A new exploit targeting Internet Explorer was announced by Microsoft yesterday, and Microsoft has released an advisory with information and workarounds. According to Microsoft's investigation so far, 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.

Security firm Symantec has also confirmed that it affects Internet Explorer versions 6 and 7.

"The exploit currently exhibits signs of poor reliability, but we expect that a fully-functional reliable exploit will be available in the near future. When this happens, attackers will have the ability to insert the exploit into Web sites, infecting potential visitors. For an attacker to launch a successful attack, they must lure victims to their malicious Web page or a Web site they have compromised. In both cases, the attack requires JavaScript to exploit Internet Explorer," Symantec said.

The security firm suggests Internet Explorer users to ensure their antivirus definitions are up to date, disable JavaScript and only visit Web sites they trust until fixes are available from Microsoft.

On completion of Microsoft's investigation, the conmpany may include providing a solution through its monthly security update release process, or an out-of-cycle security update.