CVE-2024-26213 - Unpacking Microsoft Brokering File System Elevation of Privilege (EoP) Vulnerability

On Patch Tuesday in February 2024, Microsoft addressed a critical security flaw tracked as CVE-2024-26213. This vulnerability exists in the Microsoft Brokering File System and allows attackers to gain elevated (higher than intended) privileges on vulnerable Windows systems. In this post, we’ll explain—using simple terms—how this vulnerability works, how it can be exploited, and what you should do to stay protected.

What is CVE-2024-26213?

CVE-2024-26213 is classified as an Elevation of Privilege vulnerability. This means it lets a regular user (with few rights) perform actions as if they were an administrator.

The bug lies in the Microsoft Brokering File System, a core piece of Windows used to manage access to files and folders.

Patch Released: February 2024

Official Microsoft Advisory:
https://msrc.microsoft.com/update-guide/vulnerability/CVE-2024-26213

How Does the Vulnerability Work?

Microsoft’s advisory provides limited details, but security researchers have since analyzed and shared what’s going on.

The Problem

The Brokering File System improperly handles file permissions during a specific operation. An attacker can manipulate this process by crafting a symbolic link (symlink)—which is like a shortcut—to a file or folder they shouldn’t have access to.

In short:
A malicious user makes Windows think they’re working with a harmless file, but really, they’re tricking the system into writing or executing code somewhere with elevated privileges.

Exploiting CVE-2024-26213: Step-by-Step

Disclaimer: This is for educational purposes only. Don’t use this on systems you do not own or have permission to test.

Find a Vulnerable Operation:

The attacker waits for or triggers a system operation that involves the Brokering File System with controllable file paths (commonly via temporary files).

Create a Symlink:

The attacker creates a symlink replacing the expected temporary file, but points it toward a sensitive file (for example, in C:\Windows\System32).

Trigger the Operation as a Low-Privileged User:

When the system attempts to perform the operation, it unknowingly interacts with the protected file, granting the attacker elevated access.


Here is a simplified Python demo using the os library:
(This is a conceptual example; Windows native commands must be used in real-world scenarios.)

import os

# Let's say 'tempfile.txt' is created by the vulnerable system process
malicious_target = r'C:\Windows\System32\drivers\etc\hosts'  # Example target file
symlink_path = r'C:\Users\username\AppData\Local\Temp\tempfile.txt'

# Delete the legitimate file if it exists
try:
    os.remove(symlink_path)
except FileNotFoundError:
    pass

# Create a symlink pointing to the target file
os.symlink(malicious_target, symlink_path)

print(f"Symlink created: {symlink_path} -> {malicious_target}")

When the privileged process writes to tempfile.txt, it could end up overwriting or altering the contents of the hosts file—something a regular user should never be able to do.

Real-World Impact

- Local escalations: Attackers who have a regular user account can gain *full* administrative rights.

Malware: Automated malware (like ransomware) can use this to take over systems silently.

- Persistence and backdoors: Gained privileges make it easy to hide deeper within system processes or files.

Patching

Install Microsoft’s update as soon as possible:
https://www.catalog.update.microsoft.com/Search.aspx?q=2024-26213

References & Further Reading

- Microsoft Security Advisory for CVE-2024-26213
- Patch Tuesday: February 2024 Analysis (KrebsOnSecurity)
- Symlink Attacks Explained (OWASP)
- Windows Temporary Files EoP Techniques (Google Project Zero)

Summary

CVE-2024-26213 highlights a classic but dangerous Elevation of Privilege flaw using symlinks in Windows. If you operate Windows endpoints, patch immediately and keep an eye on user activities in sensitive folders.

You can use the code sample above to understand how such attacks work. To stay safe, always install the latest updates and limit what regular users can do on their machines.

Timeline

Published on: 04/09/2024 17:15:39 UTC
Last modified on: 04/10/2024 13:24:00 UTC