Microsoft urged people to be vigilant while it investigated and prepared an emergency patch to resolve it.

Internet Explorer is used by the vast majority of the world's computer users.

"Microsoft is continuing its investigation of public reports of attacks against a new vulnerability in Internet Explorer," said the firm in a security advisory alert about the flaw.

Microsoft says it has detected attacks against IE 7.0 but said the "underlying vulnerability" was present in all versions of the browser.

Other browsers, such as Firefox, Opera, Chrome, Safari, are not vulnerable to the flaw Microsoft has identified.