Maybe not so much forgotten, but an undocumented password.
It’s an easy process. Boot into grub, press e on your usual boot line and then change the /boot/vmlinuz... entry so that it’s read-write not read-only and add on init=/bin/bash. Ctrl+X to continue booting to a bash prompt. Then change the password using passwd root, eg.
At grub boot press e
Locate the /boot/vmlinuz... line
Change the ro to rw
Add init=/bin/bash to the end of it
Press CTRL+X to continue.
At the bash prompt:
passwd root
Then reboot.