4.2. BIOS and Boot Loader Security

4.2.1. BIOS Passwords

The following are the two primary reasons for password protecting the BIOS of a computer[1]:

1. Preventing Changes to BIOS Settings — If an intruder has access to the BIOS, they can set it to boot from a diskette or CD-ROM. This makes it possible for them to enter rescue mode or single user mode, which in turn allows them to start arbitrary processes on the system or copy sensitive data.

2. Preventing System Booting — Some BIOSes allow password protection of the boot process. When activated, an attacker is forced to enter a password before the BIOS launches the boot loader.

Because the methods for setting a BIOS password vary between computer manufacturers, consult the computer's manual for specific instructions.

If you forget the BIOS password, it can either be reset with jumpers on the motherboard or by disconnecting the CMOS battery. For this reason, it is good practice to lock the computer case if possible. However, consult the manual for the computer or motherboard before attempting to disconnect the CMOS battery.

4.2.2. Boot Loader Passwords

The following are the primary reasons for password protecting a Linux boot loader:

1. 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.

2. 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.

3. 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.

The GRUB boot loader ships with Red Hat Enterprise Linux on the x86 platform. For a detailed look at GRUB, consult the chapter titled The GRUB Boot Loader in the Red Hat Enterprise Linux Reference Guide.

4.2.2.1. Password Protecting GRUB

GRUB can be configured to address the first two issues listed in Section 4.2.2 Boot Loader Passwords by adding a password directive to its configuration file. To do this, first decide on a password, then open a shell prompt, log in as root, and type:

/sbin/grub-md5-crypt

When prompted, type the GRUB password and press [Enter]. This returns an MD5 hash of the password.

Next, edit the GRUB configuration file /boot/grub/grub.conf. Open the file and below the timeout line in the main section of the document, add the following line:

password --md5 <password-hash>

Replace <password-hash> with the value returned by /sbin/grub-md5-crypt[2].

The next time the system boots, the GRUB menu does not allow access to the editor or command interface without first pressing [p] followed by the GRUB password.

Unfortunately, this solution does not prevent an attacker from booting into a non-secure operating system in a dual-boot environment. For this, a different part of the /boot/grub/grub.conf file must be edited.

Look for the title line of the non-secure operating system and add a line that says lock directly beneath it.

For a DOS system, the stanza should begin similar to the following:

title DOS lock

	Warning
	A password line must be present in the main section of the /boot/grub/grub.conf file for this method to work properly. Otherwise, an attacker can access the GRUB editor interface and remove the lock line.

To create a different password for a particular kernel or operating system, add a lock line to the stanza, followed by a password line.

Each stanza protected with a unique password should begin with lines similar to the following example:

title DOS lock password --md5 <password-hash>