This indicates an attack attempt against a Code Execution vulnerability in Apache ActiveMQ.
The vulnerability is caused by an error when the broker de-serializes a crafted Java Message Service (JMS) ObjectMessage object. A remote attacker may be able to exploit this to execute arbitrary code on the affected systems.
Apache ActiveMQ 5.x before 5.13.0
System Compromise: Remote attacker can gain control of vulnerable systems.
Refer to the vendor's website for patch or update.