Boot Attestation: Definition, Importance, and Implementation

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Quick Answer

Boot attestation is a security process that verifies the integrity of a system’s boot sequence. Precise measurements are produced from essential components such as the firmware, bootloader, and kernel during the boot sequence, and these measurements are stored in a log.

Boot attestation involves a third-party stepping in to cross-reference these logged measurements and check digital signatures. This way, we can confirm the authenticity of system components.

What is Boot Attestation?

From the moment your device boots it become vulnerable to attack. The boot process is an attractive entry point for malware because it occurs before the operating system is fully loaded and has a chance to establish its own security measures. Malware that infiltrates the boot process can quickly gain a high level of control on a compromised system.

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For conciseness, we will focus on the Windows boot process, but it is important to note that Linux can also perform boot attestation.

The loading of the operating system into memory is referred to as the boot process, and it consists of four parts.

  1. Secure Boot – A part of the UEFI firmware, responsible for initializing and launching hardware. Verifies the proper digital signature of all loaded drivers, including the bootloader.
  2. Bootloader – First program loaded from the hard drive, responsible for loading boot drivers and kernel.
  3. Trusted Boot – Verifies that the OS and system drivers are properly signed and trusted.
  4. Measured Boot – Windows reports boot measurements to the attestation service, which then verifies the state of the computer.

Boot attestation is a part of the measured boot process, so before reaching that phase several essential steps take place.

A chip on our motherboards called the Trusted Platform Module (TPM) supplies a secure environment for generating, storing, and managing cryptographic keys. The TPM possesses a unique public and private key pair referred to as the Endorsement Key (EK), which has been permanently embedded by the manufacturer. Within the TPM is a set or registers called Platform Configuration Registers (PCR) that is used to store sensitive information.

During the boot process, the TPM logs security measurements of various components and stores them in the PCR. The issue lies in the fact that these logs lack any utility unless they can be authenticated by an entity capable of analyzing them. This is where the measured boot stage comes into play.

During the measured boot stage, the TPM generates a separate key pair referred to as the Attestation Identity Key (AIK). Microsoft has simplified the attestation process for Windows users by configuring their machines to communicate with a Microsoft cloud service without requiring any manual configuration. Following the completion of the initial three stages of the boot process, a Windows system will autonomously initiate communication with the cloud service, which serves as the attestation server.

The process of boot attestation involves a series of steps:

  1. Initially, the client system sends a request to the attestation server with the public AIK key.
  2. The attestation server responds to the client’s request by providing a certificate.
  3. The client takes the provided certificate and uses it to sign the boot logs.
  4. These signed and encrypted logs are then sent back to the attestation server.
  5. Upon receiving the logs, the attestation thoroughly analyzes them.
  6. During the analysis, the attestation server references its database of expected measurements and values, and it also checks digital signatures.
  7. Based on its analysis, the attestation server assesses whether the client device’s boot process is compromised.

This entire process, from log submission to analysis and compromise determination, is known as boot attestation. It is a critical security measure that ensures the integrity of the boot process.

Why is Boot Attestation Important?

Rootkits are a specialized form of malware that excel at going undetected within your computer. Among these, a unique type called a bootkit gains entry during your system’s boot-up sequence.

UEFI bootkits take a distinctive approach compared to other malware. Instead of infiltrating software, they embed themselves into the firmware of your computer’s motherboard. This enables them to bypass traditional security defenses.

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The capabilities of bootkits are concerning. They can delete essential operating system code and files, create covert backdoor access points for attackers, steal valuable information, and even introduce additional malware throughout your system.

Antivirus software is effective in protecting against most rootkits, so what makes bootkits so dangerous?

Bootkits are a particular concern because they manage to infiltrate a system before the antivirus software can activate. Antivirus software usually resides within the operating system, which loads after the boot process. In case a bootkit makes its way into the system, we can’t rely on the antivirus software anymore, as it may have been compromised as well.

This is where boot attestation assumes its role in defending against these types of attacks. During the UEFI firmware boot-up, the Trusted Platform Module (TPM) logs measurements of essential components, creating a cryptographic record that vouches for your system’s integrity. UEFI bootkits, despite being hidden inside of the firmware will fail to pass cryptographic signature checks.

Conclusion

Boot attestation is a key factor in the measured boot process that protects our systems against stealthy bootkits and other forms of malware. Bootkits are notorious for infiltrating our systems during the boot process, striking at a vulnerable moment before our security defenses fully activate. Boot attestation uses the logs of critical components to protect the integrity of the system.