2021 Microsoft Exchange Server data breach

A global wave of cyberattacks and data breaches began in January 2021 after four zero-day exploits were discovered in on-premises Microsoft Exchange Servers, giving attackers full access to user emails and passwords on affected servers, administrator privileges on the server, and access to connected devices on the same network. Attackers typically install a backdoor that allows the attacker full access to impacted servers even if the server is later updated to no longer be vulnerable to the original exploits.

Source: Wikipedia — 2021 Microsoft Exchange Server data breach (CC BY-SA 4.0)