OpenWRT serial console is not protected by default by a password. As a principle, networking hardware should never be accessible and should be locked-down to avoid attacks.
The orginal forum thread can be read here.
A support ticket was opened here.
After OpenWRT first boot, a password is defined by the user in order to protect SSH and luci HTTP(S) access. However access to the serial console is still available without password. Very few OpenWRT users are aware that their hardware is wide open, and you should be aware and find solutions.
A workaround is to nable login that is part of the busybox package:
You may need to recompile busybox.
Edit the file /etc/inittab and replace
Single user mode is available through GRUB and allows to boot without password. An attacker is then able to change root password and reboot.
A solution would be to lock-down OpenWRT booloader process, to make sure that booting in linux single user mode is impossible. This has to be discussed and this is not yet documented.
You should know that hardware attacks on serial console pins are always possible. However, it requires time and skills.