This indicates an attack attempt to exploit a Cross-Site Scripting Vulnerability in SolarWinds Serv-U FTP Server.
The vulnerability is due to insufficient sanitizing of user supplied inputs when the vulnerable software handles a maliciously crafted HTTP request. A remote attacker may be able to exploit this to execute arbitrary script code within the context of the target user.
SolarWinds Serv-U FTP Server prior to 15.1.7 Hotfix 2
System Compromise: Remote attackers can execute arbitrary script code within the context of the target user.
Apply the most recent upgrade or patch from the vendor.