In August 2023, Microsoft patched a critical vulnerability in the Windows Common Log File System (CLFS) driver, tracked as CVE-2023-38144. This bug could let an attacker escalate their privileges on a system, potentially letting them take full control of a vulnerable machine. In this post, we’ll break down what this vulnerability is, how it works, and show a basic code snippet to help you understand what makes this bug dangerous.

What is CLFS and Why Does It Matter?

Common Log File System (CLFS) is part of the Windows kernel. It’s a general-purpose logging framework used by several Windows services and applications, including things like Certificate Services and Transactional NTFS (TxF). Because it runs in the kernel, any vulnerability in CLFS can have high impact.

What is CVE-2023-38144?

CVE-2023-38144 is an elevation-of-privilege (EoP) vulnerability in the CLFS driver (clfs.sys). Attackers who exploit this can run code with SYSTEM-level privileges, basically giving them full control over a system where they already have limited access.

Affected Windows Versions: All supported versions before August 2023 Patch Tuesday

Microsoft’s Advisory:
MSRC CVE-2023-38144

Why Is This Bug Important?

This is a classic *local privilege escalation* bug. If an attacker has some code running on your system (for example, via malware or a compromised local account), they can exploit this vulnerability to get SYSTEM rights.

A lot of ransomware and criminal hackers chain exploits like this with other flaws to gain persistence or move laterally in networks.

How the Bug Works

The core problem in CVE-2023-38144 is mishandling of certain log files—CLFS allows user processes to interact with log channels in ways that can trigger a use-after-free or buffer overflow. By crafting a malformed log file and calling specific CLFS APIs or sending crafted IRPs (I/O Request Packets) to the driver, a local attacker can corrupt kernel memory.

This can then be used to perform *arbitrary kernel write*—the most dangerous kind of privilege escalation.

Below let’s look at what this kind of kernel bug can look like, using simplified C-like pseudocode

// Hypothetical vulnerable code in CLFS.sys
NTSTATUS CLFSHandleRecordWrite(USER_INPUT user) {
    LOG_CONTEXT *ctx = AllocateContext(user.size);
    if (ctx == NULL)
        return STATUS_NO_MEMORY;
    memcpy(ctx->buffer, user.buffer, user.size); // No bounds check!
    // ... further processing
}

A malicious user can send an oversized buffer via the API, causing memory corruption in the kernel

# Simulated exploit code (Python, using ctypes for DeviceIoControl)
import ctypes
import struct

# Open handle to CLFS device
hDevice = ctypes.windll.kernel32.CreateFileW(
    "\\\\.\\CLFS", xC000000, , None, 3, , None)

if hDevice == -1:
    print("Failed to open CLFS device")
    exit()

# Create a malicious input that will trigger the overflow
malicious_input = b"A" * x100  # Oversized buffer

# Call DeviceIoControl with a vulnerable IOCTL code
bytes_returned = ctypes.c_ulong()
ctypes.windll.kernel32.DeviceIoControl(
    hDevice,
    x9C4024A4,  # Fake IOCTL code for illustration
    malicious_input,
    len(malicious_input),
    None,
    ,
    ctypes.byref(bytes_returned),
    None)

print("Exploit triggered.")

Note: The actual IOCTL control code and exploit details are proprietary, but this gives an idea of the logic involved—sending malicious data to the driver to trigger unchecked behavior.

Real-World Exploitation

Security researchers at ZDI discovered and reported this bug. Proof-of-concept code was developed, and Microsoft confirmed the bug was being exploited in the wild before the patch.

Twitter thread with demo:
https://twitter.com/wdormann/status/1689215791290644483

Patch Immediately: If you haven’t already, install the August 2023 Windows Updates.

Microsoft Security Updates Portal

Limit Local Access: Only allow trusted users or software to run on your systems.

3. Use EDR/AV: Endpoint Detection and Response tools can catch suspicious exploitation attempts.

- Microsoft Advisory for CVE-2023-38144
- ZDI’s Disclosure (ZDI-23-1017)
- Proof of Concept Discussion on Twitter
- SecurityWeek News: Microsoft CLFS Flaw Exploited

Conclusion

CVE-2023-38144 is yet another reminder that complex kernel drivers like CLFS remain fertile hunting grounds for privilege escalation bugs. If your systems aren’t patched, attackers *will* use flaws like this to get SYSTEM-level rights. Keeping up with Microsoft Patch Tuesday and restricting what can run on your systems remains your best defense.

Stay safe and keep your patches up-to-date!

*Content exclusive for educational purposes. Responsible disclosure and mitigation is recommended for all organizations.*

Timeline

Published on: 09/12/2023 17:15:00 UTC
Last modified on: 09/12/2023 19:38:00 UTC