Microsoft is issuing an emergency fix to close off a vulnerability in Microsoft’s SharePoint software that hackers have exploited to carry out widespread attacks on businesses and at least some federal agencies.

The attacks, discovered over the weekend, exploit a previously unknown vulnerability in the document-sharing software, prompting immediate action from both Microsoft and federal investigators. Newsweek reached out to Microsoft and the Cybersecurity and Infrastructure Security Agency (CISA) via email for comment.

Microsoft warned users that hackers used a security flaw in its server software to attack government agencies and businesses around the world. The federal government, along with those in Canada and Australia, has launched an investigation into the hack that targeted SharePoint servers, The Washington Post reported.