This indicates an attack attempt to exploit a Cross-Site Scripting vulnerability in Symantec Endpoint Protection Manager (SEPM).
The vulnerability is due to insufficient sanitizing of user supplied inputs in the application. A remote attacker may be able to exploit this to bypass protection mechanism to obtain session ID.
Symantec Endpoint Protection Manager and client 12.1
System Compromise: Remote attackers can execute arbitrary script code in the context of the application.
Refer to the vendor's advisory for updates: