luks_backdoor

Places a backdoor on a LUKS encrypted partition via an unprotected initramfs

Background

It's common practice to use LUKS to encrypt all partitions on a machine except for /boot, which the ESP must access in order to decrypt the rest of the partitions. This PoC shows that it is easy to modify the initramfs, stored on /boot to add a LUKS key at the same time as it decrypts the partitions.

Threat model

This attack requires physical access, and for the target to log in to decrypt the partitions at least once between planting the backdoor and using it. Examples:

• evil maid
• evil roommate
• evil administrator
• you?

How it works

This script does the following:

• mount the user-supplied ESP
• back up the initramfs (assumed to be called initramfs-linux.img)
• extract the initramfs using lsinitcpio
• modify the encrypt hook*
• repack the initramfs

* By default, the encrypt hook asks the user to decrypt the disk by simply running

cryptsetup open --type luks ${resolved} ${cryptname} ${cryptargs} ${CSQUIET}

Our modified encrypt hook now prompts the user for the password, storing it in a variable. Then, it checks if the password is valid by running the same cryptsetup open that it normally uses, and if that succeeds, it adds a new key (default: rekt).

Limitations

• Niche use-case (see Threat model)
• Noticeably longer time (approx. 2x) to decrypt, since adding the key takes a long time, in addition to the time it already takes decrypting the drive
• Easy to mitigate. See Mitigations

Mitigations

Fortunately, this attack is easy to defend against with normal security practices. A non-exhaustive list in no particular order:

• Store /boot on a separate drive kept on your person
• Encrypt /boot (Only GRUB can do this, as far as I'm aware)
• Set a BIOS password (Will help a little bit but likely not a lot if your attacker has physical access and time)
• Watch for the extra time it takes to decrypt the drive