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Securing the Windows 8 Boot Process

The Windows operating system has many features to help protect you from malware, and it does an amazingly good job. Except for apps that businesses develop and use internally, all Windows Store apps must meet a series of requirements to be certified and included in the Windows Store. This certification process examines several criteria, including security, and is an effective means of preventing malware from entering the Windows Store. Even if a malicious app does get through, the Windows 8 operating system includes a series of security features that can mitigate the impact. For instance, Windows Store apps are sandboxed and lack the privileges necessary to access user data or change system settings.

Windows 8 has multiple levels of protection for desktop apps and data, too. Windows Defender uses signatures to detect and quarantine apps that are known to be malicious. The SmartScreen Filter warns the user before allowing them to run an untrustworthy app, even if it’s recognized as malware. Before an app can change system settings, the user would have to grant the app administrative privileges by using User Account Control.

Those are just some of the ways Windows 8 protects you from malware. However, those security features protect you only after Windows 8 starts. Modern malware—and bootkits specifically—are capable of starting before Windows, completely bypassing operating system security, and remaining completely hidden.

When you run Windows 8 on a Windows 8–certified PC or any PC that supports Unified Extensible Firmware Interface (UEFI), Trusted Boot protects your PC from malware from the moment you power your PC on until your antimalware starts. In the unlikely event that malware does infect a PC, it can’t remain hidden; Trusted Boot can prove the system’s integrity to your infrastructure in a way that malware can’t disguise. Even on PCs without UEFI, Windows 8 provides even better startup security than previous versions of Windows.

First, let’s examine what rootkits are and how they work. Then, we’ll show you how Windows 8 can protect you.

The Threat: Rootkits

Rootkits are a sophisticated and dangerous type of malware that run in kernel mode, using the same privileges as the operating system. Because rootkits have the same rights as the operating system and start before it, they can completely hide themselves and other applications. Often, rootkits are part of an entire suite of malware that can bypass local logins, record passwords and keystrokes, transfer private files, and capture cryptographic data.

Different types of rootkits load during different phases of the startup process:

  • Firmware rootkits. These kits overwrite firmware of the PC’s basic input/output system or other hardware so the rootkit can start before Windows.
  • Bootkits. These kits replace the operating system’s bootloader (the small piece of software that starts the operating system) so that the PC loads the bootkit before the operating system.
  • Kernel rootkits. These kits replace a portion of the operating system kernel so the rootkit can start automatically when the operating system loads.
  • Driver rootkits. These kits pretend to be one of the trusted drivers that Windows uses to communicate with the PC hardware.

The Countermeasures

Windows 8 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 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 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, 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 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 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 PCs. In fact, an open source bootloader capable of loading Linux is already available. To begin the process of obtaining a certificate, go to http://sysdev.microsoft.com.
  • Configure UEFI to trust your custom bootloader. All Certified For Windows 8 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 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. 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 kernel before loading it. The Windows 8 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 can automatically repair the corrupted component, restoring the integrity of Windows and allowing the PC to start normally.

Early Launch Antimalware

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 antimalware 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 antimalware 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 antimalware solution; that loads later in the boot process. Windows Defender (included with Windows 8) supports ELAM, as does Microsoft System Center 2012 Endpoint Protection and several non-Microsoft antimalware apps.

Measured Boot

If a PC in your organization does become infected with a rootkit, you need to know about it. Enterprise antimalware 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 antimalware 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 allows a trusted server on the network to verify the integrity of the Windows startup process. Measured Boot uses the following process:

  1. 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 antimalware app.
  2. 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.
  3. The TPM uses the unique key to digitally sign the log recorded by the UEFI.
  4. 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 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 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, these features have the potential to eliminate kernel-level malware from your network. This is the most groundbreaking antimalware solution that Windows has ever had: It’s leaps and bounds ahead of everything else. With Windows 8, you can truly trust the integrity of your operating system.

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