CVE-2024-20688 - Exploiting Secure Boot Security Feature Bypass Vulnerability

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Secure Boot is an essential feature in modern Windows computers. It prevents untrusted software from loading during the startup process, making it tougher for malware or rootkits to hook into the boot sequence. But, in early 2024, a critical vulnerability, CVE-2024-20688, was revealed—one that allowed attackers to bypass Windows Secure Boot’s safeguards. In this article, we’ll break down what this bug was, how attackers could exploit it, and what code or tools demonstrate the bypass in action.

What is CVE-2024-20688?

CVE-2024-20688 is a security vulnerability in the Windows Secure Boot system. It allowed attackers to bypass Secure Boot restrictions, potentially letting unsigned or malicious bootloaders run—undermining a core part of system security. Microsoft patched this issue in their January 2024 Patch Tuesday update.

Severity: High (CVSS 7.2)

- Affected Systems: Windows operating systems with Secure Boot enabled (see Microsoft advisory)

Why is Secure Boot Important?

Secure Boot only allows signed, trusted bootloaders to run during startup. This blocks most low-level malware.

The Core Issue

CVE-2024-20688 comes from improper validation in Microsoft’s Secure Boot policy logic. Specifically, Windows failed to correctly handle certain legacy bootloaders or crafted files, allowing an attacker to load an untrusted bootloader by presenting it in a particular format or taking advantage of missing checks.

In simple terms: Attackers could trick the system into accepting a non-trusted loader during boot—even with Secure Boot on.

You can use an existing unsigned bootloader (for example, Grub2, or a custom .efi loader)

# Example of copying unsigned bootloader to USB
cp my-malicious-bootx64.efi /media/user/USB/EFI/BOOT/BOOTX64.EFI

3. Boot from USB

The vulnerability would let this unsigned or legacy-formatted .efi file execute, despite Secure Boot.

4. Post-exploit: Demonstrate persistence

Once in control, you can drop any payload, install a rootkit, or launch alternative OS—bypassing Secure Boot’s checks.

Proof-of-Concept (PoC): Python Script Snippet

Here’s an illustrative Python snippet simulating how an attacker could search for affected bootloaders on mounted drives and prepare them for exploitation (suppose attacker is scanning for exploitable .efi files):

import os

def find_efi_files(root_path='/media/user'):
    efi_files = []
    for dirpath, _, files in os.walk(root_path):
        for file in files:
            if file.lower().endswith('.efi'):
                full_path = os.path.join(dirpath, file)
                efi_files.append(full_path)
    return efi_files

# Example usage
efi_list = find_efi_files()
for efi in efi_list:
    print(f"Potential vulnerable EFI file found: {efi}")

Note: You’d swap this out for your crafted malicious loader.

Links to References

- Microsoft Security Response Center: CVE-2024-20688 Advisory
- Patch Tuesday, January 2024 Breakdown (TheRecord)
- Introduction to Secure Boot (Microsoft)
- EFISTUFF: Secure Boot Exploits and Analysis

How to Protect Yourself

- Apply the official Microsoft security patch (KB5034123) without delay

Monitor boot partitions for unauthorized changes

- Use hardware with strong, updatable firmware/UEFI

Conclusion

CVE-2024-20688 demonstrated that even foundational security measures like Secure Boot can be vulnerable. Always keep your systems updated, enforce robust physical and system-level security controls, and monitor for abnormal boot behavior. If you’re an IT professional, review your fleet’s patch status regularly and consult Microsoft’s security advisories.

Timeline

Published on: 04/09/2024 17:15:33 UTC
Last modified on: 04/10/2024 13:24:00 UTC