The publication of proof-of-concept exploit code affecting Internet Explorer 6 and 7 over the weekend prompted Microsoft on Monday to issue a Security Advisory.

Microsoft says that it is investigating the reported vulnerability, which affects Internet Explorer 6 Service Pack 1 on Microsoft Windows 2000 Service Pack 4, and Internet Explorer 6 and Internet Explorer 7 on Windows XP, Windows Server 2003, Windows Vista, and Windows Server 2008.

Internet Explorer 5.01 Service Pack 4 and Internet Explorer 8 are not affected.

Microsoft says that it is not aware of attacks attempting to exploit the reported vulnerability.

"The vulnerability exists as an invalid pointer reference of Internet Explorer," Microsoft's Advisory states. "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."

The fact that Internet Explorer 6 and 7 together account for about 41% of the Web browser market means attackers will be tempted to exploit the vulnerability, says Michael Sutton, VP of security research at Zscaler, a computer security company.

"Attacks such as these are also prime candidates for targeting otherwise legitimate Web sites as an attack vector," Sutton said in an e-mailed statement. "The exploit can be triggered simply via HTML code, so attackers can inject code into Web sites with weak security protections."

According to Symantec Security Response, the proof-of-concept exploit exhibits inconsistent behavior.

Symantec expects that future exploits prove more effective.

While Microsoft has yet to issue a patch for the vulnerability, its advisory offers several ways to mitigate the risk of attack.

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