I stumbled across this today, and it worked. I haven’t tested it on v8, though. Maybe give it a shot on v8 and let me know in the comments?
At the boot menu, press e to edit the existing kernel (Core).
Scroll down to the list until you see the line ro,change ro to rw and start into a bash shell. It should look like rw init=/sysroot/bin/sh.
Press Ctrl + X to start into single user mode.
In the single user mode run the commands:
chroot /sysroot
passwd root
touch /.autorelabel
Exit and reboot your system. You should be able to sign on and use the system with the new password.