2021-March. Microsoft has detected multiple 0-day exploits being used to attack on-premises versions of Microsoft Exchange Server. It is believed that HAFNIUM used these vulnerabilities to access on-premises Exchange Servers, which enabled access to email accounts and allowed the installation of additional malware to facilitate long-term access to victim environments.
Security updates to Exchange servers, troubleshooting and restoring services. Upgrade to the cloud server, to prevent a similar attack.