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

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.

Hackers with ties to the Chinese government have been linked to a recent wave of widespread attacks targeting a Microsoft SharePoint zero-day vulnerability chain.