If you’re in the field of Information Security or Systems Administration, you're probably no stranger to Secure Boot, the built-in UEFI protection designed to block unsigned or malicious code during system startup. But in early 2024, a fresh vulnerability surfaced—CVE-2024-28898—that exposes shocking weaknesses in Secure Boot’s defenses.

This long read dives deep into CVE-2024-28898, breaking down what happened, how an exploit actually works (with a code snippet!), mitigation steps, and links to trustworthy resources. Let’s get started.

What is CVE-2024-28898?

CVE-2024-28898 is a serious security vulnerability that was disclosed in March 2024 by security researchers. It allows attackers to bypass Secure Boot, potentially letting unsigned, untrusted, or even malicious software run during the early boot process.

Technical Description

At its heart, this vulnerability is a logic flaw in how Secure Boot validates bootloaders and operating systems. The Secure Boot mechanism is supposed to verify digital signatures attached to bootloaders, but due to an oversight when parsing certain crafted boot files, Secure Boot’s signature verification can be tricked into accepting malicious loaders.

Microsoft security advisory (ADV240001) and CERT/CC’s note (VU#506028) provide incident details.

The Attack

1. Craft a Malicious Bootloader: An attacker creates a bootloader (like a modified GRUB or Windows Boot Manager) with a carefully malformed digital signature.
2. Present to Secure Boot: The signed (actually, "malformed signed") bootloader is loaded by UEFI firmware.
3. Bypass Checks: UEFI Secure Boot, due to the validation flaw, misinterprets the signature, treating it as valid.
4. Execute Payload: The malicious loader hands off to malware or a rootkit before the OS even starts.

Sample Proof-of-Concept Snippet

Disclaimer: This code is a *simplified illustration* and should never be used on production systems!

Suppose malicious_loader.efi is a tampered bootloader. Attackers might use code similar to

from cryptography.hazmat.primitives.serialization import load_pem_private_key
from cryptography.hazmat.primitives.hashes import SHA256
from cryptography.hazmat.primitives.asymmetric.padding import PKCS1v15

# Load attacker's private key (mimicking a trusted one)
with open("attacker_key.pem", "rb") as key_file:
    private_key = load_pem_private_key(key_file.read(), password=None)

# Read bootloader binary
with open("malicious_loader.efi", "rb") as f:
    boot_bin = f.read()

# Create a bogus signature (malformed intentionally)
signature = private_key.sign(
    boot_bin,
    PKCS1v15(),
    SHA256()  # Exploit: Not the expected hashing algorithm
)

# Append signature to loader, exploiting Secure Boot's parsing flaw
with open("malicious_loader_signed.efi", "wb") as f:
    f.write(boot_bin + signature)

*Note: Real-world attacks involve far more complexity, but this illustrates the concept of forging signatures exploiting format or parsing weaknesses.*

References

- Microsoft Security Advisory ADV240001
- CERT/CC Vulnerability Note VU#506028
- CVE-2024-28898 on MITRE
- Original Public Disclosure on GitHub

How to Check Vulnerability Status

- Use Microsoft’s Script to scan for vulnerable boot components.

Update UEFI Firmware: Apply UEFI or BIOS updates from your system vendor.

2. Deploy OS Patches: Windows and major Linux distributions have released security patches which BLOCK known-bad bootloaders.
3. Revoke Old Bootloaders: Use Microsoft’s Secure Boot DBX (revoked signatures) update (how-to here).
4. Audit Boot Order: Remove external and removable boot options, disabling boot from USB/CD when possible.

Conclusion

The CVE-2024-28898 Secure Boot bypass is a textbook example of how even the most foundational system protections can fail if implementation has oversights. Stay on top of firmware and OS updates, and use the references above to keep your systems protected.

Timeline

Published on: 04/09/2024 17:15:48 UTC
Last modified on: 04/10/2024 13:24:00 UTC