The Biden administration is warning about cyberattacks against Microsoft’s Exchange Server, which the Big Tech company says are being carried out by a sophisticated hacker group based in China and backed by the Chinese government.

Microsoft detected “multiple zero-day exploits being used to attack on-premises versions of Microsoft Exchange Server in limited and targeted attacks” and that its Threat Intelligence Center had attributed the cyber campaign with “high confidence” to a hacker group dubbed “Hafnium,” the company announced this week. Microsoft said the hacker group was “state-sponsored” and operating out of China, an assessment it said was based on “observed victimology, tactics, and procedures.” The Microsoft Exchange Server handles the company’s email, calendar, scheduling, contact, and collaboration services.