This indicates an attack attempt to exploit an Insecure Deserialization Vulnerability in Microsoft SharePoint Server.
This vulnerability is due to improper input validation of the workflow rules file used to generate custom workflows. A remote, authenticated attacker could exploit this vulnerability by sending a crafted request to the target server. Successful exploitation could result in the execution of code in the security context of the server process.
Microsoft SharePoint Enterprise Server 2013 Service Pack 1
Microsoft SharePoint Enterprise Server 2016
Microsoft SharePoint Foundation 2013 Service Pack 1
Microsoft SharePoint Server 2019
Microsoft SharePoint Server Subscription Edition
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.