petri[.]com/microsoft-warns-of-active-exchange-server-exploits-patches-released
krebsonsecurity[.]com/2021/03/at-least-30000-u-s-organizations-newly-hacked-via-holes-in-microsofts-email-software/
wired[.]com/story/china-microsoft-exchange-server-hack-victims/
It started in late January with a network security company detecting cyber attacks breaching on premises / in-house Exchange servers using zero day [previously unknown] vulnerabilities. Microsoft released emergency patches March 3rd -- since then the Chinese group responsible has gone wild, automating the attacks, with at least 10s of thousands of victims worldwide.
Exchange Server is the Microsoft product for email systems -- there are both on premises & in the cloud versions -- Exchange Server in the cloud is unaffected.
wikipedia[.]org/wiki/Microsoft_Exchange_Server
Loads of city / town governments, school systems, hospitals & healthcare systems, small to mid-sized businesses etc. are effected, and while applying the patches from Microsoft closes the door that let the hackers in initially, that does not remove them from the compromised systems. There are not enough security experts in the world to tackle that last part, removing the backdoors and possibly other tools the hackers installed once they were in. Since this is on a previously unimagined scale, one that the original hacking group perhaps cannot manage, it wouldn't be unheard of for other hackers &/or hacking groups to try and take advantage of the backdoors that have been installed. We could for example see all sorts of ransomware attacks.