An Internet Explorer flaw made public two months ago is now being used in online attacks.
The flaw, which has not yet been patched, has been used in "limited, targeted attacks", Microsoft said in an update to its security advisory on the issue.
Google concurred, and offered a few more details. "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 authorisation.
The flaw lies in the Windows MHTML (Mime HTML) parsing software used by Internet Explorer, and affects all currently supported versions of Windows. It was disclosed on the Full Disclosure mailing list in January.
Microsoft has released a Fixit tool 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.
Google isn't saying who exactly was targeted in this latest incident, but Chinese activist groups have been the focus of cyber attacks in the past. This may be another example of an ongoing and methodical effort to track and steal information from pro-democracy and Tibetan activists.
Now that the flaw is being exploited in attacks, the pressure is mounting on Microsoft to produce a reliable patch for the issue that can be pushed out to hundreds of millions of customers.
"For now, we recommend concerned users and corporations seriously consider deploying Microsoft's temporary Fixit to block this attack until an official patch is available," Google said.