Password protection for the BIOS (or BIOS equivalent) and the boot loader can prevent unauthorized users who have physical access to systems from booting using removable media or attaining root privileges through single user mode. But the security measures one should take to protect against such attacks depends both on the sensitivity of the information the workstation holds and the location of the machine.
For instance, if a machine is used in a trade show and contains no sensitive information, than it may not be critical to prevent such attacks. However, if an employee's laptop with private, unencrypted SSH keys for the corporate network is left unattended at that same trade show, it could lead to a major security breach with ramifications for the entire company.
On the other hand, if the workstation is located in a place where only authorized or trusted people have access, then securing the BIOS or the boot loader may not be necessary at all.
The following are the two primary reasons for password protecting the BIOS of a computer:
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.
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.
Other architectures use different programs to perform low-level tasks roughly equivalent to those of the BIOS on x86 systems. For instance, Intel® Itanium™ computers use the Extensible Firmware Interface (EFI) shell.
For instructions on password protecting BIOS-like programs on other architectures, refer to the manufacturer's instructions.
The following are the primary reasons for password protecting a Linux boot loader:
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.
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.
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:
When prompted, type the GRUB password and press
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.
The next time the system boots, the GRUB menu does not allow access
to the editor or command interface without first pressing
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
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>
Since system BIOSes differ between manufacturers, some may not support password protection of either type, while others may support one type but not the other.
GRUB also accepts unencrypted passwords, but it is recommended that an md5 hash be used for added security.