Inside CVE-2026-1337: How the Windows CLFS Driver Zero-Day Is Being Weaponized by Ransomware Groups

On June 3rd, 2026, Microsoft confirmed active exploitation of CVE-2026-1337, a critical privilege escalation vulnerability in the Windows Common Log File System (CLFS) driver. Within 72 hours of public disclosure, our OSINT engine detected at least three distinct ransomware groups incorporating the exploit into their initial access toolkits.

What Is the CLFS Driver?

The Common Log File System (clfs.sys) is a kernel-mode driver present in every modern Windows installation. It provides a general-purpose logging infrastructure used by applications, services, and the OS itself. Because it runs in kernel space and is always loaded, it's a high-value target for privilege escalation.

The Vulnerability

CVE-2026-1337 is a use-after-free vulnerability triggered when the CLFS driver processes a malformed Base Log File (BLF). An attacker with local code execution can craft a BLF that causes the driver to reference freed memory, leading to arbitrary kernel write primitives.

Exploitation Chain Observed in the Wild

Our telemetry shows the following kill chain being used by the "BlackMesh" ransomware group: (1) Initial access via phishing email with malicious Office macro, (2) CLFS exploit executed to gain SYSTEM privileges, (3) Credential dumping via LSASS, (4) Lateral movement via SMB, (5) Ransomware deployment.

Remediation

Microsoft released an emergency out-of-band patch on June 5th, 2026. Apply KB5040442 immediately. If patching is not immediately possible, consider disabling the CLFS driver via Group Policy — note this will break applications that depend on CLFS logging.

reg add "HKLM\SYSTEM\CurrentControlSet\Services\CLFS" /v "Start" /t REG_DWORD /d 4 /f

Monitor for suspicious BLF file creation in %TEMP% and %APPDATA% directories. YARA rules and Sigma detections are available in our GitHub repository.