CVE-2026-2636: Windows CLFS.sys Driver DoS Vulnerability

CVE-2026-2636 Overview

CVE-2026-2636 is a Denial of Service vulnerability in the Windows Common Log File System (CLFS) driver (CLFS.sys). This vulnerability is caused by improper handling of invalid use of special elements (CWE-159), which leads to an unrecoverable inconsistency in the CLFS driver. This condition forces a call to the KeBugCheckEx function, allowing an unprivileged local user to trigger a system crash (Blue Screen of Death).

Critical Impact
An unprivileged local attacker can exploit this vulnerability to cause a complete system crash, resulting in denial of service. The vulnerability allows disruption of system availability without requiring elevated privileges.

Affected Products

Windows 11 2024 LTSC (prior to September 2025 cumulative update)
Windows Server 2025 (prior to September 2025 cumulative update)
Windows 11 23H2 and earlier versions (remain vulnerable)

Discovery Timeline

2026-02-25 - CVE-2026-2636 published to NVD
2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-2636

Vulnerability Analysis

This vulnerability affects the Windows Common Log File System driver (CLFS.sys), a kernel-mode component responsible for managing transaction logs. The flaw stems from improper handling of invalid use of special elements, classified under CWE-159. When the driver encounters specially crafted input containing malformed special elements, it fails to properly validate or sanitize this data, leading to an unrecoverable inconsistency within the driver's internal state.

The vulnerability requires local access to the target system but can be exploited by users with low privileges. No user interaction is required for exploitation. The impact is limited to availability—the attacker cannot leverage this flaw to read sensitive data or modify system integrity.

Root Cause

The root cause lies in the CLFS.sys driver's failure to properly handle invalid or malformed special elements in log file operations. When the driver processes input containing these malformed elements, it enters an inconsistent state from which it cannot recover. This forces the driver to invoke the KeBugCheckEx function, which initiates a system bugcheck (BSOD) to prevent further corruption or unpredictable behavior.

Attack Vector

The attack vector is local, meaning an attacker must have some level of authenticated access to the target system. The exploitation involves interacting with the CLFS driver through standard Windows APIs or by manipulating Common Log File System log files. An unprivileged user can craft specific input that triggers the improper handling condition, causing the kernel driver to crash the entire system.

The vulnerability mechanism involves sending specially crafted requests to the CLFS driver that contain invalid special elements. When the driver attempts to process these elements, it fails to handle them correctly, resulting in a state inconsistency. The driver's error handling code then determines the system must be halted to prevent further issues, calling KeBugCheckEx and triggering an immediate Blue Screen of Death.

For additional technical details, see the Fortra Security Advisory FR-2026-001.

Detection Methods for CVE-2026-2636

Indicators of Compromise

System crash events with bugcheck codes related to CLFS driver failures
Unexpected Blue Screen of Death events on systems with unprivileged user activity
Windows Event Log entries indicating CLFS.sys driver errors or crashes
Repeated system restarts without obvious cause on affected Windows versions

Detection Strategies

Monitor Windows Event Logs for kernel driver crash events, particularly those involving CLFS.sys
Implement system availability monitoring to detect unexpected reboots or crashes
Deploy endpoint detection solutions capable of identifying suspicious interactions with CLFS APIs
Review crash dump files for evidence of CLFS driver state inconsistencies

Monitoring Recommendations

Enable Windows Error Reporting to capture crash dump data for analysis
Configure centralized logging to aggregate system crash events across the environment
Set up alerts for systems running vulnerable Windows versions that experience repeated crashes
Monitor for any unusual patterns of system instability on Windows 11 23H2 and earlier versions

How to Mitigate CVE-2026-2636

Immediate Actions Required

Apply the September 2025 cumulative update for Windows 11 2024 LTSC and Windows Server 2025
Upgrade Windows 11 23H2 systems to Windows 25H2 or later, which was released with the patch
Prioritize patching systems that are accessible to multiple users or have less trusted user accounts
Implement application whitelisting to restrict which applications can interact with low-level system drivers

Patch Information

Microsoft silently fixed this vulnerability in the September 2025 cumulative update for Windows 11 2024 LTSC and Windows Server 2025. Windows 25H2, released in September 2025, includes the fix by default. Windows 11 23H2 and earlier versions remain vulnerable and should be upgraded to a patched version. Refer to the Fortra Security Advisory FR-2026-001 for additional details.

Workarounds

Restrict local access to systems running vulnerable Windows versions to trusted users only
Implement strict user account controls to minimize the number of accounts with local system access
Consider isolating systems running Windows 11 23H2 or earlier that cannot be immediately upgraded
Monitor for and promptly investigate any system crash events on vulnerable systems

bash

# Verify Windows version and patch status
winver
# Check for September 2025 cumulative update installation
wmic qfe list | findstr "KB"
# Review system crash history
wevtutil qe System /q:"*[System[Provider[@Name='Microsoft-Windows-Kernel-Power']]]" /c:10 /f:text