Using IT maintenance software from Solarwinds, the attackers were believed to have entered the computer networks of US government agencies, including those of the Department of Finance and Energy, for espionage purposes.
The attack was discovered late last year.
The exact extent of the information captured has not been made public to this day.
The US government and IT security researchers suspect hackers with connections to the Russian foreign intelligence service were behind the attack.
Moscow always denied the allegations.
Microsoft calls the hacker group "Nobelium".
Recently, she has focused on companies that set up and maintain cloud services for other companies.
They probably hoped to use the service providers 'access to their customers' computer systems, explained the Microsoft experts.
Numerous ransom demands
Online criminals had already been successful with such an attack method in the summer.
Using a vulnerability at the IT service provider Kaseya, they were able to encrypt customers' computers and demand a ransom.
Since July 1, Microsoft has informed 609 customers about almost 23,000 attacks by “Nobelium”, it said.
The success rate of the attacks is in the low single-digit percentage range.
"The latest activity is another indication that Russia is trying to achieve long-term, systematic access to technology supply chains - and to create a mechanism by which targets of interest to the Russian government can be monitored now or in the future," wrote the Microsoft experts.