CriticalCVE-2021-43226

    Windows Common Log File System Driver Elevation of Privilege Vulnerability (CVE-2021-43226)

    Published Last modified In CISA KEV since Sources: CVE.org, pwn.watch

    What is affected, what to do

    Windows Common Log File System Driver Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. Update Microsoft Windows 10 Version 1809 to version 10.0.17763.2366, Microsoft Windows Server 2019 to version 10.0.17763.2366, Microsoft Windows Server 2019 (Server Core installation) to version 10.0.17763.2366. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1809 lt 10.0.0..10.0.17763.2366
    • Microsoft Windows Server 2019 lt 10.0.0..10.0.17763.2366
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.0..10.0.17763.2366
    • Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1977
    • Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1415
    • Microsoft Windows Server 2022 lt 10.0.0..10.0.20348.405
    • Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1415
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19041.1415
    • Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1415
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19042.1415
    • Microsoft Windows 11 version 21H2 lt 10.0.0..10.0.22000.376
    • Microsoft Windows 10 Version 21H2 lt 10.0.0..10.0.19044.1415
    • Microsoft Windows 10 Version 1507 lt 10.0.0..10.0.10240.19145
    • Microsoft Windows 10 Version 1607 lt 10.0.0..10.0.14393.4825
    • Microsoft Windows Server 2016 lt 10.0.0..10.0.14393.4825
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.0..10.0.14393.4825
    • Microsoft Windows 7 lt 6.1.0..6.1.7601.25796
    • Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25796
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20207
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.0..6.0.6003.21309
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.0..6.0.6003.21309
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.0..6.1.7601.25796
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.0.0..6.1.7601.25796
    • Microsoft Windows Server 2012 lt 6.2.0..6.2.9200.23545
    • Microsoft Windows Server 2012 lt 6.2.0..6.2.9200.23540
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.0..6.2.9200.23545
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.0..6.2.9200.23540
    • Microsoft Windows Server 2012 R2 lt 6.3.0..6.3.9600.20207
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.0..6.3.9600.20207
    Fixed in
    • Microsoft Windows 10 Version 1809 10.0.17763.2366
    • Microsoft Windows Server 2019 10.0.17763.2366
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.2366
    • Microsoft Windows 10 Version 1909 10.0.18363.1977
    • Microsoft Windows 10 Version 21H1 10.0.19043.1415
    • Microsoft Windows Server 2022 10.0.20348.405
    • Microsoft Windows 10 Version 2004 10.0.19041.1415
    • Microsoft Windows Server version 2004 10.0.19041.1415
    • Microsoft Windows 10 Version 20H2 10.0.19042.1415
    • Microsoft Windows Server version 20H2 10.0.19042.1415
    • Microsoft Windows 11 version 21H2 10.0.22000.376
    • Microsoft Windows 10 Version 21H2 10.0.19044.1415
    • Microsoft Windows 10 Version 1507 10.0.10240.19145
    • Microsoft Windows 10 Version 1607 10.0.14393.4825
    • Microsoft Windows Server 2016 10.0.14393.4825
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4825
    • Microsoft Windows 7 6.1.7601.25796
    • Microsoft Windows 7 Service Pack 1 6.1.7601.25796
    • Microsoft Windows 8.1 6.3.9600.20207
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21309
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21309
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25796
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25796
    • Microsoft Windows Server 2012 6.2.9200.23545
    • Microsoft Windows Server 2012 6.2.9200.23540
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23545
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23540
    • Microsoft Windows Server 2012 R2 6.3.9600.20207
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20207

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    7.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    7.8

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org, pwn.watch

    3.3 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 88.4

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    226.72 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    7.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    3.3 %
    Moderate — model estimates 1-10% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    88.4
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    CISA assessment (SSVC)
    Exploitation: active · not automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 16 December 2021

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This bug requires local access with low privileges but no user interaction. Its role is the post-exploitation stage: once attackers have a foothold (phishing, stolen VPN or RDP credentials), they use it to reach SYSTEM, disable EDR, harvest credentials and deploy ransomware. CISA explicitly confirms use in ransomware campaigns. Exploitation likelihood is well above average, and impact on confidentiality, integrity and availability is complete. Because a fix has existed since 2021, any vulnerable host points to a gap in patch management. Treat such systems as high priority, especially multi-user hosts and unsupported builds in critical-infrastructure-adjacent environments.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Verify patch level: The clfs.sys (Common Log File System) flaw was fixed in Microsoft's December 2021 security updates. Query your inventory (MECM/Intune/WSUS reporting) for OS builds below these thresholds: 10.0.10240.19145 (1507), 10.0.14393.4825 (1607), 10.0.17763.2366 (1809), 10.0.18363.1977 (1909), 10.0.19041.1415 (2004) and 10.0.19042.1415 (20H2). Anything below these builds is vulnerable. Look up the per-build KB number in the Microsoft Security Update Guide.
      • Deploy the latest cumulative update: Push the current Windows cumulative update to every flagged host. It contains the 2021 fix plus later CLFS hardening. Start with multi-user systems: RDS/Citrix hosts, jump servers and admin workstations.
      • Check server equivalents: Builds 14393 and 17763 map to Windows Server 2016 and 2019. Include server estates on those build lines in scope, even though the product list only names Windows 10.
      • Isolate unsupported builds: Hosts on 1507, 1909, 2004 or 20H2 without LTSC/ESU coverage will receive no further fixes. Segment them now and schedule their migration.
      • Prioritise hosts with suspected compromise: Any unpatched host with existing initial access (phishing hit, EDR alert) may already have been taken over at SYSTEM level. Take a forensic image and rotate credentials cached on it (local admins, service accounts).

    2. Runbook step 2

      Mitigation layers

      • No workaround for the driver: CLFS is a core Windows component and cannot realistically be disabled. Patching is the only complete fix. The controls below only raise the bar for exploitation or limit its impact.
      • Application control: Use WDAC or AppLocker to block unsigned binaries and scripts from user-writable paths (%TEMP%, %APPDATA%, C:\Users\Public, Downloads). Local privilege-escalation exploits are typically launched from these locations.
      • Least privilege (IAM):
        • Remove local admin rights from end users.
        • Restrict interactive and RDP logon on servers to the groups that need it.
        • Manage local admin passwords with Windows LAPS.
      • Limit what SYSTEM access buys an attacker: Enable the following so that SYSTEM does not translate directly into credential theft or EDR shutdown:
        • LSA protection (RunAsPPL)
        • Credential Guard
        • HVCI/memory integrity
        • EDR/Defender tamper protection
        • ASR rule "Block credential stealing from LSASS"
      • Network segmentation: Place legacy systems in a dedicated VLAN with no SMB/RDP reach to domain controllers and file servers. This caps lateral movement after a successful escalation.

    3. Runbook step 3

      Detection rules

      • Sysmon EID 11 (FileCreate): Detect non-system processes creating .blf files (CLFS base logs) in user-writable paths.
        • Sigma logic: TargetFilename|endswith: '.blf' AND TargetFilename|contains: ['\Users\Public\','\AppData\Local\Temp\','\ProgramData\'] AND Image not in your allowlist of legitimate CLFS consumers.
      • Process ancestry (EDR/Sysmon EID 1, Event ID 4688): Look for processes running as NT AUTHORITY\SYSTEM whose parent ran at medium integrity in a user context. Pay particular attention to cmd.exe, powershell.exe or rundll32.exe spawned beneath unknown binaries in %TEMP%.
        • KQL: DeviceProcessEvents | where AccountSid == "S-1-5-18" and InitiatingProcessIntegrityLevel == "Medium"
      • Kernel crashes: System log Event ID 1001 (BugCheck) or 41 referencing clfs.sys. Failed exploit attempts often bluescreen the host. Repeated crashes on a single host are a strong indicator of an exploitation attempt.
      • Post-escalation follow-on activity:
        • LSASS access (Sysmon EID 10, TargetImage ending in lsass.exe, GrantedAccess 0x1010/0x1FFFFF)
        • Security services being stopped (Event ID 7036/7040)
        • vssadmin delete shadows (a classic precursor to ransomware encryption)

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

    Windows Common Log File System Driver Elevation of Privilege Vulnerability

    Source: CISA KEV Catalog, NVD (NIST), CVE.org (MITRE), cvelistv5, pwn.watch

    Vendors and products

    Affected operating systems

    Affected operating systems
    WindowsMicrosoftWindows 10 Version 150710.0.10240.19145
    WindowsMicrosoftWindows 10 Version 160710.0.14393.4825
    WindowsMicrosoftWindows 10 Version 180910.0.17763.2366
    WindowsMicrosoftWindows 10 Version 190910.0.18363.1977
    WindowsMicrosoftWindows 10 Version 200410.0.19041.1415
    WindowsMicrosoftWindows 10 Version 20H210.0.19042.1415
    WindowsMicrosoftWindows 10 Version 21H110.0.19043.1415
    WindowsMicrosoftWindows 10 Version 21H210.0.19044.1415
    WindowsMicrosoftWindows 11 version 21H210.0.22000.376
    WindowsMicrosoftWindows 76.1.7601.25796
    WindowsMicrosoftWindows 7 Service Pack 16.1.7601.25796
    WindowsMicrosoftWindows 8.16.3.9600.20207
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 16.1.7601.25796
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)6.1.7601.25796
    WindowsMicrosoftWindows Server 2008 Service Pack 26.0.6003.21309
    WindowsMicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)6.0.6003.21309
    WindowsMicrosoftWindows Server 20126.2.9200.23545
    WindowsMicrosoftWindows Server 2012 (Server Core installation)6.2.9200.23545
    WindowsMicrosoftWindows Server 2012 R26.3.9600.20207
    WindowsMicrosoftWindows Server 2012 R2 (Server Core installation)6.3.9600.20207
    WindowsMicrosoftWindows Server 201610.0.14393.4825
    WindowsMicrosoftWindows Server 2016 (Server Core installation)10.0.14393.4825
    WindowsMicrosoftWindows Server 201910.0.17763.2366
    WindowsMicrosoftWindows Server 2019 (Server Core installation)10.0.17763.2366

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited

    References & sources

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2021-43226","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2025-10-04T03:55:25.776895Z"}
      {"id":"CVE-2021-43226","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2021-12-16T00:00:00+00:00"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

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    <a href="https://intel.neosec.eu/en/cve/CVE-2021-43226"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-43226.svg?lang=en" alt="CVE-2021-43226 – NEOSEC Intel" height="20"></a>