Listen to this Post
The vulnerability resides in the Windows Common Log File System (CLFS) driver, a core component that manages transaction logging. Due to improper validation of user‑supplied data within the driver’s memory management routines, a local attacker with low privileges can craft a malicious CLFS log file. When the driver processes this specially crafted log file, it triggers a buffer overflow or a use‑after‑free condition. The corrupted kernel memory allows the attacker to execute arbitrary code with SYSTEM‑level privileges, bypassing all security restrictions. No user interaction is required beyond the attacker having local access. The flaw affects a wide range of Windows versions, including Windows 10, Windows 11, Windows Server 2016, 2019, and 2022. Although Microsoft patched the issue in December 2021, the vulnerability remained actively exploited, and CISA added it to the Known Exploited Vulnerabilities catalog in October 2025. Attackers frequently chain this elevation‑of‑privilege flaw with a remote code execution vulnerability to gain an initial foothold, then move laterally and deploy ransomware such as LockBit.
dailycve form
Platform: Windows OS
Version: Multiple versions
Vulnerability: Elevation of privilege
Severity: High
date: 2021-12-14
Prediction: Patch: 2021-12-14
What Undercode Say:
Check CLFS driver version (example)
driverquery /v | findstr "CLFS"
Detect potential exploitation via system event logs
Get-WinEvent -LogName "System" | Where-Object { $_.Id -eq 41 }
Minimal PoC code (educational – triggers UAF condition)
include <windows.h>
int main() {
HANDLE hLog = CreateLogFile(L"malicious.clfs", ...);
// Trigger free and subsequent reuse
...
}
Exploit
The attacker creates a specially crafted CLFS log file that forces the driver to use a memory region after it has been freed (use‑after‑free). This corrupts kernel memory, allowing the attacker to overwrite critical structures and elevate privileges to SYSTEM. Public proof‑of‑concept exploits are available, and the vulnerability has been weaponized in real‑world ransomware campaigns (LockBit, MimiKatz).
Protection from this CVE
- Apply the official Microsoft security update (released December 14, 2021).
- Implement Application Control policies and Windows Defender Exploit Guard.
- Enforce the principle of least privilege and restrict local logon rights.
- Monitor for unusual privilege escalation events and CLFS‑related crashes.
Impact
Successful exploitation grants SYSTEM‑level access, enabling the attacker to fully compromise the host, disable security tools, access sensitive data, deploy ransomware, and move laterally across the network.
🎯Let’s Practice Exploiting & Learn Patching For Free:
Sources:
Reported By: www.cve.org
Extra Source Hub:
Undercode
