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Security Services

Microsoft Internet Explorer 9 Container Cache Uninitialized Memory Vulnerability



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


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

The vulnerability occurs when accessing an array of container elements during HTML parsing. IE fails to fully initialize an array element, which is later accessed when iterating through a cache of container elements. Uninitialized values are used to calculate an offset into an additional array of structures. Later on during processing, this pointer is used to make an indirect function call. This can lead 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 has 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 uninitialized memory block. Labs testing has demonstrated that this is possible and that it can be performed reliably. Despite the ability to control the memory, exploitation of this vulnerability is further complicated by the need to know the base address of mshtml.dll. Without knowing this value, the code will exit without using the invalid memory. Since this vulnerability only affects IE 9 (which only runs on Windows Vista and Windows 7), ASLR will make guessing the pointer value difficult. Based on this, exploitation without an additional vulnerability to determine the address space layout will be difficult.


Internet Explorer 9 is vulnerable.


Several workarounds involving disabling or restricting site access to ActiveScript can be employed to mitigate this vulnerability, including the following:

  • 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-2012-1522 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems.


04/25/2012 Initial Vendor Notification
04/25/2012 Initial Vendor Reply
07/10/2012 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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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.