On most systems you can press a hotkey in grub to edit the Linux command line that will be booted and in about 7 keystrokes gain access to any unlocked filesystem. Asking how you can break into a system you physically control is like asking how many ways you could break into a house supposing you had an hour alone with a crowbar the answers are legion. No machine in someone else’s hand which is unlocked can possibly be deemed secure.
Even dumber no installer will create such an insecure configuration because the people that design Linux installers are smarter than you.