Secure the Windows 8.1 boot process

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The countermeasures

Windows 8.1 supports four features to help prevent rootkits and bootkits from loading during the startup process:

• Secure Boot. PCs with UEFI firmware and a Trusted Platform Module (TPM) can be configured to load only trusted operating system bootloaders.
• Trusted Boot. Windows checks the integrity of every component of the startup process before loading it.
• Early Launch Anti-Malware (ELAM). ELAM tests all drivers before they load and prevents unapproved drivers from loading.
• Measured Boot. The PC’s firmware logs the boot process, and Windows can send it to a trusted server that can objectively assess the PC’s health.
  

Figure 1 shows the Windows 8.1 startup process.

Figure 1. Secure Boot, Trusted Boot, and Measured Boot block malware at every stage

Secure Boot and Measured Boot are only possible on PCs with UEFI 2.3.1 and a TPM chip. Fortunately, all PCs certified for use with Windows 8.1 have these components, and many PCs designed for earlier versions of Windows have them, as well.

The sections that follow describe Secure Boot, Trusted Boot, ELAM, and Measured Boot.

Secure Boot

When a PC starts, it first finds the operating system bootloader. PCs without Secure Boot simply run whatever bootloader is on the PC’s hard drive. There’s no way for the PC to tell whether it’s a trusted operating system or a rootkit.

When a PC equipped with UEFI starts, the PC first verifies that the firmware is digitally signed, reducing the risk of firmware rootkits. If Secure Boot is enabled, the firmware examines the bootloader’s digital signature to verify that it hasn’t been modified. If the bootloader is intact, the firmware starts the bootloader only if one of the following conditions is true:

• The bootloader was signed using a trusted certificate. In the case of PCs certified for Windows 8.1, Microsoft’s certificate is trusted.
• The user has manually approved the bootloader’s digital signature. This allows the user to load non-Microsoft operating systems.
  

All x86-based Certified For Windows 8.1 PCs must meet several requirements related to Secure Boot:

• They must have Secure Boot enabled by default.
• They must trust Microsoft’s certificate (and thus any bootloader Microsoft has signed).
• They must allow the user to configure Secure Boot to trust other bootloaders.
• They must allow the user to completely disable Secure Boot.
  

These requirements help protect you from rootkits while allowing you to run any operating system you want. You have three options for running non-Microsoft operating systems:

• Use an operating system with a certified bootloader. Because all Certified For Windows 8.1 PCs must trust Microsoft’s certificate, Microsoft offers a service to analyze and sign any non-Microsoft bootloader so that it will be trusted by all Certified For Windows 8.1 PCs. In fact, an   open source bootloader capable of loading Linux is already available. To begin the process of obtaining a certificate, go to   [url]http://sysdev.microsoft.com[/url].
• Configure UEFI to trust your custom bootloader. All Certified For Windows 8.1 PCs allow you to trust a noncertified bootloader by adding a signature to the UEFI database, allowing you to run any operating system, including homemade operating systems.
• Turn off Secure Boot. All Certified For Windows 8.1 PCs allow you to turn off Secure Boot so that you can run any software. This does not help protect you from bootkits, however.
  

To prevent malware from abusing these options, the user must manually configure the UEFI firmware to trust a noncertified bootloader or to turn off Secure Boot. Software cannot change the Secure Boot settings. For more information about Secure Boot, read the blog,   Protecting the pre-OS environment with UEFI.

Like most mobile devices, ARM-based Certified For Windows RT devices, such as the Microsoft Surface RT device, are designed to run only Windows 8.1. Therefore, Secure Boot cannot be turned off, and you cannot load a different operating system. Fortunately, there is a large market of ARM devices designed to run other operating systems.

Trusted Boot

Trusted Boot takes over where Secure Boot leaves off. The bootloader verifies the digital signature of the Windows 8.1 kernel before loading it. The Windows 8.1 kernel, in turn, verifies every other component of the Windows startup process, including the boot drivers, startup files, and ELAM. If a file has been modified, the bootloader detects the problem and refuses to load the corrupted component. Often, Windows 8.1 can automatically repair the corrupted component, restoring the integrity of Windows and allowing the PC to start normally.

Early Launch Anti-Malware

Because Secure Boot has protected the bootloader and Trusted Boot has protected the Windows kernel, the next opportunity for malware to start is by infecting a non-Microsoft boot driver. Traditional anti-malware apps don’t start until after the boot drivers have been loaded, giving a rootkit disguised as a driver the opportunity to work.

ELAM can load a Microsoft or non-Microsoft anti-malware driver before all non-Microsoft boot drivers and applications, thus continuing the chain of trust established by Secure Boot and Trusted Boot. Because the operating system hasn’t started yet, and because Windows needs to boot as quickly as possible, ELAM has a simple task: Examine every boot driver and determine whether it is on the list of trusted drivers. If it’s not trusted, Windows won’t load it.

An ELAM driver isn’t a full-featured anti-malware solution; that loads later in the boot process. Windows Defender (included with Windows 8.1) supports ELAM, as does   Microsoft System Center 2012 Endpoint Protection and several non-Microsoft anti-malware apps.

Measured Boot

If a PC in your organization does become infected with a rootkit, you need to know about it. Enterprise anti-malware apps can report malware infections to the IT department, but that doesn’t work with rootkits that hide their presence. In other words, you can’t trust the client to tell you whether it’s healthy.

As a result, PCs infected with rootkits appear to be healthy, even with anti-malware running. Infected PCs continue to connect to the enterprise network, giving the rootkit access to vast amounts of confidential data and potentially allowing the rootkit to spread across the internal network.

Working with the TPM and non-Microsoft software, Measured Boot in Windows 8.1 allows a trusted server on the network to verify the integrity of the Windows startup process. Measured Boot uses the following process:

• The PC’s UEFI firmware stores in the TPM a hash of the firmware, bootloader, boot drivers, and everything that will be loaded before the anti-malware app.
• At the end of the startup process, Windows starts the non-Microsoft remote attestation client. The trusted attestation server sends the client a unique key.
• The TPM uses the unique key to digitally sign the log recorded by the UEFI.
• The client sends the log to the server, possibly with other security information.
  

Depending on the implementation and configuration, the server can now determine whether the client is healthy and grant the client access to either a limited quarantine network or to the full network.

Figure 2 illustrates the Measured Boot and remote attestation process.

Figure 2. Measured Boot proves the PC’s health to a remote server

Windows 8.1 includes the application programming interfaces to support Measured Boot, but you’ll need non-Microsoft tools to implement a remote attestation client and trusted attestation server to take advantage of it. For an example of such a tool, download the   TPM Platform Crypto-Provider Toolkit from Microsoft Research or Microsoft Enterprise Security MVP Dan Griffin’s   Measured Boot Tool.

Measured Boot uses the power of UEFI, TPM, and Windows 8.1 to give you a way to confidently assess the trustworthiness of a client PC across the network.

Summary

Secure Boot, Trusted Boot, and Measured Boot create an architecture that is fundamentally resistant to bootkits and rootkits. In Windows 8.1, these features have the potential to eliminate kernel-level malware from your network. This is the most groundbreaking anti-malware solution that Windows has ever had: It’s leaps and bounds ahead of everything else. With Windows 8.1, you can truly trust the integrity of your operating system.