If you don’t want any one to access your system without permission, you should confirm that you don’t have recovery mode enabled in GRUB boot loader or it must be password protected. Otherwise any one can get into root console as and change the password to own the box.
Here is how you add password to your GRUB prompt.
Login as root
Enter grub mode:
GNU GRUB version 0.97 (640K lower / 3072K upper memory)
[ Minimal BASH-like line editing is supported. For the first word, TAB
lists possible command completions. Anywhere else TAB lists the possible
completions of a device/filename.]