Chinese hackers have been using a vulnerability in a popular Microsoft email application to break into computers and networks, officials at the company announced on Tuesday. Microsoft said that it had detected a hacker group it dubbed “Hafnium” exploiting four undisclosed vulnerabilities in Microsoft Exchange Server. Hafnium hackers, the company claimed, are likely linked to, or sponsored by, the Chinese government and researchers say they have previously targeted American “infectious disease researchers, law firms, higher education institutions, defense contractors, policy think tanks and [non-governmental organizations].”

It’s unclear which organizations and entities Hafnium hackers targeted but cybersecurity researchers and the National Security Agency have all recommended Microsoft Exchange users patch their software immediately to prevent future break-ins.