This indicates an attack attempt on a Cross-Site Scripting (XSS) vulnerability in Emby Media Server.
A remote, unauthenticated attacker can exploit this vulnerability by sending a crafted request to the target server. Successful exploitation results in the execution of arbitrary script code in the browser.
Emby Media Server 4.0
System Compromise : Remote attackers can execute arbitrary script code within the context of the target user's browser
Refer to the vendor supplied advisory for updates: