This indicates an attack attempt to exploit a Remote Code Execution Vulnerability in Microsoft SharePoint 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 SharePoint Enterprise Server 2016
Microsoft SharePoint Server 2019
Microsoft SharePoint Foundation 2013 Service Pack 1
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.