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Microsoft is investigating reports of a serious security flaw in Internet Explorer, but has not yet seen malicious code that exploits the reported flaw, the company said today.

Security experts earlier this week warned that code exploiting a newly discovered security hole in IE is circulating on the Internet. The code exploits a buffer overflow vulnerability in IE 6 and has been confirmed on PCs running Windows XP with Service Pack 1 and Windows 2000, according to Danish Security company Secunia.

The U.S. Computer Emergency Readiness Team issued an alert similar to the Secunia advisory. CERT warns that aside from the Web browser, applications such as e-mail clients that rely on browser controls may also be vulnerable. Attackers could gain complete control over a victim's computer by exploiting the flaw, according to Secunia and CERT.

Microsoft is investigating the possible vulnerability, the company says in a statement. However, while Secunia and CERT are raising an alarm over code exploiting the vulnerability being publicly available, Microsoft says it has not seen that yet. "We have not been made aware of any active exploits of the reported vulnerabilities or customer impact at this time, but we are aggressively investigating the public reports," the company says.

In June, IE's global browser usage share was 95 percent, according to the Web analytics firm WebSideStory. However, being the most used browser makes it the browser most likely to be attacked as well. For more information, see " Is It Time to Ditch IE?"

The flaw lies in the way IE handles the "src" and "name" attributes of the "frame" and "iframe" HTML (Hypertext Markup Language) elements, according to the CERT alert. A user could be attacked via a Web page containing malicious code or through an HTML e-mail message.

There is no patch for this flaw, but computers running Windows XP Service Pack 2 appear to be protected, according to Secunia and CERT.

Upon completing its investigation, Microsoft says, it will take the appropriate action to protect Windows users. This may include providing a fix through its monthly patch release process or through an out-of-cycle security update, the company says.