Cybersecurity reporter Brian Krebs said in a report on Friday that at least 30,000 Microsoft customers had been affected by the attack.
Microsoft, meanwhile, has blamed China for the hacking, claiming state support for the attack, which was carried out using email services.
In this case, the hackers have taken advantage of the weakness of Microsoft's exchange service. Exchange is the organization's mail and calendar server. Government and military contractors, starting with small businesses, use Microsoft's exchange services.