The Justice Department announced a “court-authorized operation” by the FBI to copy and remove “malicious web shells” from hundreds of U.S. computers in response to the massive cyberattacks against Microsoft’s Exchange Server, which the Big Tech company has assessed are being carried out by a sophisticated Chinese state-backed hacker group and others.

Microsoft detected “multiple zero-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks,” the company said in an early March announcement, adding that its Threat Intelligence Center attributed the cybercampaign with “high confidence” to a hacker group dubbed “Hafnium.” Microsoft said the hacker group was “state-sponsored” and operating out of China. The Microsoft Exchange Server handles the company’s email, calendar, scheduling, contact, and collaboration services.