A sort of computer virus.
It is able to spread and has a maximum depth of 3.
It won't spread to other hosts but is able to travel up between folders (don't know why).
As soon as you run it, it begins the process, so be careful, I guess.
Deleting the cartridge,
Renaming everything important to you so that the name has a -(hyphen), including folders,
Or putting the cart in a folder with nothing in it.
UFNR doesn't (in this case) stand for anything.
Definition of computer virus (for the nerdy ones): a piece of code that can duplicate itself, with usually a harmful intention or effect. UFNR can spread but doesn't really do anything else.