This indicates an attack attempt against a Cross-Site Scripting vulnerability in Magento.
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.
Magento Commerce 2.1 prior to 2.1.17
Magento Commerce 2.2 prior to 2.2.8
Magento Commerce 2.3 prior to 2.3.1
System Compromise: Remote attackers can execute arbitrary script code in the context of the affected site.
Apply the latest update from the vendor.