This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in Apache ActiveMQ.
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 script code within the context of the application.
Apache ActiveMQ before version 5.15.5
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected system.
Apply the most recent upgrade or patch from the vendor.