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 yesterday.

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 US Computer Emergency Readiness Team issued an alert similar to the Secunia advisory. CERT warns that aside from the web browser, applications such as email 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.

There is no patch for the present 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.