The user "ftpuser" is a user account present in the factory settings of the FRITZ!Box that can be used to access storage connected to the FRITZ!Box. Of course you can also use other user accounts to access storage connected to the FRITZ!Box instead of the user "ftpuser".

The "ftpuser" is granted "Access to NAS contents" and has read and write permission for all storage available on the FRITZ!Box. By default, the password for "ftpuser" is the same as the password for the FRITZ!Box user interface.

If you do not use the "ftpuser", you can delete this user account under "System > FRITZ!Box Users" in the user interface. If needed, you can always create a new user account with the name "ftpuser" in the FRITZ!Box and assign this user the right to "Access to NAS contents".