// back

Microsoft Internet Explorer HTML Layout Engine Uninitialized Memory Vulnerability

12.08.09

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/

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.

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.

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.

WORKAROUND

Disabling Active Scripting will prevent this vulnerability from being exploited.

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

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.

DISCLOSURE TIMELINE

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

CREDIT

This vulnerability was reported to iDefense by team509.

Get paid for vulnerability research
http://labs.idefense.com/methodology/vulne rability/vcp.php

Free tools, research and upcoming events
http://labs.idefense.com/

LEGAL NOTICES

Copyright © 2009 Verisign, Inc.

Permission is granted for the redistribution of this alert electronically. It may not be edited in any way without the express written consent of iDefense. If you wish to reprint the whole or any part of this alert in any other medium other than electronically, please e-mail customer service for permission.

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 author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.