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Security Services

Microsoft Internet Explorer 9 CTreePos Use-after-Free Vulnerability



Internet Explorer is Microsoft's web browser for the Windows platform.


Remote exploitation of a use-after-free vulnerability in Microsoft Corp.'s Internet Explorer 9 could allow an attacker to execute arbitrary code with the privileges of the current user. The vulnerability occurs when handling block type elements in Internet Explorer 9. When several types of elements are stylized with certain CSS properties, it is possible to trigger a use-after-free vulnerability when the DOM tree is torn down (during a refresh, navigation or modification). JavaScript is not required to trigger the teardown, but it does facilitate exploitation. The use-after-free condition can lead to a memory corruption, which leads to 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 Web page that an attacker created. 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.

To exploit this vulnerability, it is necessary to control heap data in the chunk that is freed and reused. Testing has demonstrated that this can be difficult due to the window of time between free and reuse.


This vulnerability exists in Internet Explorer 9.


Several workarounds involving disabling or restricting site access to ActiveScript can be employed to make exploitation more difficult; however, this will not prevent the vulnerability from being exploited if the user refreshes the page manually. The following workarounds can be employed to disable scripting:

  • Set Internet and Local intranet security zone settings to "High" to block ActiveX Controls
  • and Active Scripting in these zones. Configure Internet Explorer to prompt before running Active Scripting or to disable Active Scripting in the Internet and Local intranet security zone; add trusted sites to the Internet Explorer trusted sites zone.
Keep in mind that disabling or restricting scripting can severely impact the usability of the browser.


Microsoft Corp. has released patches which addresses this issue. For more information, consult their advisory at the following URL:


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2012-1539 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems.


08/08/2012 Initial Vendor Notification
08/08/2012 Initial Vendor Reply
11/13/2012 Coordinated Public Disclosure


This vulnerability was reported to iDefense by Jose A. Vazquez of

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Copyright © 2012 Verisign, Inc.

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