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BIOS: Overview and Security
Basic Input/Output System (BIOS), also known as the system BIOS or ROM BIOS, is a standard defining a firmware interface
By: Albert Fruz
Jan. 3, 2014 09:00 AM
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 rein over the systems that we are hired to protect. An attack vector that many people forget to consider is the boot process, 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 PCs 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 preboot hardware initializations before handing over control to the bootloader. This was fine 30 years ago, when software was simpler and attacks were not very predominant, thus the BIOS 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.
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)
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 and 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 PCs 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.
Why BIOS Is in Blue
Under certain conditions, setting the highest bit of the background color may cause the text to blink instead of making the background color intensified. In this context the highest bit of the background color should be kept low according to the BIOS color attribute distribution. As a result the Blue color which comprises value '1′ in hexadecimal is generally used for an uninterrupted BIOS display with intensified background with clear text.
Top BIOS Manufacturers
Role of BIOS
Conventional BIOS (Legacy BIOS)
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 the remaining firmware in BIOS and if any is corrupted recovers those. The boot block then initializes almost all the hardware associated with the 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 that have their own BIOS software. This Option ROMS could inform the BIOS about its functionality, and then it could be called later on in 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 device's priority. If any storage device has a valid data that relates to MBR, it is selected. MBR then points to a corresponding boot loader of an operating system and thus in turns loads the operating system.
In a conventional Boot process, the System Management Mode (SMM) can be initiated by using SMI handlers and ACPI table's code. System Management Mode is a 32-bit mode that runs on high-privileged mode that can override almost all the hardware security mechanisms of the protected mode. In order to change to SMM mode, BIOS loads SMI handlers and initializes the ACPI tables and codes
Legacy BIOS Boot Process
Unified Extensible Firmware Interface (UEFI)
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's generally placed 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
Windows 8 uses UEFI
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
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
User Initiated Attack
Network Based or Organizational Attack
How Do We Mitigate Common BIOS Threats
In order to overcome the malicious attacks on BIOS, we can implement following methods:
Automated Authentication Method
Rollback Prevention Method
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 the lesser version does not contain any vulnerability.
Physical Authentication Method
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