This indicates an attack attempt to exploit an Insecure Deserialization Vulnerability in MuleSoft Runtime.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker can exploit this to execute arbitrary code within the context of the application, via a crafted HTTP request.
MuleSoft Runtime prior to version 3.8
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.