Microsoft on Tuesday warned that a Windows vulnerability exposed by rival Google was being exploited by a hacking group suspected of cyber attacks on US political institutions.
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 in a blog post.
Spear phishing involves tricking people with personally crafted email messages that get them to click on links or files booby-trapped with malicious code that infects machines, letting hackers steal information or take control.
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.
However, 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 countered that it gave Microsoft seven days to fix the flaw, as per its policy regarding critical flaws, before making it public on Monday.