TrueCrypt Security Model bypass exploiting wrong BIOS API usage

2008-09-03 / 2008-09-04
Credit: iViZ
Risk: Low
Local: Yes
Remote: No
CWE: CWE-200

CVSS Base Score: 2.1/10
Impact Subscore: 2.9/10
Exploitability Subscore: 3.9/10
Exploit range: Local
Attack complexity: Low
Authentication: No required
Confidentiality impact: Partial
Integrity impact: None
Availability impact: None

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 - ----------------------------------------------------------------------- [ iViZ Security Advisory 08-003 25/08/2008 ] - ----------------------------------------------------------------------- iViZ Techno Solutions Pvt. Ltd. - ----------------------------------------------------------------------- * 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 - --[ 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: -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.7 (MingW32) iD8DBQFIsnPvkcchhz1mT/IRAvK9AJ9NB+/8OwAm+/pHA0ZaKLjgqu/umwCgtfoJ QTYYHLIVb80eudsZrlSBx04= =mttT -----END PGP SIGNATURE-----


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