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Microsoft Internet Explorer 'onreadystatechange' Use After Free Vulnerability



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:


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.


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.


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.


iDefense is currently unaware of any workarounds for this issue.


Microsoft Corp. has released a patch which addresses this issue. Information about downloadable vendor updates can be found by clicking on the URLs shown.


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 (, which standardizes names for security problems.


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


This vulnerability was reported to iDefense by wushi of team509.

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