NamelessMC is a free, easy to use & powerful website software for Minecraft servers. In affected versions an admin can add the ability to have users fill out an additional field and users can inject javascript code into it that would be activated once a staffer visits the user's profile on staff panel. As a result an attacker can execute javascript code on the staffer's computer.

This issue has been addressed in version 2.1.3 and all users are advised to upgrade. There are no known workarounds for this vulnerability.