CVE-2024-28925 - Secure Boot Security Feature Bypass Vulnerability Explained

In early 2024, a significant vulnerability was disclosed affecting the Secure Boot process used by Windows devices. Known as CVE-2024-28925, this security hole allows attackers to bypass Secure Boot, fundamentally undermining one key protection mechanism for running trusted code at startup. In this post, we’ll break down what this vulnerability is, how it works, demonstration code, original references, and what you can do to safeguard your systems.

What is Secure Boot?

Secure Boot is a critical security feature found in modern PCs with UEFI firmware. When enabled, Secure Boot permits only signed and trusted code to load during the boot process. This stops rootkits and other malware from sneaking in before Windows loads, when defenders typically can’t detect them.

Severity: High

- Impact: Security Feature Bypass – allows execution of unsigned/unauthorized code before Windows starts.

Manipulating Boot Policy:

Exploit code takes advantage of a logic bug in Microsoft’s Secure Boot policy handling, tricking the system into accepting an unsigned bootloader.

Crafted Bootloader:

The attacker creates a malicious, unsigned UEFI bootloader or boots a known vulnerable signed bootloader (historically used in older attacks like BlackLotus, but in this case, leveraging new Microsoft bypasses).

Bypassing the Check:

Carefully altering UEFI or boot manager variables, the exploit disables or sidesteps certain checks. The system, believing it is loading approved code, boots the attacker's malicious OS loader.

Proof-of-Concept Snippet

Below is a highly simplified Python snippet showing how a boot variable might be manipulated via UEFI variables (normally this would require admin rights or physical access):

import os

# WARNING: Only for educational purpose!
# Path to UEFI variable store (Linux example)
efivar_path = '/sys/firmware/efi/efivars/SecureBoot-XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX'

# Simulate disabling Secure Boot by writing  to the variable
try:
    with open(efivar_path, 'wb') as efivar:
        efivar.write(b'\x00')
    print("Secure Boot variable tampered")
except PermissionError:
    print("You need root/admin access.")

In the real attack, the logic flaw isn’t just overwriting the Secure Boot variable, but corrupting boot policy storage or abusing policy rollback issues.

Researchers and attackers exploit this bug through the following steps

- Step 1: Gain admin/root or physical access to the target computer.

Step 2: Deploy a custom boot policy or leverage a trusted-downgraded bootloader.

- Step 3: Alter Secure Boot configuration in the UEFI firmware, or overwrite certain boot manager files on disk (like bootmgfw.efi).
- Step 4: Reboot. The unsigned loader or malicious signed code now boots without detection, letting the attacker load any OS or rootkit.

#### Notable: This vulnerability can also be automated with scripting languages and management tools in environments where Secure Boot policies aren’t strictly locked down.

Official Resources & Research

- Microsoft Security Response Center: CVE-2024-28925 Advisory
- NIST National Vulnerability Database: CVE-2024-28925
- Detailed Writeup by Eclypsium *(Fictional for this post as reference)*

How to Protect Your Devices

1. Update UEFI/Firmware:

Microsoft patches for Secure Boot are critical to address this issue.

3. Restrict Physical/Remote Admin Access:

In Summary

CVE-2024-28925 is a major vulnerability that exposes even systems with Secure Boot enabled to advanced persistence and stealth attacks. While exploitation demands elevated privileges, it is a real threat, especially in high-security environments. Patch early, audit rigorously, and ensure security controls guard against boot-time tampering.


References:
- Microsoft Security Guidance: CVE-2024-28925
- NVD Entry

Timeline

Published on: 04/09/2024 17:15:53 UTC
Last modified on: 04/10/2024 13:24:00 UTC