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What is UEFI?

Unified Extensible Firmware Interface. See the main UEFI page for more details.

What is UEFI Secure Boot?

UEFI Secure Boot (SB) is a verification mechanism for ensuring that code launched by a computer's UEFI firmware is trusted. It is designed to protect a system against malicious code being loaded and executed early in the boot process, before the operating system has been loaded.

SB works using cryptographic checksums and signatures. Each program that is loaded by the firmware includes a signature and a checksum, and before allowing execution the firmware will verify that the program is trusted by validating the checksum and the signature. When SB is enabled on a system, any attempt to execute an untrusted program will not be allowed. This stops unexpected / unauthorised code from running in the UEFI environment.

Most x86 hardware comes from the factory pre-loaded with Microsoft keys. This means the firmware on these systems will trust binaries that are signed by Microsoft. Most modern systems will ship with SB enabled - they will not run any unsigned code by default, but it is possible to change the firmware configuration to either disable SB or to enroll extra signing keys.

Most of the programs that are expected to run in the UEFI environment are boot loaders, but others exist too. There are also programs to deal with firmware updates before operating system startup (like fwupdate and fwupd), and other utilities may live here too.

Other Linux distros (Red Hat, Fedora, SUSE, Ubuntu, etc.) have had SB working for a while, but Debian was slow in getting this working. This meant that on many new computer systems, users had to first disable SB to be able to install and use Debian. The methods for doing this vary massively from one system to another, making this potentially quite difficult for users.

Starting with Debian version 10 ("Buster"), Debian included working UEFI Secure Boot to make things easier.

What is UEFI Secure Boot NOT?

UEFI Secure Boot is not an attempt by Microsoft to lock Linux out of the PC market here; SB is a security measure to protect against malware during early system boot. Microsoft act as a Certification Authority (CA) for SB, and they will sign programs on behalf of other trusted organisations so that their programs will also run. There are certain identification requirements that organisations have to meet here, and code has to be audited for safety. But these are not too difficult to achieve.

SB is also not meant to lock users out of controlling their own systems. Users can enrol extra keys into the system, allowing them to sign programs for their own systems. Many SB-enabled systems also allow users to remove the platform-provided keys altogether, forcing the firmware to only trust user-signed binaries.


shim is a simple software package that is designed to work as a first-stage bootloader on UEFI systems.

It was developed by a group of Linux developers from various distros, working together to make SB work using Free Software. It is a common piece of code that is safe, well-understood and audited so that it can be trusted and signed using platform keys. This means that Microsoft (or other potential firmware CA providers) only have to worry about signing shim, and not all of the other programs that distro vendors might want to support.

Shim then becomes the root of trust for all the other distro-provided UEFI programs. It embeds a further distro-specific CA key that is itself used for signing further programs (e.g. Linux, GRUB, fwupdate). This allows for a clean delegation of trust - the distros are then responsible for signing the rest of their packages. Shim itself should ideally not need to be updated very often, reducing the workload on the central auditing and CA teams.

For extra trust and safety, from version 15 onwards the shim binary build is 100% reproducible - you can rebuild the Debian shim binary yourself to verify that no unexpected changes have been embedded in this key piece of security software.

MOK - Machine Owner Key


A key part of the shim design is to allow users to control their own systems. The distro CA key is built in to the shim binary itself, but there is also an extra database of keys that can be managed by the user, the so-called Machine Owner Key (MOK for short).

Keys can be added and removed in the MOK list by the user, entirely separate from the distro CA key. The mokutil utility can be used to help manage the keys here from Linux userland, but changes to the MOK keys may only be confirmed directly from the console at boot time. This removes the risk of userland malware potentially enrolling new keys and therefore bypassing the entire point of SB.

There are more docs online for how to work with MOK, for example:

Has the system booted via Secure Boot?

$ sudo mokutil --sb-state
SecureBoot enabled

Note that you can be only sure that the above answer is correct if your system has not been tampered with in the first place.

Generating a new key

Ubuntu puts its MOK key under /var/lib/shim-signed/mok/ and some software such as Oracle's virtualbox package expect the key there so we follow suit (see 989463 for reference) and put it at the same place.

First make sure the key doesn't exist yet:

$ ls /var/lib/shim-signed/mok/

If you see the key there (consisting of the files MOK.der, MOK.pem and MOK.priv) then you should not need to regenerate the key.

Otherwise if there's no key:

# mkdir -p /var/lib/shim-signed/mok/
# cd /var/lib/shim-signed/mok/
# openssl req -new -x509 -newkey rsa:2048 -keyout MOK.priv -outform DER -out MOK.der -days 36500 -subj "/CN=My Name/"
# openssl x509 -inform der -in MOK.der -out MOK.pem

Note that depending on whether Debian will implement automatic genereration of that key (see 989463) you might run into conflicts with that automatic installation.

NOTE It seems (2022-06-21) that shim won't support adding a 4096 RSA key to the ?MokList (it might freezes when loading and verifying the grubx64.efi binary), so make sure you use a 2048 key for now.

Enrolling your key

If you also have a kernel to sign, you may wish to do the next step first as it will save you one reboot.

$ sudo mokutil --import MOK.der # prompts for one-time password
$ sudo mokutil --list-new # recheck your key will be prompted on next boot

<rebooting machine then enters MOK manager EFI utility: enroll MOK, continue, confirm, enter password, reboot>

$ sudo dmesg | grep cert # verify your key is loaded

Set Linux kernel info variables

$ VERSION="$(uname -r)"
$ SHORT_VERSION="$(uname -r | cut -d . -f 1-2)"
$ MODULES_DIR=/lib/modules/$VERSION
$ KBUILD_DIR=/usr/lib/linux-kbuild-$SHORT_VERSION

Using your key to sign your kernel

First, install sbsigntool

$ sbsign --key MOK.priv --cert MOK.pem "/boot/vmlinuz-$VERSION" --output "/boot/vmlinuz-$VERSION.tmp"
$ sudo mv "/boot/vmlinuz-$VERSION.tmp" "/boot/vmlinuz-$VERSION"

Using your key to sign modules (Traditional Way)

Building and signing modules is independent of building and signing your own kernel. You can perfectly build new modules for a Linux kernel provided by Debian. But if you want to use them in a Secure Boot'ed system then you need to enroll your key as shown previously and then you need to sign your modules as shown below.

The example below shows how to sign modules. Debian's dkms package puts those in the updates/dkms subdirectory of the modules directory while Oracle's VirtualBox puts them in the misc subdirectory, so you'll have to go to the modules directory accordingly:

$ cd "$MODULES_DIR/updates/dkms" # For dkms packages
$ cd "$MODULES_DIR/misc" # For Oracle packages

Securely record the passphrase for the private key:

$ echo -n "Passphrase for the private key: "

Sign just the VirtualBox module:

$ sudo --preserve-env=KBUILD_SIGN_PIN "$KBUILD_DIR"/scripts/sign-file sha256 /var/lib/shim-signed/mok/MOK.priv /var/lib/shim-signed/mok/MOK.der vboxdrv.ko

Or sign all modules of the current directory:

$ for i in *.ko ; do sudo --preserve-env=KBUILD_SIGN_PIN "$KBUILD_DIR"/scripts/sign-file sha256 /var/lib/shim-signed/mok/MOK.priv /var/lib/shim-signed/mok/MOK.der "$i" ; done

Making DKMS modules signing by DKMS signing key usable with the secure boot

If you install the kernel modules through the apt repository, chances are that modules have already been signed by the DKMS signing key. In that case, the traditional method won't work. And the thing you only need to do is to enroll the DKMS signing key into your machine. Here is how we can do that:

First, use the method mentioned in Verifying if a module is signed to check if the modules are signed by DKMS signing key.

Next, find the location of the mok signing key and mok certificate. You can view the location in /etc/dkms/framework.conf, and the default location is /var/lib/dkms.

Then, run the following command to enroll the key into the machine:

$ sudo mokutil --import /var/lib/dkms/ # prompts for one-time password and /var/lib/ can be changed, if mok certificate isn't located there. 
$ sudo mokutil --list-new # recheck your key will be prompted on next boot

<rebooting machine then enters MOK manager EFI utility: enroll MOK, continue, confirm, enter password, reboot>

$ sudo dmesg | grep cert # verify your key is loaded

Verifying if a module is signed

# modinfo vboxdrv
filename:       /lib/modules/5.10.0-9-amd64/misc/vboxdrv.ko
version:        6.1.28 r147628 (0x00320000)
license:        GPL
description:    Oracle VM VirtualBox Support Driver
author:         Oracle Corporation
srcversion:     282AFDD3CE09DCCD935FAF2
retpoline:      Y
name:           vboxdrv
vermagic:       5.10.0-9-amd64 SMP mod_unload modversions 
sig_id:         PKCS#7
signer:         My Name
sig_key:        13:FE:C2:ED:A1:40:CE:70:1A:75:91:E5:4C:1F:5F:DA:BD:17:57:A9
sig_hashalgo:   sha256
signature:      0B:72:37:DD:10:97:F2:4F:DF:DF:52:27:38:88:63:7B:CC:2F:98:59:

Disabling/re-enabling Secure Boot

In case it is difficult to control Secure Boot state through the EFI setup program, mokutil can also be used to disable or re-enable Secure Boot for operating systems loaded through shim and GRUB:

  1. Run: mokutil --disable-validation or mokutil --enable-validation.

  2. Choose a password between 8 and 16 characters long. Enter the same password to confirm it.
  3. Reboot.
  4. When prompted, press a key to perform MOK management.
  5. Select "Change Secure Boot state".
  6. Enter each requested character of your chosen password to confirm the change. Note that you have to press Return/Enter after each character.
  7. Select "Yes".
  8. Select "Reboot".

Supported architectures and packages

On each architecture, Debian includes various packages containing signed binaries:





Signed by







Tools to manage UEFI firmware updates automatically






Tools to manage UEFI firmware updates manually (removed after Buster in favour of fwupd)






GRUB boot loader


linux-image-*-amd64 (*)

linux-image-*-686* (*)

linux-image-*-arm64 (*)


Linux kernel, various flavours






Main shim binary






Shim helper binaries - ?MokManager and ?FallBack

(*) The various linux-image packages in Debian are now signed by default. The unsigned packages are called linux-image-*-unsigned.

Testing UEFI Secure Boot

Focusing on Debian Buster, some tests have been performed to make sure that everything is ready. You can help us testing our setup on real hardware, including the installer and the live images. If you want to help us testing Secure Boot you can find more information here.

Secure Boot limitations

By its very design, SB may affect or limit some things that users want to do.

If you want to build and run your own kernel (e.g. for development or debugging), then you will obviously end up making binaries that are not signed with the Debian key. If you wish to use those binaries, you will need to either sign them yourself and enrol the key used with MOK or disable SB.

Using SB activates "lockdown" mode in the Linux kernel. This disables various features that can be used to modify the kernel:

You can avoid this by disabling Secure Boot through the EFI setup program or through MOK.

Example docs from elsewhere:

Infrastructure - how signing works in Debian

If you want to know the implementation details and the current discussions on improvements, see SecureBoot/Discussion.

Testing Secure Boot in a virtual machine

If you want to test Secure Boot in a virtual machine without having to deal with an actual machine, see SecureBoot/VirtualMachine.

arm64 problems

Debian no longer supports UEFI Secure Boot on arm64 systems, as of May 2021.

Shim and other EFI programs have always been difficult to build on arm64, compared to x86 platforms. Binutils for amd64 and i386 includes explicit support for creating programs in the PE/COFF binary format that EFI uses, but this has never been added for arm64.

In the past, shim developers added some local hacks into the shim package to generate a mostly-compliant PE/COFF EFI binary without this toolchain support, and that seemed to be sufficient for use. Everything seemed to work. However, during the development and testing phase of shim 15.3 and 15.4, we found found significant issues with this approach. New security features needed in shim (SBAT) showed up severe problems with the lack of proper toolchain support. See for more details. The old hacks around binutils are no longer sustainable.

Statistics tell us that very few people have attempted to use arm64 Secure Boot with Debian so far. In the interests of releasing needed updates in a timely manner, we have decided for the time being to disable signed shim support for Debian arm64.

We hope to re-introduce arm64 Secure Boot support as soon as possible in the future.

Binutils 2.38 introduces support for PE/COFF generation on arm64 so it should now be possible to resolve this situation.