Since November I have been releasing details on all vulnerabilities I
found that I have not released before. This is the 37th entry in the
series. This information is available in more detail on my blog at
http://blog.skylined.nl/20161221001.html. There you can find a repro
that triggered this issue in addition to the information below, as well
as a Proof-of-Concept exploit that attempts to prove exploitability.

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MSIE 11 MSHTML CPasteCommand::ConvertBitmaptoPng heap-based BOF
===============================================================
(MS14-056, CVE-2014-4138)

Synopsis
--------
A specially crafted web-page can trigger an out-of-bounds write in
Microsoft Internet Explorer 11. Code that handles pasting images from
the clipboard uses an incorrect buffer length, which allows writing
beyond the boundaries of a heap-based buffer. An attacker able to
trigger this vulnerability can execute arbitrary code.

Known affected software, attack vectors and potential mitigations
-----------------------------------------------------------------
* **Microsoft Internet Explorer 11.0.9600.16521**

  An attacker would need to get a target user to open a specially
  crafted web-page. In order to trigger the issue, the web-page needs
  to either programmatically copy/paste an image using Javascript or
  get the user to do this (for instance by tricking the user into
  typing keyboard shortcuts such as CTRL+C/CTRL+V) . By default, MSIE
  prompts the user to allow or disallow programmatically copy/pasting
  the first time a website tries to do this, so user-interaction is
  normally required in such cases. Disabling the `Allow Programmatic
  clipboard access` setting in `Internet Options` -> `Security
  Settings` -> [Choose a zone] -> `Scripting` should prevent websites
  from programmatically copy/pasting an image. Disabling execution of
  scripts on web-pages altogether will have the same effect. Please
  note that neither option prevents a website from social engineering
  the user into typing a keyboard shortcut to copy/paste the image.

Details
-------
When an image is pasted in MSHTML, it gets converted from BMP format to
PNG. This is done in the `MSHTML!CPasteCommand::ConvertBitmaptoPng`
function. This function incorrectly uses the size of the original BMP
image to allocate memory for storing the converted PNG image. The PNG
image will be smaller than the BMP under most circumstances, but if a
specially crafted image leads to the original BMP image being smaller
than the converted PNG, the function will write PNG data beyond the
bounds of the allocated memory.

Exploit
-------
An attacker looking to exploit this issue will commonly attempt to get
the memory allocated to store the PNG image in a location that is
followed by a pre-allocated memory block that contains information the
attacker would like to modify. Using the buffer overflow, the attacker
can overwrite this pre-allocated memory block with attacker controlled
data. Depending on the type of the pre-allocated memory, this could
allow the attacker to read or modify arbitrary information within the
process and take control of execution flow. No attempt was made to
create a Proof-of-Concept that shows this level of control.

Time-line
---------
* 8 May 2014: This vulnerability was submitted to ZDI.
* 9 June 2014: This vulnerability was acquired by ZDI.
* 23 June 2014: This vulnerability was disclosed to Microsoft by ZDI.
* 14 October 2014: This vulnerability was address by Microsoft in
  MS14-056.
* 21 December 2016: Details of this vulnerability are released.

Cheers,

SkyLined