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. Additional information can be found at Security at home.

Mitigating Factors:

• Internet Explorer 8 is not affected.
• Protected Mode in Internet Explorer 7 in Windows Vista limits the impact of the vulnerability.
• By default, Internet Explorer on Windows Server 2003 and Windows Server 2008 runs in a restricted mode that is known as Enhanced Security Configuration. This mode sets the security level for the Internet zone to High. This is a mitigating factor for Web sites that you have not added to the Internet Explorer Trusted sites zone.
• An attacker who successfully exploited this vulnerability could gain the same user rights as the local user. Users whose accounts are configured to have fewer user rights on the system could be less affected than users who operate with administrative user rights.
• By default, all supported versions of Microsoft Outlook, Microsoft Outlook Express, and Windows Mail open HTML e-mail messages in the Restricted sites zone. The Restricted sites zone helps mitigate attacks that could try to exploit this vulnerability by preventing Active Scripting and ActiveX controls from being used when reading HTML e-mail messages. However, if a user clicks a link in an e-mail message, the user could still be vulnerable to exploitation of this vulnerability through the Web-based attack scenario.

More at source:

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