The user "ftpuser" is a user account used in the FRITZ!Box in older FRITZ!OS versions that could be used to access storage connected to the FRITZ!Box.

The "ftpuser" was granted "Access to NAS contents" by default and had read and write permission for all storage available on the FRITZ!Box. For security reasons, the "ftpuser" could not access the FRITZ!Box over the internet. By default, the password for "ftpuser" was the same as the password for the FRITZ!Box user interface.

If a user account "ftpuser" is still present in your FRITZ!Box, you can delete the "ftpuser" under "System > FRITZ!Box Users" in the user interface.