Microsoft has warned that a group of hackers linked to attacks on the Democratic National Committee had exploited a vulnerability in all Windows PCs and that it would not be able to fully mend this for another week.
The flaw was disclosed publicly on Monday by Google, provoking a sharp rebuke from Microsoft about the dangers of revealing flaws like this before fixes are available.
"Hackers that Microsoft researchers call "Strontium" launched a low-volume spear-phishing campaign aimed at a specific set of people", Windows and Devices Group executive Vice President, Terry Myerson said, reported by AFP.
Spear phishing is an e-mail spoofing fraud attempt that targets a specific organization, seeking unauthorized access to confidential data. Spear phishing attempts are not typically initiated by "random hackers" but are more likely to be conducted by perpetrators out for financial gain, trade secrets or military information.
The vulnerability exposed this week let hackers slip into computers through web browsing software, eventually installing "back doors" through which they could enter as they please.
Strontium, given an array of names by cyber security researchers, has been described as a sophisticated hacking operation with ties to Russia.
The group has also been suspected of involvement in hacks of political parties in the US presidential race.
In a threat report, Microsoft described Strontium as having the markings of being backed by a nation state, but did not conclude which one.
The US technology titan did take aim at Google, saying that a "responsible" member of the industry would have put users first and waited for a patch to be ready before making the vulnerability public.
"Google's decision to disclose these vulnerabilities before patches are broadly available and tested is disappointing, and puts customers at increased risk," Myerson said.
Google gave Microsoft seven days to fix the flaw, as per its policy regarding critical flaws, before making it public on Monday.
"This vulnerability is particularly serious because we know it is being actively exploited," threat analysis group, Google, Neel Mehta and Billy Leonard said, reports AFP.
Google said it told Microsoft and Adobe about the vulnerabilities on October 21, and an update to Flash addressing the weakness was released five days later.
Microsoft said people using the Edge browser on fully updated Windows 10 operating systems should be protected from the attack.
"We have coordinated with Google and Adobe to investigate this malicious campaign and to create a patch for down-level versions of Windows," Myerson said.
A Microsoft threat report described Strontium as a group that usually targets government agencies, diplomatic institutions and military organizations, as well as affiliated private sector organisations such as defense contractors and public policy research institutes.