Tens of thousands of organizations in Europe, Asia and the United States have been affected by a recently discovered vulnerability in Microsoft's email service Exchange, according to data released by
The existence of the vulnerability was revealed by Microsoft on Tuesday.
The company also accused China of exploiting the leak.
Hackers would have been able to steal the contents of e-mail boxes as a result.
How many organizations have actually fallen victim is unclear, according to a source from
This may be a relatively small number, although that number can increase if vulnerable companies, governments and organizations do not plug the leak.
In addition to the Chinese hackers, others could also use the information to break into Outlook or Exchange email servers.
"Microsoft continues to see more and more use of these vulnerabilities in attacks targeting non-botched systems," Microsoft wrote in its blog on Friday.
In addition to Hafnium (the Chinese hackers), this is also done by "several malicious parties", according to the company.
Beijing denies being behind hacking attacks.
Microsoft has made resources available to help affected organizations defend themselves against attacks.
The company offers, among other things, methods with which organizations can protect themselves against an attack or close the leak to a limited extent.
Organizations can also check whether they have been hacked.