This indicates an attack attempt to exploit a Security Bypass Vulnerability in Microsoft Sharepoint.
The vulnerability is due to improper validation of website controls. A remote attacker can exploit this to execute arbitrary code within the context of the target system.
Microsoft SharePoint Enterprise Server 2016
Microsoft SharePoint Foundation 2010 Service Pack 2
Microsoft SharePoint Foundation 2013 Service Pack 1
Microsoft SharePoint Server 2019
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.