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.

If exploited, attackers can gain full access to SharePoint content and potentially pivot to Outlook, Teams, and OneDrive. Learn how to protect your SharePoint server from compromise.

Security researchers say Microsoft customers should take immediate action to defend against the ongoing cyberattacks, and must assume they have already been compromised.