Anatomy of BIOS security

Ryan Mazerik
July 31, 2013 by
Ryan Mazerik

Computer security has become much harder to manage in recent years, and this is due to the fact that attackers continuously come up with new and more effective ways to attack our systems. As attackers become increasingly sophisticated we as security professionals must ensure that they do not have free reign over the systems that we are hired to protect. An attack vector that many people forget to consider is the boot process itself which is almost completely controlled by the BIOS.

The BIOS is a privileged piece of software that is generally ignored by day to day users and thus they are usually unable to comprehend the importance of it in our computers. The Basic Input/Output System was first invented by Gary Kildall for use in his operating system CP/M and this became what we now know as the conventional BIOS system. The BIOS appeared in IBM compatible PC's around 1975 and was used extensively in the CP/M operating system. This was later used in the MSDOS systems where it was known as DOS BIOS. These systems were only responsible for basic pre boot hardware initializations before handing over control to the bootloader. This was fine thirty years ago, when software was simpler and attacks were not very predominant, thus the BIOS itself was not designed with security in mind. However in today's world this is no longer the case. BIOS security lacks several features that make it vulnerable to external attack.

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These are some notable attacks carried out against BIOS systems:

Chernobyl Attack (1998) – Also known as CIH or Spacefiller was the first major attack on BIOS systems. This virus installs on the windows memory and hooks into file access calls and infects all the currently executing programs. Then the virus tries to flash the BIOS rom by filling it with zeros. The other payload infects the Master Boot Record (MBR) by filling the first megabyte of the hard disk with zeros.

Mebromi (2012) - Is made up of a BIOS rootkit, MBR rootkit, Trojan downloader and PE infector. This Trojan deletes a specific registry value and checks for the BIOS manufacturer. If it's Award BIOS, it then infects the BIOS ROM and in turn infects the Master BOOT Record (MBR) and alters it allowing the execution of an infected program at each Operating System start-up.

We attempt to prevent such attacks by outlining several attack vectors and also suggest several mechanisms for the mitigation of attacks against the BIOS.

BIOS (Basic Input Output System)

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Basic Input/Output System (BIOS), also known as the system BIOS or ROM BIOS is a standard defining a firmware interface. BIOS software is built into the PC, and is the first software run by a PC when powered on. The fundamental purposes of the BIOS are to initialize and test the system hardware components, and to start the boot loader or an operating system from a secondary storage device. It also takes care of essential system functions such as power management and temperature regulation. It provides an abstraction layer for the underlying hardware by providing a consistent way for operating systems & application programs to interact with various input/output devices.

Changes in system hardware are abstracted by the BIOS from programs that use BIOS services instead of directly accessing the hardware. BIOS software is stored on a non-volatile ROM chip on the motherboard. Its unique design makes it compatible for particular models of computer, interfacing with various devices that make up the complementary chipset of the system. In modern PC's the BIOS contents are stored on an EEPROM chip.

An EEPROM chip or Electronically Erasable Programmable Read only memory is a type of non-volatile memory used by many electronic devices that requires small amounts of data to be stored for quick access. The contents of an EEPROM chip can be flashed i.e. they can be overwritten with new data. This allows BIOS software to be easily upgraded to add new features and bug fixes. This feature is also one of the reasons that BIOS chipsets are vulnerable to attack. A BIOS screen looks like this:

Why BIOS is in Blue colour?

Most BIOS Screens will be Blue in colour, this is due to how the BIOS Manufacturers implement general BIOS colour attributes. BIOS Colour Attributes are 8 bit values where the lower 4 bits represent the character colour and the higher 4 bits represent the background colour. In BIOS, to print a white character in blue background the 'BIOS colour attribute' would be set to a hexadecimal value of 0x1F.

Under certain conditions, setting the highest bit of the background colour may cause the text to blink instead of making the background colour intensified. In this context the highest bit of the background colour should be kept low according to the BIOS colour attribute distribution. As a result the Blue colour which comprises value '1' in hexadecimal is generally used for an uninterrupted BIOS display with intensified background with clear text.

Top BIOS manufacturers

BIOS software is developed by several companies around the world and are usually deeply integrated with the system motherboard. Several of the most popular BIOS manufacturers are:

  • American Mega Trends
  • WinBond
  • Phoenix
  • AMI
  • IBM
  • Award

Role of BIOS

The BIOS has an essential role in the boot process of the computer also known as bootstrapping. It initializes system hardware manages ACPI and regulates CPU temperatures during the booting process. The major responsibilities of the BIOS are listed below:

  1. Establish Trust: The BIOS is responsible for verifying the integrity of all the hardware components in the system and also to authenticate them before use. This is done with the help of Core Root of Trust Measurement (CRTM), which basically checks if the hardware is valid and that its integrity has not been compromised.
  2. Test Hardware: The secondary functionality of the BIOS is to initialize and test the hardware present on the computer before it's used. Hardware such as the motherboard, chipset and memory are included in this test. This is generally carried out during POST (Power-On-Self-Test).
  3. Load additional Modules: Several devices present on the computer may require additional firmware for its proper functioning. The BIOS ensures that such additional firmware modules are loaded and executed. These may be stored in the BIOS chip itself or some secondary storage device.
  4. Boot Device Selection: After the above steps have been carried out the BIOS starts to detect a valid boot device e.g. USB drives, Hard disk etc. Once such a device has been found it executes the bootloader found on that device.
  5. Start Operating System: After this the actual bootstrapping process begins, the bootloader starts to execute and begins to load the OS kernel into memory. Once the kernel has been initialized the BIOS transfers full control to the Operating System.

BIOS overview

System BIOS can be of two types namely

  • Legacy BIOS
  • BIOS based upon the UEFI specification

Conventional BIOS (Legacy BIOS)

The Legacy BIOS or conventional BIOS is the tried and true BIOS type which has been around for years. It's generally a 16bit program that is flashed onto a ROM chip and placed in the motherboard of the computer. This type of BIOS is very outdated and more vulnerable to attack therefore it's advisable to use a newer and more stable specification.

The key component in conventional BIOS is a boot block. This part is logically separated from other parts of the BIOS and initially executed during the BIOS boot process. Then the boot block checks the integrity of remaining firmware in BIOS and if any is corrupted, then recovers those. The boot block then initializes almost all the hardware's associated with system by using a Power-On-Self-Test (POST). During this procedure low level hardware components like Memory, CPU, Chipset etc. are initialized.

After this process, it then loads other option ROMS like Video Cards, SCSI Controller Cards, and Network Boot ROM etc. that has its own BIOS software. This Option ROMS could inform the BIOS about its functionality, and then it could be called later on the boot process depending on the order the user had selected. Then the BIOS checks the Master BOOT Record (MBR) in the order of the boot devices priority. If any storage device has a valid data that relates to MBR, then it is selected. MBR then points to a corresponding boot loader of an operating system and thus in turns loads the operating system.

In conventional Boot process System Management Mode (SMM) can be initiated by using SMI handlers and ACPI table's code. System Management Mode is a 32-bit mode which runs on high-privileged mode that can override almost all the hardware security mechanisms of protected mode. In order to change to SMM mode BIOS load SMI handlers and initializes the ACPI tables and codes

Legacy BIOS Boot Process

When a computer is first powered on the BIOS is the first piece of software that is executed, the boot block then executes a POST (Power-On-Self-Test) thereby ensuring that all the hardware on the system is valid and accounted. After the POST screen the user has the option to load the BIOS screen or to continue booting the current Operating System instead, this is done by pressing a pre-designated key on the keyboard this key may vary depending on the BIOS manufacturer. The BIOS then checks if any additional pieces of firmware have to be loaded for individual devices on the computer if so, then these modules are loaded and executed.

Unified Extensible Firmware Interface (UEFI)

UEFI (Unified Extensible Firmware Interface) is a specification that was first designed by Intel in the 1990 for its Itanium range of computer systems. It was originally called the EFI specification and was intended to be a better replacement for legacy BIOS systems. UEFI has several advantages over the conventional BIOS and is radically different from these older systems.

The UEFI specification defines a programmable software interface that lies between the device firmware and the Operating system. It provides an almost OS like interface to device firmware. Depending on the manufacturer it may lie on top of the BIOS but it is generally place in the /EFI/ directory on some form of non-volatile memory. This may either be a NAND chip on the motherboard, a hard drive or even on a network share.

Differences between UEFI and Legacy BIOS

There are several differences between conventional BIOS and UEFI systems many of them add greater functionality and power to the computer. It also provides a more efficient and secure booting mechanism.

  1. Larger Address Space: Conventional BIOS were forced to work in 16bit mode with a maximum of 1mb addressable space. UEFI allows running in 32 and 64bit mode allowing larger and more sophisticated programs to be run by the UEFI.
  2. Support for Larger File Systems: Traditional BIOS only supports booting of disks which have MBR partitions. MBR partitioning schemes only support 4 partitions per disk and a maximum size of 2TB. UEFI supports the booting of GPT partitions (GUID partitions) which allow the booting of extremely large disks up to 8Zb.
  3. Improved Security Capabilities: The UEFI specification also improves upon the security aspects of the older BIOS systems. It supports several security features such as secure boot, it also has provisions for providing basic cryptographic and public key infrastructure.
  4. CPU independent design: UEFI has employed a CPU independent design methodology, i.e. it can run on many different types of architectures. The code is available is compiled differently for the required platform.
  5. Powerful Execution Environment: The UEFI specification provides a much more powerful execution environment for computers. It allows special features such as booting over a network, using the mouse, ACPI control and even browsing the web.
  6. Improved Performance: UEFI compliant operating systems have been seen to have a significant performance boost not just during the boot process but also during running and powering off the system.
  7. Windows 8 uses UEFI

    UEFI though supported by several operating system vendors for years has not seen widespread adoption until the release of Windows 8. Windows 8 has tried to incorporate the best parts of UEFI into their latest operating system release particularly the secure boot feature.

    Secure Boot

    One of UEFI's most interesting features is called Secure Boot, which allows you to boot only an authenticated OS kernel. Windows 8 relies heavily on this method to ensure that only authenticated firmware with a validated kernel image can be booted. This is quite different from older bootstrapping methodologies where any kind of bootloading code can be loaded and executed by the BIOS.

    In secure a boot before the BIOS gives full control to the OS, bootloader makes sure that the firmware has been signed. This is done with the help of cryptographic signatures that are embedded on the firmware by the OEM. During the boot process the firmware will compare the platform key with the key present in the firmware of each device. This comparison is carried out between a database of authenticated valid keys, if the key is allowed then the firmware is allowed to execute otherwise it is rejected.

    This allows only authenticated devices to be loaded and ensures that malicious bootloader code is not loaded and executed. The safe boot mechanism in windows 8 significantly reduces the chances of boot sector viruses and bootkits from launching and affecting the boot process of the machine.

    UEFI Boot Process

    The UEFI boot process is much like the boot process in conventional BIOS with a few minor changes. The process is divided into stages which take place sequentially and ends with the complete handover of control to the operating system. UEFI booting runs in 32 bit or 64 bit protected mode on the CPU not in 16bit mode unlike legacy BIOS.

    UEFI also starts with a small amount of code that begins the execution of the entire booting process this phase is called the Security phase (SEC) and it acts as the core root of trust. This is followed by the Pre-EFI initialization (PEI) this mode is similar to the Legacy bios pre boot initialization phase in which device firmware is checked before boot. Then the Driver execution environment is started where the actual initialization of extra device drivers takes place, devices such as network cards and graphic cards are checked in this phase.

    The boot device is selected during the BDS (Boot Device Selection) phase. This then transfers control to the bootloader that is located in a GPT partition, the bootloader handles the loading of the OS kernel into memory.

    Common BIOS threats

    BIOS is always written to a non-volatile storage device such as an EEPROM, this allows the contents of the ROM to be overwritten to introduce bug fixes and updates for the particular BIOS version. However this also has great potential for misuse such that, malicious programs may also have the ability to modify the contents of the ROM disk if given enough access.

    User Initiated Attack

    This type of attack is carried out by end user who uses an unauthenticated file to update the bios. This can be carried out by an end user who doesn't have prior knowledge about the update file or a user with malicious intent.

    Malware Attack

    Malware attacks can be used to exploit vulnerability in bios. The attacker opens a backdoor to system and cause a bios crash using a vulnerable update version of bios.

    Network Based or Organisational Attack

    This is the large scale and crucial attack on organisational basis. An attacker who gets access to a compromised update server can carry out an organisational wide attack and infect all systems by rolling out all authorised bios versions to malicious ones.

    How do we mitigate common BIOS Threats

    This section describes the security measures that an organisation should implement in order to secure the BIOS. Since vulnerability in BIOS is crucial point to a system, it is important that every organisation should follow predefined guidelines to secure the BIOS structure. The following method can be implemented in an enterprise structure to enhance BIOS security

    In order to overcome the malicious attacks on BIOS, we can implement following methods:

    • Digital Authentication Method

    • Rollback Prevention Method

    • Physical Authentication Method

    Automated Authentication Method

    In this method, the authenticity of BIOS can be ensured through digital signatures. Here bios updates should only be installed if its authenticity is verified. Here, digital signature embedded update images by BIOS manufactures will be the last level of authorisation. This process can be automated by using a signature verification algorithm that ensures the validity of the digital signatures. This digital authentication method must be integrated by providing strong security features.

    Rollback Prevention Method

    Implement a mechanism that ensures the update images of BIOS such that, it should not be rolled back to previous versions. This method ensures that if an update image is to be installed, it should be ensured that its version number is greater than the current one. This can ensure that the bios version is not roll backed to a previous image that contains vulnerability.

    But in some cases if the current higher version has to be rolled back to a previous lesser version. i.e., if the current updated version of the bios contains vulnerability and there are no higher version updates to be installed and the earlier lesser version is stable than the current one. In this case the corresponding authority has to ensure that lesser version do not contain any vulnerability.

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    Physical Authentication Method

    This method provides the authenticity of updated images by ensuring the physical presence of the corresponding authority (system administrator). Here the authorities can verify the update images and update the BIOS, if the image is a valid one. This method can be used as a subsidiary to digital authentication method by providing as a recovery mechanism in situations like bios crash.


    Ryan Mazerik
    Ryan Mazerik

    Ryan has over 10yrs of experience in information security specifically in penetration testing and vulnerability assessment. He used to train and mentor consultants of these offerings to expand security delivery capabilities.He has strong passion in researching security vulnerabilities and taking sessions on information security concepts.