JavaScript pre-processing can be used by the attacker to gain access to the file system (read-only access on behalf of user "zabbix") on the Zabbix Server or Zabbix Proxy, potentially leading to unauthorized access to sensitive data.
An unauthenticated user can create a link with reflected Javascript code inside the backurl parameter and send it to other authenticated users in order to create a fake account with predefined login, password and role in Zabbix Frontend.