iDefense Security Advisory 03.30.10
http://labs.idefense.com/intelligence/vulnerabilities/ 
Mar 30, 2010

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 use after free vulnerability in Microsoft
Corp.'s Internet Explorer could allow an attacker to execute arbitrary
code with the privileges of the current user.

The vulnerability occurs when an HTML object with an
'onreadystatechange' event handler is not properly freed. This event is
used to perform actions when the state of some HTML object changes; for
example, when a form has data input. Specifically, when certain
properties of the object are changed, the event handler function object
is freed, but a reference to it remains. When the object is later
accessed, this invalid memory is treated as an object pointer, and one
of its members is used to make an indirect function call. This results
in the execution of arbitrary code.

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.

IV. DETECTION

iDefense has confirmed the existence of this vulnerability in Internet
Explorer version 6. Previous versions may also be affected, however,
Internet Explorer 7 and 8 are not affected.

V. WORKAROUND

iDefense is currently unaware of any workarounds for this issue.

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/ms10-018.mspx 

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2010-0491 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

07/21/2009  Initial Vendor Notification
07/21/2009  Initial Vendor Reply
03/30/2010  Coordinated Public Disclosure

IX. CREDIT

This vulnerability was reported to iDefense by wushi of team509.

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X. LEGAL NOTICES

Copyright =A9 2010 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
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.