If running the Trivial File Transfer Protocol (TFTP) service is necessary,
it should be configured to change its root directory at startup. To do so,
find the path for the tftp systemd service:
$ sudo systemctl show tftp | grep ExecStart=
ExecStart={ path=/usr/sbin/in.tftpd ; argv[]=/usr/sbin/in.tftpd -s /var/lib/tftpboot ; ignore_errors=no ; start_time=[n/a] ; stop_time=[n/a] ; pid=0 ; code=(null) ; status=0/0 }e
and ensure the ExecStart line on that file includes the -s option with a subdirectory:
ExecStart=/usr/sbin/in.tftpd -s