The flaw in Microsoft's Internet Explorer could allow criminals to take control of people's computers and steal their passwords, internet experts say.
Microsoft is investigating the problem and preparing an emergency software patch to resolve it, it says.
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 version seven of the browser - its most widely used edition.
But the company warned that other versions were also potentially vulnerable.
source : BBC