Disable auto-login on 16.04 when connecting via serial console?

Question

I have an ARM server running Ubuntu 16.04. I am connecting to the machine via the serial port.

When I reboot the system, it automatically logs in as root, without the usual username/password prompt. This is what I see after it completes the usual bootup stuff:

Ubuntu 16.04.3 LTS ubuntu ttyAMA0

ubuntu login: root (automatic login)

Last login: Thu Mar  1 08:07:07 UTC 2018 on ttyAMA0
Welcome to Ubuntu 16.04.3 LTS (GNU/Linux 4.8.0-32-t99-perf aarch64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

  Get cloud support with Ubuntu Advantage Cloud Guest:
    http://www.ubuntu.com/business/services/cloud

50 packages can be updated.
1 update is a security update.


root@ubuntu:~# 

How can I configure it so it prompts for the username/password instead?

Answer 1

@Pablo Bianchi's comment was close... It turns out the file I needed to edit was:

/lib/systemd/system/serial-getty@.service

Specifically change this line:

ExecStart=-/sbin/agetty --autologin root --keep-baud 115200,38400,9600 %I $TERM

to:

ExecStart=-/sbin/agetty --keep-baud 115200,38400,9600 %I $TERM

Hurrah for grepping blindly and arriving at a solution!