CISA added CVE-2021-43226 to its Known Exploited Vulnerabilities (KEV) catalog on October 6, 2025, signaling confirmed real-world exploitation and forcing rapid mitigation requirements for U.S. federal and critical-infrastructure organizations. The vulnerability resides in the Microsoft Common Log File System (CLFS) driver (clfs.sys / related components) and is tracked as a local privilege-escalation flaw that can allow an authenticated, local user to escalate to SYSTEM privileges. CISA’s KEV listing and contemporaneous reporting make clear this is no longer theoretical the inclusion moves the issue into an urgent operational posture.

From a technical and operational risk perspective, the flaw stems from improper validation/handling of user-supplied data in the CLFS driver’s memory routines, which attackers can trigger by crafting malicious CLFS log files to produce buffer-overflow or similar memory corruption conditions that lead to arbitrary code execution at elevated privileges. The weakness affects multiple Windows releases (Windows 10, Windows 11 and supported Server SKUs are named in public guidance), and because the exploit requires only local, authenticated access with standard user rights, it is particularly dangerous when paired with common initial-access vectors (phishing, remote code execution footholds, or stolen credentials). Public reporting also notes proof-of-concept exploit code has appeared in underground forums, increasing the chance of opportunistic and ransomware-style post-exploitation activity.

Detection and short-term mitigations should be treated as urgent. CISA has set a mandatory remediation deadline (October 27, 2025) under BOD-22-01 for covered entities, and organizations should immediately prioritize deploying Microsoft’s security updates via Windows Update or WSUS, starting with domain controllers, file servers, and other high-value hosts. For systems that cannot be patched instantly, implement application control policies (whitelisting), enable Windows Defender Exploit Guard / Attack Surface Reduction rules, and restrict local write/execution privileges where possible. Concurrently, hunt for indicators such as anomalous CLFS-related activity, unexpected kernel-mode driver loads, and privilege escalation events security teams should review relevant Event IDs and unusual service/driver behavior to detect exploitation attempts.