Tried, Tested and Proven

Security Advisory 05 – 011 – BIOS code logic error

Vulnerable System:

MediaPartner 5.0

Vulnerability discovery and development:

The Portcullis R&D team discovered this vulnerability. Whilst assessing a secured (software access control) laptop environment the team discovered it was possible to cause any system running the vulnerable version of the BIOS to become unstable and no longer boot. Once the coding error was known it could be exploited to cause a near permanent Denial of Service attack.

Affected Systems:

The fault was initially found on a Toshiba Satellite Pro A60 running the ACPI BIOS version 1.60. This is a fundamental error that could be exploited in any workstation based on the 1.60 version of the ACPI BIOS. All hard disks, including those encrypted by software or hardware, will be affected. Note that later versions of the ACPI (v1.7 and v1.8), also appear to contain the same coding error however, Portcullis have not tested these versions.


There is a programming error in the BIOS when the Master Boot Record (MBR) is searched for the bootable disk partition. The error in the BIOS code results in only the first slot in the MBR partition table being tested for the active partition. Where the active partition is not described in the first slot of the MBR table, the BIOS ignores the remaining slots and searches other devices for a boot mechanism, and in consequence fails to start the Operating System.

The BIOS seeks to validate the contents of the MBR and the boot sector in the active partition to achieve confidence in the integrity of the boot mechanism. At various points on the start-up sequence the BIOS reads the MBR from disk and tests the data therein. In at least one case, the BIOS sets out to test all 4 entries in the MBR partition table but due to treating an absolute pointer as one relative to the start of the MBR, it drops out of the loop after the first iteration. Slots 2, 3 and 4 are never examined and the active partition not found. Specifically, the 512 byte MBR end-of-table is tested by comparing the pointer to see if it has reached end-of-table at 510 bytes. Unfortunately, in the ACPI version the pointer is set to MBR-start plus start-of-table within MBR. Therefore, the pointer begins with a value larger than the size of the MBR. The result is that the first slot fails, when testing for the active partition byte, the test will show an end-of-table result: the subsequent slots are ignored.

An example of the problem is shown when the MBR is read into boot address 0000:7C00h and then tested for an active partition.

xor bx, bx ;zero BX

mov es, bx

mov bx, 7C00h ;buffer address ES:BX set to 0000:7C00h

mov cx, 1 ;set sector and cylinder value

xor dh, dh ;and head value to read cylinder 0, head 0, sector 1

mov dl, 80h ;set first hard disk drive

mov ax, 0201h ;set read 1 sector

int 13h ;call BIOS routine to read MBR and assume success

add bx, 446 ;ES:BX point to start of partition table in MBR (BX =



cmp es:[bx], 80h ;is this slot bootable?

je ActiveOk ;yes, drop out of loop

add bx, 16 ;increment by length of slot in table (BX becomes 7C00h+446+16)

cmp bx, 510 ;end of slot table reached? error!

should be cmp bx, 7C00h + 510

jb NextSlot ;no, look at next entry - branch never taken

stc ;set no active partition found status



clc ;set active partition found status


The system will not boot. Standard analysis tools will not identify the contents of the MBR partition as invalid. Unless the help-desk engineer is aware of this BIOS error, he/she will be unable to diagnose the fault. Therefore, the impact is denial of service: the system will not boot and yet there is nothing wrong with the contents of the hard disk.


The error in the BIOS code means that any workstation using this version of the BIOS, can be configured such that the bootable partition is defined below the first slot in the MBR partition table and will not boot. An attack at any time during an operating session can leave the workstation subsequently unable to reboot. The nature of the fault means that it is very difficult to identify and may leave the workstation inoperable for an extended period of time.

Vendor notified:

Toshiba were notified initially by email, as no response was received this was followed up by telephone on Friday 4-Feb-2005 and again on Wednesday 23-Feb-2005.

Vendor response:

The A60 laptop works with their supplied configuration. Fault not recognised. Reference supplied by Toshiba is 1-585 92244. When the fault was reported again on 23-Feb-2005 a new fault reference was supplied 1-165 162202 but no technical response was provided by Toshiba.


Either, the BIOS code must be corrected by the manufacturer, or a monitoring device installed to detect any re-configuration that would exploit the vulnerability. Validating and correcting the contents of the MBR at system shutdown is a minimum requirement in these circumstances.


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