This indicates an attack attempt to exploit a Remote Code Execution in Microsoft Exchange Server.
The vulnerability is due to insufficient sanitization when handling a malicious request. A remote attacker may be able to exploit this to disclose data or execute arbitrary code within the context of the application, via a crafted HTTP request.
Microsoft Exchange Server 2016 Cumulative Update 21
Microsoft Exchange Server 2016 Cumulative Update 22
Microsoft Exchange Server 2019 Cumulative Update 10
Microsoft Exchange Server 2019 Cumulative Update 11
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.