CVE-2021-43226: Windows Common Log File System Driver Elevation of Privilege

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 17 September 2026. Confidence: medium.

high priorityCVSS 7.8 with local low-privilege exploitation, KEV listing and known ransomware use make this a high-priority patch despite the local attack vector.

What it is

The Windows Common Log File System (CLFS) driver contains an elevation of privilege flaw. A local attacker with low privileges can exploit it to gain full control of confidentiality, integrity and availability on the host. Because CLFS is a core Windows component present across desktop and server releases, unpatched systems remain exposed.

Impact

An attacker gains SYSTEM-level privileges from a low-privileged local account, enabling full control of the machine and a foothold for further lateral movement or ransomware deployment.

Attack surface

Reached locally (AV:L) with low privileges required (PR:L) and no user interaction (UI:N). No network vector or authentication bypass is described in the record.

Exploitation

Listed in CISA KEV with a due date of 2025-10-27 and flagged for known ransomware campaign use. EPSS 30-day probability is 0.03072 (86.98th percentile), indicating elevated but not top-tier predicted activity.

What to do

• Apply the Microsoft security update for CVE-2021-43226 across all affected Windows 10, Windows 11, Windows 7, Windows 8.1, Windows RT 8.1 and Windows Server versions.
• Follow CISA BOD 22-01 guidance and remediate or discontinue use of affected products where patching is not possible.
• Restrict local interactive logon and administrative rights to reduce the pool of accounts that can reach the CLFS driver.
• Monitor for and block known ransomware tooling and post-exploitation behavior on endpoints that cannot be patched immediately.

Detection

• Alert on unexpected processes spawning from or interacting with clfs.sys and on SYSTEM-level process creation from low-privileged parent processes.
• Monitor Windows event logs for privilege escalation indicators such as token manipulation and suspicious service or driver activity.
• Hunt for known ransomware precursor behavior on hosts running unpatched Windows builds listed in the affected product set.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.