iDefense Security Advisory 12.08.09
http://labs.idefense.com/intelligence/vulnerabilities/ 
Dec 08, 2009

I. BACKGROUND

Internet Explorer is a graphical web browser developed by Microsoft
Corp. that has been included with Microsoft Windows since 1995. For
more information about Internet Explorer, please the visit following
website: http://www.microsoft.com/ie/ 

II. DESCRIPTION

Remote exploitation of a memory corruption vulnerability in Microsoft
Corp.'s Internet Explorer could allow an attacker to execute arbitrary
code with the privileges of the current user.

The vulnerability exists due to an uninitialized stack variable in the
'CLayout::EnsureDispNode' method. This method is called to recalculate
the location of various HTML elements within the page. This function
passes a 'CDispNodeInfo' object to another function,
'CLayout::GetDispNodeInfo', which is supposed to initialize the object
passed in. However, the function fails to properly initialize a flags
value that is used later to determine how many "extra" bytes to
allocate for a heap buffer. This eventually leads to undersized buffer
being allocated to hold a 'CDispClipNode' object in the
'CLayout::EnsureDispNodeCore' function. The vulnerability manifests
itself when the 'CDispNode::SetExpandedClipRect' function attempts to
use the invalid "extra size" to calculate an offset into the object,
and manipulate a bit at this location. This corrupts the objects VTABLE
by setting the 2nd bit to 1, which can lead to the execution of
arbitrary code when this pointer is later accessed.

III. ANALYSIS

Exploitation of this vulnerability results in the execution of arbitrary
code with the privileges of the user viewing the web page. To exploit
this vulnerability, a targeted user must load a malicious webpage
created by an attacker. An attacker typically accomplishes this via
social engineering or injecting content into compromised, trusted
sites. After the user visits the malicious web page, no further user
interaction is needed.

The successful exploitation of this vulnerability is dependent upon the
version of mshtml.dll present on the targeted host. Since the
vulnerability does not allow an attacker to directly control the
corrupted VTABLE's value, exploitation depends upon the existing value
of this pointer. For example, on Internet Explorer 6, with mshtml.dll
version 6.00.2900.3527, the vulnerability is not exploitable because
the corrupted VTABLE address is not a mappable userland address.

IV. DETECTION

iDefense confirmed the existence of this vulnerability in Internet
Explorer versions 6 and 7. Internet Explorer versions 5 and 8 do not
appear to be affected.

V. WORKAROUND

Disabling Active Scripting will prevent this vulnerability from being
exploited.

VI. VENDOR RESPONSE

Microsoft Corp. has released a patch which addresses this issue.
Information about downloadable vendor updates can be found by clicking
on the URLs shown.
http://www.microsoft.com/technet/security/bulletin/ms09-072.mspx 

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2009-3672 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for 
security problems.

VIII. DISCLOSURE TIMELINE

06/09/2009  Initial Vendor Notification
06/09/2009  Initial Vendor Reply
12/08/2009  Coordinated Public Disclosure

IX. CREDIT

This vulnerability was reported to iDefense by team509.

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X. LEGAL NOTICES

Copyright =A9 2009 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
 There are no warranties with regard to this information. Neither the
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