Introduction

Updating the UEFI revocation database prevents starting EFI binaries with known security issues, and is typically no longer done from a firmware update due to the risk of the machine being “bricked” if the bootloader is not updated first.

This plugin also checks if the UEFI dbx contains all the most recent revoked checksums. The result will be stored in an security attribute for HSI.

Firmware Format

The daemon will decompress the cabinet archive and extract a firmware blob in EFI_SIGNATURE_LIST format.

See https://www.uefi.org/sites/default/files/resources/UEFI%20Spec%202_6.pdf for details.

This plugin supports the following protocol ID:

• org.uefi.dbx2

Comparing Versions

Earlier versions of fwupd used the org.uefi.dbx protocol which counted the number of Microsoft hashes in the local dbx file and in the firmware payload.

The equivalent versions are given below:

x64

Old Version	New Version
9	20100307
13	20140413
77	20160809
190	20200701
211	20210401
217	20220801
220	20230301
371	20230501

aarch64

Old Version	New Version
19	20200729
21	20210401
22	20220801
26	20230501

i386

Old Version	New Version
41	20200701
55	20210401
57	20230301
89	20230501

GUID Generation

These devices use the GUID constructed of the uppercase SHA256 of the X509 certificates found in the system KEK and optionally the EFI architecture. e.g.

• UEFI\CRT_{sha256} (quirk-only)
• UEFI\CRT_{sha256}&ARCH_{arch}

…where arch is typically one of IA32, X64, ARM or AA64

Additionally, the last listed dbx SHA256 checksum is added as a quirk-only GUID so that the version can be corrected even when fwupd is operating 100% offline.

• UEFI\CSUM_{sha256}

Metadata

Microsoft actually removes checksums in some UEFI dbx updates, which is probably a result of OEM pressure about SPI usage — but local dbx updates are append-only. This means that if you remove hashes then you can have a different number of dbx checksums on your machine depending on whether you went A→B→C→D or A→D…

In these cases we look at the last-entry dbx checksum and compare to the set we know, either from the quirk files, local metadata, or remote metadata from the LVFS.

The org.linuxfoundation.dbx.*.firmware components will match against a hash of the system PK. The latest cabinet archive can also be installed into the vendor-firmware remote found in /usr/share/fwupd/remotes.d/vendor/firmware/ which allows the version-fixup to work even when offline — although using the LVFS source is recommended for most users.

The last-entry checksum can be found from the fwupdtool firmware-parse DBXUpdate-$VERSION$.x64.bin efi-signature-list command.

Update Behavior

The firmware is deployed when the machine is in normal runtime mode, but it is only activated when the system is restarted.

Vendor ID Security

The vendor ID is hardcoded to UEFI:Microsoft for all devices.

External Interface Access

This plugin requires:

• read/write access to /sys/firmware/efi/efivars

Version Considerations

This plugin has been available since fwupd version 1.5.0 with org.uefi.dbx2 being available in fwupd 1.9.27 (from the 1.9.x series) and 2.0.4 (from the 2.0.x series).