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Secure Booting Windows 8 With UEFI

In September 22 blog post, Steven Sinofsky explains how the company is protecting pre-OS environment in Windows 8 with UEFI. “The UEFI (Unified Extensible Firmware Interface) secure boot protocol is the foundation of an architecturally neutral approach to platform and firmware security. Based on the Public Key Infrastructure (PKI) process to validate firmware images before they’re allowed to execute, secure boot helps reduce the risk of boot loader attacks. Microsoft relies on this protocol in Windows 8 to improve platform security for our customers,” explains Sinofsky.

“UEFI has a firmware validation process, called secure boot, which is defined in Chapter 27 of the UEFI 2.3.1 specification. Secure boot defines how platform firmware manages security certificates, validation of firmware, and a definition of the interface (protocol) between firmware and the operating system.”

Sinofsky writes, “In most PCs today, the pre-operating system environment is vulnerable to attacks by redirecting the boot loader handoff to possible malicious loaders. These loaders would remain undetected to operating system security measures and antimalware software. Windows 8 addresses this vulnerability with UEFI secure boot, and using policy present in firmware along with certificates to ensure that only properly signed and authenticated components are allowed to execute.

Secure boot is only a part of the Windows 8 Platform Integrity story.”

Hers is a quick summary:

  • UEFI allows firmware to implement a security policy
  • Secure boot is a UEFI protocol not a Windows 8 feature
  • UEFI secure boot is part of Windows 8 secured boot architecture
  • Windows 8 utilizes secure boot to ensure that the pre-OS environment is secure
  • Secure boot doesn’t “lock out” operating system loaders, but is is a policy that allows firmware to validate authenticity of components
  • OEMs have the ability to customize their firmware to meet the needs of their customers by customizing the level of certificate and policy management on their platform
  • Microsoft does not mandate or control the settings on PC firmware that control or enable secured boot from any operating system other than Windows

Sinfosky explains the how does UEFI work:

“Powering on a PC starts the process of executing code that configures the processor, memory, and hardware peripherals in preparation for the operating system to execute. This process is consistent across all platforms, regardless of underlying silicon architectures (x86, ARM, etc.).

Shortly after the system is powered on, and before handoff to the OS loader occurs, the firmware will check the signature of firmware code that exists on hardware peripherals such as network cards, storage devices, or video cards. This device code, called Option ROMs, will continue the process of configuration by ensuring that the peripheral is prepared for handoff to the operating system.

During this part of the boot process firmware will check for an embedded signature inside of the firmware module, much like an application, and if that signature matches against a database of signatures in firmware, then that module is allowed to execute. These signatures are stored in databases in firmware. These databases are the “Allowed” and “Disallowed” lists that determine if the booting process can continue.

This figure represents the hierarchy of signatures and keys in a system with secure boot. The platform is secured through a platform key that the OEM installs in firmware during manufacturing. This is the process used today on most shipping systems, regardless of whether they are based on UEFI, or legacy BIOS. (Applications like firmware update utilities will use the platform key to protect the firmware image.) Other keys are used by secure boot to protect access to databases that store keys to allow or disallow execution of firmware.

The Allowed database contains keys that represent trusted firmware components and, more importantly, operating system loaders. Another database contains hashes of malware and firmware, and blocks execution of those malware components. The strength of these policies is based on signing firmware using Authenticode and Public Key Infrastructure (PKI). PKI is a well-established process for creating, managing, and revoking certificates that establish trust during information exchange. PKI is at the core of the security model for secure boot,” explainded Sinofsky.

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