Microsoft has released a critical patch for a security flaw in its SharePoint software. Hackers actively exploited this vulnerability, targeting businesses and US government agencies. The company issued the fix between July 19 and 20.

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.