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Introduction

In early 2022, Microsoft patched a critical security vulnerability in the Common Log File System (CLFS) driver of Windows. The vulnerability, tracked as CVE-2022-21981, allowed attackers to use the CLFS.sys driver to elevate privileges on the system. This post explains what this bug is, why it matters, and how attackers could exploit it. If you’re a Windows user or admin, you’ll want to understand the basics—even if you’re not a security pro.

What is CLFS.sys?

Common Log File System (CLFS) is a Windows component used by the operating system and some applications to create and manage log files. It’s a kernel driver found at C:\Windows\System32\Drivers\clfs.sys. Because CLFS operates in kernel mode, any bugs in it can be dangerous.

About CVE-2022-21981

- CVE: CVE-2022-21981

Patch Date: February 8, 2022

- Note: This is a different vulnerability from CVE-2022-22000, which affects the same driver but is a separate issue.

What’s the Problem?

The vulnerability allowed a local attacker to send carefully crafted requests to the CLFS driver, causing it to perform unsafe actions in memory (a "use-after-free" or similar bug). In practice, this meant a regular user or malware could get SYSTEM-level privileges.

Who Is Affected?

All supported Windows versions before the February 2022 patch.

Program interacts directly with the CLFS driver, sending malformed requests.

3. Due to a bug in CLFS, an internal object is freed but still referenced—a *use-after-free* occurs.
4. Attacker arranges for their own code/data to reoccupy (“spray”) that freed memory.
5. When CLFS accesses the freed object, it actually reads/writes from the attacker's memory.

Example Pseudocode (For Educational Purposes)

Below is a *simplified* pseudocode sample. This won’t work directly, but shows the logic.

// Open a handle to the driver
HANDLE hClfs = CreateFile("\\\\.\\clfs", ...);

// Prepare malformed input to trigger the bug
BYTE maliciousInput[] = { /* crafted bytes here */ };

// Send malicious input via DeviceIoControl
DWORD bytesReturned;
DeviceIoControl(
    hClfs,
    MALICIOUS_IOCTL_CODE,  // a specific control code
    maliciousInput,
    sizeof(maliciousInput),
    NULL,
    ,
    &bytesReturned,
    NULL
);

// After bug is triggered, try to gain SYSTEM privileges
// (e.g., by stealing token, replacing process ID, etc.)
EscalateToSystem();

DeviceIoControl is the key user-kernel interface for many driver exploits.

### Original Advisory & Patch Details

Microsoft rated this as EXploitation More Likely.

- Patch released in February 2022 Patch Tuesday.

Technical Write-Ups:  
- Zero Day Initiative – ZDI-22-299  
- ZecOps Research: CLFS.sys Vulns (Related vulnerabilities)

Final Thoughts

CVE-2022-21981 is a reminder that even obscure Windows drivers can expose your entire system if flawed. Attacks like this are sophisticated, but once weaponized, can be used by malware or hackers to take over computers. Always keep your systems patched and stay aware of the latest security news.

References

- Microsoft CVE-2022-21981 Advisory
- Zero Day Initiative ZDI-22-299
- ZecOps CLFS Vulns Overview
- Microsoft Patch Tuesday (February 2022)


*This post is based on public records and research, but is written exclusively for understanding CVE-2022-21981 in simple terms. All exploit code here is not functional and is for awareness only.*

Timeline

Published on: 02/09/2022 17:15:00 UTC
Last modified on: 05/23/2022 17:29:00 UTC