Microsoft Security Advisory (977981)
Vulnerability in Internet Explorer Could Allow Remote Code Execution
Published: November 23, 2009
Version: 1.0
Microsoft is investigating new public reports of a vulnerability in Internet Explorer. This advisory contains information about which versions of Internet Explorer are vulnerable as well as workarounds and mitigations for this issue.
Our investigation so far has shown that Internet Explorer 5.01 Service Pack 4 and Internet Explorer 8 on all supported versions of Microsoft Windows are not affected, and that Internet Explorer 6 Service Pack 1 on Microsoft Windows 2000 Service Pack 4, and Internet Explorer 6 and Internet Explorer 7 on supported editions of Windows XP, Windows Server 2003, Windows Vista, and Windows Server 2008 are affected.
The vulnerability exists as an invalid pointer reference of Internet Explorer. It is possible under certain conditions for a CSS/Style object to be accessed after the object is deleted. In a specially-crafted attack, Internet Explorer attempting to access a freed object can lead to running attacker-supplied code.
At this time, we are aware of no attacks attempting to use this vulnerability against Internet Explorer 6 Service Pack 1 and Internet Explorer 7. We will continue to monitor the threat environment and update this advisory if this situation changes. On completion of this investigation, Microsoft will take the appropriate action to protect our customers, which may include providing a solution through our monthly security update release process, or an out-of-cycle security update, depending on customer needs.
We are actively working with partners in our Microsoft Active Protections Program (MAPP) and our Microsoft Security Response Alliance (MSRA) programs to provide information that they can use to provide broader protections to customers. In addition, we’re actively working with partners to monitor the threat landscape and take action against malicious sites that attempt to exploit this vulnerability.
Microsoft continues to encourage customers to follow the "Protect Your Computer" guidance of enabling a firewall, applying all software updates and installing anti-virus and anti-spyware software.
Workarounds
Microsoft has tested the following workarounds. Although these workarounds will not correct the underlying vulnerability, they help block known attack vectors. When a workaround reduces functionality, it is identified in the following section.
* Set Internet and Local intranet security zone settings to "High" to prompt before running 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
* Enable DEP for Internet Explorer 6 Service Pack 2 or Internet Explorer 7
We just released Security Advisory 977981 concerning an issue affecting Internet Explorer 6 and Internet Explorer 7 that could lead to remote code execution. At this time, we are not aware of any active attacks seeking to use this vulnerability. Our teams are currently working to develop an update and we will take appropriate action to protect customers when the update has met the quality bar for broad distribution. That may include releasing the update out of band.
I want to point out that Internet Explorer 8 is not affected on any platform and that running Protected Mode in Internet Explorer 7 on Windows Vista mitigates this issue. We provide more guidance and workarounds in the advisory so I encourage customers to review it right away.