This indicates an attack attempt to exploit a remote Code Execution vulnerability in Fastjson.
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application.
Fastjson version 1.2.24 and prior.
System Compromise: Remote attackers can gain control of vulnerable systems
Apply the most recent upgrade or patch from the vendor.