This indicates an attack attempt to exploit an Insecure Deserialization Vulnerability in VMware vCenter.
This vulnerability is due to insufficient validation of user-supplied data. A specially-crafted HTTP request by an attacker can lead to remote code execution.
VMware vCenter Server prior to 6.5 Update 3t
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.