an object, when in fact it
data for another type of value, such as a string or number. The garbage
collector code will use this data as a virtual function table (vftable)
in order
to make a virtual function call. An attacker has enough control over
this data
to allow execution of arbitrary code.

Known affected software and attack vectors
+ **Microsoft Internet Explorer 11** (all versions before the June 2016

  An attacker would need to get a target user to open a specially crafted
  webpage. Disabling JavaScript should prevent an attacker from
triggering the
  vulnerable code path.

I've created two separate html files that can be used to reproduce this
and shows control over a 32-bit vftable pointer in x86 versions of MSIE or a
partial control over a 64-bit vftable pointer in x64 versions.

<!DOCTYPE html>
<meta http-equiv="X-UA-Compatible" content="IE=7">
  oElement = document.createElement("IMG");
  var oAttr = document.createAttribute("loop");
  oAttr.nodeValue = oElement;
  oElement.loop = 0x41424344; // Set original value data to 44 43 42 41
  oElement.setAttributeNode(oAttr); // Replace oElement with original value data
  CollectGarbage(); // Use original value data as address 0x41424344 of a vftable

(I've had to use xcript rather than script because Gmail refused to send it
otherwise, see for the reason.)

When `setAttributeNode` is used to set an attribute of a HTML element,
and the
`Attr` node's `nodeValue` is not a valid value, this `nodeValue` is set
to the
value the attribute had before the call. This can happen for instance
when you
try to set an attribute that must have a string or number value by using an
`Attr` node with a HTML element as its `nodeValue` (as this is not a
string or
number). The HTML element in `nodeValue` is replaced with the string or
value the attribute had before the call to `setAttributeNode`.

If the `Attr` node is then removed using `removeAttributeNode` and the
collector runs, the code appears to assume the nodeValue still contains an
object, rather than the string or number it has been changed into. This
the code to use the data for the string or number value as if it was a C++
object. It attempts to determine a function pointer for a method from the
object's virtual function table before calling this function using the

If the previous value is a string, the character data from the string is
to calculate the function pointer. If the previous value is a number,
the value
of the number is used. This provides an attacker with a large amount of
over the function pointer and may allow execution of arbitrary code.

I build a "scanner" to analyze this issue and help create two
files that show control over the vftable pointer. More details and the
for these can be found on my blog at