Preventing Access to Single User Mode — If attackers can boot the system into single user mode, they are logged in automatically as root without being prompted for the root password.

Preventing Access to the GRUB Console — If the machine uses GRUB as its boot loader, an attacker can use the use the GRUB editor interface to change its configuration or to gather information using the cat command.

Preventing Access to Non-Secure Operating Systems — If it is a dual-boot system, an attacker can select at boot time an operating system, such as DOS, which ignores access controls and file permissions.