Who am I ?

Scope of this presentation

We present a new class of vulnerabilities,

Affecting multiple pre-boot authentication software under x86 and x64 architectures,

Exploitable without physical access.

Limitations : we will focus on password based authentication solely.

Contents

(Technically) defining pre-boot authentication

Password leakage under Windows

Password leakage under *nix

Rebooting in spite of a pre-boot authentication

Examples of vulnerable software

Mitigating those vulnerabilities

I - (Technically) defining pre-boot

authentication

Boot sequence overview

Taxonomy of pre-boot authentication software

BIOS API for user inputs

BIOS internals for keyboard management

BIOS keyboard buffer Remanence...

Verifying this bug exists “in real life”

Password chaining

I-1) Boot sequence overview

I-2) Taxonomy of pre-boot authentication softwares

Bios Passwords

Bootloader Passwords (Vista's Bitlocker, Grub or Lilo, and most others pre-boot authentication software : Truecrypt, Diskcryptor...)

Early kernel stage passwords – typically before decompression (eg: suspend2 hibernation patch for GNU/Linux)

I-3) BIOS API for user inputs (1/2)

Interruption 0x16 invoked via functions :

ah=0x00 , “Get keystroke” : returns the keystroke scancode in AH and its ASCII code in AL.

ah=0x01 , “Check for keystroke” : idem, but the Zero Flag is set if no keystroke is available in the Bios keyboard buffer.

I-4) BIOS internals for keyboard management

I-5) BIOS keyboard buffer

Remanance... (1/3)

Filling the BIOS keyboard buffer (with the keyboard) :

I-5) BIOS keyboard buffer

Remanence... (2/3)

Reading the BIOS keyboard buffer (using int 0x16, ah=0x00 or 0x01) :

I-5) BIOS keyboard buffer

Remanence... (3/3)

Who is supposed to clear the keyboard buffer ?

I-6) Verifying this bug exists “in real

life” (1/2) :

We want to check the authentication routines in the BIOS themselves (aka: BIOS Passwords)

We will write a small USB-bootable OS in 16b asm to read the content of the BIOS keyboard buffer in Real Mode (sploitOS.S)

I-6) Verifying this bug exists “in real

life” (2/2) :

Results :

Most BIOS Passwords are vulnerable (more on this later).

... if the BIOS Programmers themselves do not clear the BIOS keyboard buffer... just imagine third party programmers...

I-7) Password chaining :

Let's now imagine we have two authentication devices in a raw (asking for pass1 and pass2 respectively)....

What happens in the BIOS keyboard buffer ?

The passwords are concatenated ! So we can retrieve both ;)

[p][a][s][s][1][Enter][p][a][s][s][2][Enter]