Throughout November, I plan to release details on vulnerabilities I
found in web-browsers which I've not released before. This is the second
entry in that series.

The below information is also available on my blog at
http://blog.skylined.nl/20161102001.html. There you can find a repro
that triggered this issue in addition to the information below.

Follow me on http://twitter.com/berendjanwever for daily browser bugs.

MSIE 11 MSHTML CView::CalculateImageImmunity UAF
================================================

(The fix and CVE number for this bug are not known)

Synopsis
--------
Setting the `listStyleImage` property of an Element object causes MSIE
11 to allocate 0x4C bytes for an "image context" structure, which
contains a reference to the document object as well as a reference to
the same `CMarkup` object as the document. When the element is removed
from the document/document fragment, this image context is freed on the
next "draw". However, the code continues to use the freed context almost
immediately after it is freed.

Known affected versions, attack vectors and mitigations
-----------------------
+ Microsoft Internet Explorer 11

  An attacker would need to get a target user to open a specially
  crafted webpage. As far as can be determined, disabling JavaScript
  should prevent an attacker from triggering the vulnerable code path.

Exploit
-------
I tried a few tricks to see if there was an easy way to reallocate the
freed memory before the reuse, but was unable to find anything. I do not
know if there is a way to cause further reuse of the freed memory later
on in the code. Running the repro as-is without page heap does not
appear to trigger crashes. It does not appear that there is enough time
between the free and reuse to exploit this issue.

Repro.html:

<script>
  var oDocumentFragment = document.createDocumentFragment(),
      oElement = document.createElement('x');
  oDocumentFragment.appendChild(oElement);
  oElement.style.listStyleImage = "url(x)";
  oDocumentFragment.removeChild(oElement);
</script>



Timeline
--------
* May 2014: This vulnerability was found through fuzzing.
* June 2014: This vulnerability was submitted to ZDI.
* July 2014: ZDI rejects the submission.
* November 2016: The issue does not reproduce in the latest build of
  MSIE 11.
* November 2016: Details of this issue are released.

Unfortunately, my records of what happened after ZDI rejected the issue
are patchy. It appears that I did not pursue reporting the issue
anywhere else, but Microsoft does appear to have patched the issue, as I
can no longer reproduce it.

Cheers,

SkyLined