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CVE-2016-3247 Microsoft Edge CTextExtractor::GetBlockText OOB read details



Throughout November, I plan to release details on vulnerabilities I
found in web-browsers which I've not released before. This is the
fourteenth entry in that series. Unfortunately I won't be able to
publish everything within one month at the current rate, so I may
continue to publish these through December and January.

The below information is available in more detail on my blog at
http://blog.skylined.nl/20161118002.html.

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

Microsoft Edge CTextExtractor::GetBlockText OOB read
=====================================
(MS16-104, CVE-2016-3247)

Synopsis
--------
A specially crafted web-page can cause an integer underflow in Microsoft
Edge. This causes `CTextExtractor::GetBlockText` to read data outside of
the bounds of a memory block.

Known affected software, attack vectors and mitigations
-------------------------------------------------------
* Microsoft Edge 11.0.10240.16384
  An attacker would need to get a target user to open a specially
  crafted web-page. JavaScript is not necessarily required to trigger
  the issue.

Description
-----------
Though I did not investigate thoroughly, I did find out the following:
* The root cause appears to be an integer underflow in a 32-bit
  variable used in `CTextExtractor..GetBlockText` as an index to read a
  `WCHAR` in a string buffer. This index is decreased once too often
  and becomes -1, or a very large positive number depending on how it
  is used.
* This does not result in a crash on 32-bit systems, as an integer wrap
  causes the code to read one `WCHAR` before the start of the buffer,
  which is normally also in allocated memory.
* On 64-bit systems, the 32-bit -1 value is interpreted as 0xFFFFFFFF,
  a very large positive value. As this is an index into a `WCHAR`
  string, it gets multiplied by two and added to the start of the
  buffer to find the location of a `WCHAR` to read. This causes the OOB
  read to be around 8Gb (!!) beyond the address at which the buffer is
  allocated.
* The crash happens in code that appears to be rendering the web-page,
  which does not immediately offer an obvious way of extracting
  information using this bug.

Exploit
-------
This is where it gets interesting, as the OOB read happens
approximately 0x2`00000000 bytes after the address at which the buffer
is allocated. This presents us with a problem: how to store some
information that we'd be interested in reading at such a large offset
from the original allocation?

As one might come to expect from me, I used a heap spray. But it needed
to be a special kind of heap spray as I did not want to actually have to
allocate 8Gb of RAM. However, about ten years ago (boy, time flies!) I
developed a heap spray that uses significantly less RAM than a
traditional heap spray does; in practice probably about 33% in most
cases, but theoretically much more in ideal situations. I've been
meaning to blog about it, but never found the time to do so until today:
you can read all about it here:

http://blog.skylined.nl/20161118001.html

That said, I have not actually looked at whether it is possible to
exfiltrate useful information using this bug. However, I did write a
Proof-of-Concept that attempts to make sure something is allocated in
the area where the OOB read happens. This PoC uses these heap spray
tricks to spray the heap while minimizing memory use. The
Proof-of-Concept uses about ~5.3Gb to allocate the memory at around 8Gb
distance from the buffer (up to ~10Gb to be sure). When you load the PoC
in a 64-bit version of Edge, you may notice that, unlike the original
repro, it will not crash Edge (even though it does trigger the
issues): the heap spray has allocated the memory that the out-of-bounds
read accesses, and this prevents an access violation exception.
Refreshing the page is likely to screw up the precise allocation process
needed and will probably cause a crash.

This proves that it is theoretically possible to allocate information at
the address used by the code. All that is left is prove that the
information read by the code can be exfiltrated somehow, and you have a
working exploit. This is left as an exercises to the reader.

Time-line
---------
* June 2016: This vulnerability was found through fuzzing.
* June 2016: This vulnerability was submitted to ZDI and iDefense.
* July 2016: This vulnerability was acquired by ZDI.
* September 2016: This vulnerability was addressed by Microsoft in
  MS16-104.
* November 2016: Details of this issue are released.

Cheers,

SkyLined

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