[IVIZ-08-003] TrueCrypt Security Model bypass exploiting wrong BIOS API usage

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

[ iViZ Security Advisory 08-003 25/08/2008 ]

iViZ Techno Solutions Pvt. Ltd.
http://www.ivizsecurity.com

• Title: TrueCrypt Security Model bypass exploiting wrong BIOS API
  usage
• Date: 25/08/2008
• Software: TrueCrypt

• –[ Synopsis:

  The password checking routine of TrueCrypt fails to sanitize the
  BIOS keyboard buffer before AND after reading passwords.

• –[ Affected Software:

  • TrueCrypt 5.0 (possibly older versions also)

• –[ Technical description:

  Truecrypt's pre-boot authentication routines use the BIOS API to
  read user input via the keyboard. The BIOS internally copies the
  keystrokes in a RAM structure called the BIOS Keyboard buffer
  inside the BIOS Data Area. This buffer is not flushed after use,
  resulting in potential plain text password leakage once the OS
  is fully booted, assuming the attacker can read the password at
  physical memory location 0x40:0x1e. It is also possible for a root
  user to reboot the computer by instrumenting the BIOS keyboard
  buffer in spite of the full disk encryption.

• –[ Impact:

  1) Plain text password disclosure.
  Required privileges to perform this operation are OS dependent,
  from unprivileged users under Windows (any), to root under most
  Unix.

  2) A privileged attacker able to write to the MBR and knowing the
  password (for instance thanks to 1), is able to reboot the computer
  in spite of the password prompted at boot time (and in spite of
  disk encryption) by initializing the BIOS keyboard buffer with the
  correct password (using an intermediary bootloader that will in turn
  run TrueCrypt).

• –[ Full Technical Whitepaper

  http://www.ivizsecurity.com/research/preboot/preboot_whitepaper.pdf

• –[ Vendor response:

  • Vendor denies the vulnerability
  • Fixed in updated versions

• –[ Credits:

  This vulnerability was discovered by Security Researcher
  Jonathan Brossard from iViZ Techno Solutions Pvt. Ltd.

• –[ Disclosure timeline:

  • First private disclosure to vendor on July 29th 2008
  • First vendor reply on July 29th 2008, denying the
    vulnerability.
  • First Public disclosure at Defcon 16 on August 10th 2008

• –[ Reference:

  http://www.ivizsecurity.com/security-advisory.html
  -----BEGIN PGP SIGNATURE-----
  Version: GnuPG v1.4.7 (MingW32)

iD8DBQFIsnPvkcchhz1mT/IRAvK9AJ9NB+/8OwAm+/pHA0ZaKLjgqu/umwCgtfoJ
QTYYHLIVb80eudsZrlSBx04=
=mttT
-----END PGP SIGNATURE-----