The flaw, which has not yet been patched, has been used in "limited, targeted attacks," Microsoft said Friday in an update to its .

"We've noticed some highly targeted and apparently politically motivated attacks against our users," Google said in blog post. "We believe activists may have been a specific target. We've also seen attacks against users of another popular social site."

The attack is triggered when the victim is tricked into visiting a maliciously encoded Web page -- what's known as a Web drive-by attack. It gives the attacker a way of hijacking the victims browser and accessing Web applications without authorization.

The flaw lies in the Windows mshtml.dll software library used by Internet Explorer, and affects all currently supported versions of Windows.

Microsoft has released a that users can download to repair the problem, but has not said when, or even if, it plans to push out a comprehensive security update to all users.