How To Password Protect GRUB Bootloader In Linux

Password Protect GRUB Bootloader In Linux

There is an old adage “boot access==root access”. I fully concur with this fact. Any one who can access the boot loader can also get the root access to your system easily. We already have posted a guide that described how to reset root password in Linux. That tutorial has taught you how to easily reset or recover root user password via Grub bootloader. Once someone has physical access and bootloader access to a machine, there is no way to stop them. That’s why we need to password protect Grub Bootloader to add an extra security layer. If you’re a Linux administrator, you should know how to secure your bootloader.

This guide addresses how to protect Grub bootloader with password in CentOS. I have tested this guide on CentOS 6.x and CentOS 7.x systems.

Password Protect GRUB Bootloader

In older Linux distributions like CentOS 6.x, RHEL 6.x, Grub is the default bootloader. This section describes how to set grub password in CentOS 6.x systems.

Before doing any changes, it is always recommended to backup the GRUB configuration file

cp /etc/grub.conf /etc/grub.conf.bak

First, we need to encrypt the password. To do so, log in to your centos system as root user and create a file called grub as shown below.

touch grub

Next, encrypt the password using “md5crypt” command like below. Run the following command from the Terminal and press ENTER key.

grub-md5-crypt >grub

Enter your password twice. Please note that you won’t see anything on your screen. Just type password and press ENTER, and re-type the same password and press ENTER.

The password has been encrypted. Next, we need to add this password in /etc/grub.conf file.

Now, let us add the password in the grub.conf file. To do so, open the two files grub and /etc/grub.conf files.

vi /root/grub /etc/grub.conf

The above command will open the both files in vi editor.

You will see the encrypted password like below. Move the cursor point and place it in-front of the password. Then, type yy to copy the password.

Password:
Retype password:
$1$Ch0NF/$0XsWw8.EW31vRjm5zsnPb/

And then, type :n (colon n). This will switch you to the next file i.e /etc/grub.conf.

You will there see a line labeled. After the splashimage=(hd0,0)/grub/splash.xpm.gz line, press p to paste the encrypted password from the previous file.

Then, press and add the line password –md5 before the encrypted password as shown below.

password --md5 $1$I2w2s1$EPZtrLn/h2M4qfh48ZL8O0

Refer the following screenshot for more clarification.

Then, press ESC and type :wq to save and exit.

Reboot your system.

Now, look at the Grub boot menu. You can’t edit grub menu without entering the password first.

To edit the Grub menu, press p. You will be asked to enter the password. Just enter the password to unlock the Grub boot menu.

Now, you can make any changes you want in the grub boot menu.

Password Protect GRUB2 Bootloader

In RHEL 7 and its clones like CentOS 7, Scientific Linux 7, Grub2 is the default bootloader. Protecting Grub2 bootloader with password is different than grub bootloader.

First, create the encrypted password using the following command as root user:

grub2-mkpasswd-pbkdf2

Sample output would be:

Enter password: 
Reenter password: 
PBKDF2 hash of your password is grub.pbkdf2.sha512.10000.62C46DE64C6BDE39A440092F521F86E24F486F5F9FE58B38D1CA9DDA27D6DDA6A6F5615836537B31AF2D06D2C5A8C6BE26709269A08E81286357501882016523.FD91A05503B0538FBA4CF9783A13727C43917E63528FF9FFC9917E4780B9C420CEDAE98451CF9256BA77AC144FA6734CA193D1E4183AC71E1F297BD7868FFC4B

We have now encrypted password to secure the Grub2 bootloader. As you might already know, It is not recommended to edit and paste the newly generated password directly in the grub2 main configuration file. Instead, we should add the password in a custom Grub2 menu file which is found in /etc/grub.d/ directory, and finally update the Grub2 main configuration file i.e /etc/grub.cfg.

Make a copy of the custom Grub2 menu file:

cp /etc/grub.d/40_custom /etc/grub.d/40_custom.bak

Then, Edit the custom Grub2 menu config file as root user:

vi /etc/grub.d/40_custom

Add the following lines. Make sure you have pasted the correct password which we generated earlier.

set superusers="root"
password_pbkdf2 root grub.pbkdf2.sha512.10000.62C46DE64C6BDE39A440092F521F86E24F486F5F9FE58B38D1CA9DDA27D6DDA6A6F5615836537B31AF2D06D2C5A8C6BE26709269A08E81286357501882016523.FD91A05503B0538FBA4CF9783A13727C43917E63528FF9FFC9917E4780B9C420CEDAE98451CF9256BA77AC144FA6734CA193D1E4183AC71E1F297BD7868FFC4B

Press ESC and type :wq to save and close the file.

Now, it is time to update the Grub2 main configuration file.

Make sure you have backup copy of the Grub2 main config file.

cp /boot/grub2/grub.cfg /boot/grub2/grub.cfg.bak

Update Grub2 bootloader configuration file using command:

grub2-mkconfig -o /boot/grub2/grub.cfg

Sample output:

Generating grub configuration file ...
Found linux image: /boot/vmlinuz-3.10.0-327.22.2.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-327.22.2.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-327.13.1.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-327.13.1.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-123.9.3.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-123.9.3.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-123.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-123.el7.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-e250d471d5594282ba042c653cfa0172
Found initrd image: /boot/initramfs-0-rescue-e250d471d5594282ba042c653cfa0172.img
done

You can verify if the password has been set correctly in the /etc/grub2.cfg file as shown below.

cat /etc/grub2.cfg

Sample output would be:

We’re all set. Reboot your system to verify if the bootloader has been secured with password.

After restarting the system, try to edit Grub2 bootloader. To do so, press e.

You’ll be asked to enter the user name and password which we have defined in the earlier step.

If you entered the correct username and password, you’ll be able to edit the Grub2 bootloader.

You know now how to password protect the Grub and Grub2 bootloader in Linux. This will add extra security to your Linux server.

And, that’s all for now folks. We will be posting more useful guides like this in the days to come. Keep visiting.

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  • Jane Doe

    Very informative and useful article!
    Thank you !!
    I’ll try to figure out how to do it on an Arch system.

    • SK

      Glad to know that I helped you. Cheers!

  • Gregory Pittman

    On my system you have to be root to edit the grub.cfg file, so I’m not sure what this adds to the situation.

  • Remco Siderius

    And what if you boot from a USB stick, or CD / DVD? And thus bypassing GRUB?
    Can’t you access all files then? Can’t you even reinstall GRUB then?
    The only way to prevent acces I can think of, is encrypting the root file system (or at least /home)
    Am I wrong?

    • SK

      You’re right. I didn’t say protecting GRUB alone will protect your system. There are other ways too. I will posting more guides related to Linux security.