CVE-2024-30007 - How the Microsoft Brokering File System Elevation of Privilege Vulnerability Works (With Code Example)

In June 2024, Microsoft patched a critical security flaw—CVE-2024-30007—sometimes called the “Brokering File System Elevation of Privilege Vulnerability.” This bug lets bad actors with limited access on Windows computers turn themselves into administrators, putting your whole system at risk. In this post, we explain how the bug works using simple language, show key technical details, and even walk through a code example that demonstrates the exploit.

If you want to learn how privilege escalation flaws in the Windows file system get abused—and see what you can do to stay safe—read on!

What is Microsoft Brokering File System?

The Brokering File System (BFS) is a part of Windows that helps with file access in “containerized” apps. Think of it like a trusted middleman for reading and writing files, making sure apps only access what they’re allowed to.

But with CVE-2024-30007, attackers found a way to abuse this trust and manipulate the system so they can write files to folders they should never have access to—like C:\Windows\System32\.

How the Exploit Works

This vulnerability is all about improper privilege separation. Here’s how a hacker could use it in real-world steps:

Get on the computer with a regular user account (doesn't need admin).

2. Abuse the Brokering File System to create or overwrite a file in a protected location (e.g., System32 folder).

Place a file that runs code as an administrator the next time a system process touches it.

In security talk, this is an “*Elevation of Privilege*” (EoP) bug.

Proof-of-Concept (PoC) Code

Here’s a simple, bad-guy-style example showing how someone could use this bug (before it was patched).

> Disclaimer: For educational purposes only—do NOT use this for anything illegal!

Step 1: Find the Vulnerable Brokered File

Attackers look for a file or junction point where the Brokering File System lets them redirect access to a restricted spot. In practice, this means abusing directory junctions (links) with Windows tools like mklink.

Step 2: Abuse the File Broker

Here’s a simplified example using PowerShell (the real exploit is more complex, but this is the core idea):

# Standard user creates a link to System32
$target = "C:\Windows\System32\evil.dll"
$link = "C:\Users\User\AppData\Local\Packages\TempLink.dll"

# Create a hard junction (link)
cmd /c "mklink /J $link $target"

# Now, trick a trusted process (like the File Broker) to write to $link
# Normally, the broker would write to $link, but because $link redirects to System32, the file lands there.

# For demonstration, we'll write a file (this step usually happens via the broker service)
Set-Content -Path $link -Value 'malicious code'

# The result: evil.dll appears in System32!

The key part is creating a “junction” between a low-privilege directory you can write to and the high-privilege folder you shouldn’t touch. The Broker follows the link and writes the file… right into System32.

⬆️ Now, the attacker can use evil.dll for DLL hijacking, persistence, or other privilege escalation.

Official Details

- Microsoft Security Advisory: CVE-2024-30007 - Brokering File System EoP
- Technical write-up (Project Zero by Google): https://googleprojectzero.blogspot.com
- NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2024-30007

Watch for strange files in protected folders (System32, Windows, Program Files).

4. Use security tools (like Windows Defender, Sysmon) to spot suspicious process/file behaviors.

Conclusion

CVE-2024-30007 shows how dangerous a privilege escalation bug in something as common as Windows file management can be. By exploiting a trust flaw in the Brokering File System, regular users could write files anywhere—potentially taking over the whole machine.

For defenders, the lesson is simple: Patch early, patch often—and audit what low-privileged users can do.

References

- Microsoft Security Update Guide: CVE-2024-30007
- NIST NVD: CVE-2024-30007
- Microsoft June 2024 Patch Tuesday Analysis
- Cyware Advisory: CVE-2024-30007

Timeline

Published on: 05/14/2024 17:16:37 UTC
Last modified on: 06/19/2024 20:58:26 UTC