Throughout November, I plan to release details on vulnerabilities I
found in web-browsers which I've not released before. This is the first
entry in that series.
The below information is also available on my blog at
http://blog.skylined.nl/20161101001.html. There you can find a repro
that triggered this issue in addition to the information below.
Follow me on twitter.com/berendjanwever for daily browser bugs.

MSIE 9 MSHTML CAttrArray use-after-free
=======================================
(MS14-056, CVE-2014-4141)

Synopsis
--------
A specially crafted webpage can cause Microsoft Internet Explorer to
reallocate a memory buffer in order to grow it in size. The original
buffer will be copied to newly allocated memory and then freed. The code
continues to use the freed copy of the buffer.

Known affected versions, attack vectors and mitigations
-------------------------------------------------------
+ Microsoft Internet Explorer 9

  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.

Analysis
--------
The CAttrArray object initially allocates a CImplAry buffer of 0x40
bytes, which can store 4 attributes. When the buffer is full, it is
grown to 0x60 bytes. A new buffer is allocated at a different location
in memory and the contents of the original buffer is copied there. The
repro causes the code to do this, but the code continues to access the
original buffer after it has been freed.

Exploit
-------
If an attacker was able to cause MSIE to allocate 0x40 bytes of memory
and have some control over the contents of this memory before MSIE
reuses the freed memory, there is a chance that this issue could be used
to execute arbitrary code. I did not attempt to write an exploit for
this vulnerability myself.

Timeline
--------
* April 2014: This vulnerability was found through fuzzing.
* July 2014: This vulnerability was submitted to [ZDI][].
* July 2014: ZDI reports a collision with a report by another
  researcher. (From the credits given by Microsoft and ZDI, I surmise
  that it was Peter 'corelanc0d3r' Van Eeckhoutte of Corelan who
  reported this issue.
* October 2014: Microsoft release MS14-056, which addresses this issue.
* November 2016: Details of this issue are released.


Cheers,

SkyLined