CriticalCVE-2021-36955

    Windows Common Log File System Driver Elevation of Privilege Vulnerability (CVE-2021-36955)

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

    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 1507 to version 10.0.10240.19060, Microsoft Windows 10 Version 1607 to version 10.0.14393.4651, Microsoft Windows 10 Version 1809 to version 10.0.17763.2183. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1507 lt 10.0.10240.0..10.0.10240.19060
    • Microsoft Windows 10 Version 1607 lt 10.0.14393.0..10.0.14393.4651
    • Microsoft Windows 10 Version 1809 lt 10.0.17763.0..10.0.17763.2183
    • Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1801
    • Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1237
    • Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1237
    • Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1237
    • Microsoft Windows 7 lt 6.1.0..6.1.7601.25712
    • Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25712
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20120
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.7601.0..6.1.7601.25712
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.1.7601.0..6.1.7601.25712
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.6003.0..6.0.6003.21218
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.6003.0..6.0.6003.21218
    • Microsoft Windows Server 2012 lt 6.2.9200.0..6.2.9200.23462
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.9200.0..6.2.9200.23462
    • Microsoft Windows Server 2012 R2 lt 6.3.9600.0..6.3.9600.20120
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.9600.0..6.3.9600.20120
    • Microsoft Windows Server 2016 lt 10.0.14393.0..10.0.14393.4651
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.14393.0..10.0.14393.4651
    • Microsoft Windows Server 2019 lt 10.0.17763.0..10.0.17763.2183
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.17763.0..10.0.17763.2183
    • Microsoft Windows Server 2022 lt 10.0.20348.0..10.0.20348.230
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19041.1237
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19042.1237
    Fixed in
    • Microsoft Windows 10 Version 1507 10.0.10240.19060
    • Microsoft Windows 10 Version 1607 10.0.14393.4651
    • Microsoft Windows 10 Version 1809 10.0.17763.2183
    • Microsoft Windows 10 Version 1909 10.0.18363.1801
    • Microsoft Windows 10 Version 2004 10.0.19041.1237
    • Microsoft Windows 10 Version 20H2 10.0.19042.1237
    • Microsoft Windows 10 Version 21H1 10.0.19043.1237
    • Microsoft Windows 7 6.1.7601.25712
    • Microsoft Windows 7 Service Pack 1 6.1.7601.25712
    • Microsoft Windows 8.1 6.3.9600.20120
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25712
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25712
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21218
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21218
    • Microsoft Windows Server 2012 6.2.9200.23462
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23462
    • Microsoft Windows Server 2012 R2 6.3.9600.20120
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20120
    • Microsoft Windows Server 2016 10.0.14393.4651
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4651
    • Microsoft Windows Server 2019 10.0.17763.2183
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.2183
    • Microsoft Windows Server 2022 10.0.20348.230
    • Microsoft Windows Server version 2004 10.0.19041.1237
    • Microsoft Windows Server version 20H2 10.0.19042.1237

    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

    4.1 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 90.6

    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
    229.72 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
    4.1 %
    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
    90.6
    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 4 February 2025

    Response and mitigation

    Why act now?

    Prioritisation rationale

    CVE-2021-36955 is a low-complexity local privilege escalation. It needs only low privileges and no user interaction, and it gives the attacker full control of the host. CISA flags it for known ransomware campaign use. Operators typically chain it after initial access (phishing, stolen VPN or RDP credentials) to reach SYSTEM, harvest credentials and disable defences ahead of encryption. Its exploitation likelihood is notably high for a local-only bug, and CLFS is a repeatedly targeted component. The fix has been available for years, so any unpatched host is a high-priority finding. Multi-user systems and Windows 7 without ESU in particular should be treated as a material vulnerability-management gap under NIS2.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the September 2021 Patch Tuesday security update, or any later cumulative update. Any host on a current cumulative update is already fixed. Look up the per-build KB numbers in the MSRC advisory for CVE-2021-36955. Treat builds below 10.0.10240.19060 (1507), 10.0.14393.4651 (1607), 10.0.17763.2183 (1809), 10.0.18363.1801 (1909) and 10.0.19041/19042/19043.1237 (2004/20H2/21H1) as vulnerable.
      • Run a patch-compliance query across the full client and server estate (Intune, ConfigMgr/WSUS, Defender Vulnerability Management). Prioritise RDS/VDI hosts, jump hosts and build servers, since these concentrate many interactive low-privilege users. For Server SKUs, check the vendor advisory's affected-products list.
      • Locate remaining Windows 7 machines. They only get the fix with Extended Security Updates (ESU). Without ESU, disconnect them or move them into a quarantine VLAN with no domain trust.
      • Treat any host with confirmed exploitation as fully compromised. The bug yields SYSTEM. Rotate the local admin password (LAPS), reset cached domain credentials and invalidate Kerberos tickets valid on that host.
      • On an EDR privilege-escalation alert on an unpatched host: network-isolate the host via EDR and capture memory and disk before any reboot.

    2. Runbook step 2

      Mitigation layers

      • No vendor workaround: Microsoft lists no configuration-based mitigation for CVE-2021-36955. Disabling clfs.sys is not viable because OS components depend on it. Compensating controls must therefore stop attackers from running low-privilege code in the first place.
      • Application control (WDAC/AppLocker): block unsigned executables and scripts from user-writable paths such as %TEMP%, %APPDATA%, C:\Users\Public and C:\ProgramData. CLFS exploits are usually dropped as standalone EXEs or DLLs into exactly these locations.
      • Endpoint hardening: enable Defender ASR rules, notably "Block executable files unless they meet prevalence, age or trusted list criterion" and "Block process creations originating from PSExec and WMI commands". Enforce EDR tamper protection, because attackers routinely kill security tooling once they hold SYSTEM.
      • IAM and least privilege: restrict interactive logon on servers to the appropriate admin tier via GPO (SeInteractiveLogonRight / SeRemoteInteractiveLogonRight). Set "Deny log on locally" for service accounts. Enforce tiering so that a local escalation does not expose Domain Admin credentials.
      • Credential protection: enable Credential Guard and LSA Protection (RunAsPPL) to make post-escalation LSASS dumping harder.
      • Network segmentation: ring-fence unpatched legacy hosts. Block client-to-client SMB/RDP/WinRM (445, 3389, 5985/5986) to limit lateral movement after escalation.

    3. Runbook step 3

      Detection rules

      • Sysmon EID 11 / EDR file events: creation of .blf or CLFS container files by non-system processes in user-writable paths. Sigma shape: TargetFilename|endswith: '.blf' AND TargetFilename|contains: ['\Users\Public\', '\AppData\Local\Temp\', '\ProgramData\'] AND Image|not startswith: 'C:\Windows\'.
      • Integrity-level jump: a process running at System integrity with a Medium/Low-integrity parent that is not services.exe, wininit.exe or a known installer, which indicates token theft. KQL: DeviceProcessEvents | where ProcessIntegrityLevel == "System" and InitiatingProcessIntegrityLevel in ("Medium","Low") and InitiatingProcessFolderPath !startswith @"C:\Windows\"
      • Security log 4688 / Sysmon EID 1: cmd.exe or powershell.exe running as NT AUTHORITY\SYSTEM whose parent image lives in %TEMP% or %APPDATA%. Process-creation auditing with command line must be enabled for this.
      • Crash telemetry: Event ID 1001 (BugCheck) or WER reports referencing clfs.sys point to failed exploitation attempts. Investigate repeated occurrences on the same host.
      • Post-escalation activity: Sysmon EID 10 targeting lsass.exe (GrantedAccess 0x1010/0x1410), Event 7045 (new service installed), and vssadmin delete shadows or wbadmin delete catalog, which are classic ransomware pre-encryption steps.

    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), cvelistv5, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    MicrosoftWindows 10 Version 1507
    • 10.0.10240.0 – < 10.0.10240.19060
    • 10.0.10240.19060
    MicrosoftWindows 10 Version 1607
    • 10.0.14393.0 – < 10.0.14393.4651
    • 10.0.14393.4651
    MicrosoftWindows 10 Version 1809
    • 10.0.17763.0 – < 10.0.17763.2183
    • 10.0.17763.2183
    MicrosoftWindows 10 Version 1909
    • 10.0.0 – < 10.0.18363.1801
    • 10.0.18363.1801
    MicrosoftWindows 10 Version 2004
    • 10.0.0 – < 10.0.19041.1237
    • 10.0.19041.1237
    MicrosoftWindows 10 Version 20H2
    • 10.0.0 – < 10.0.19042.1237
    • 10.0.19042.1237
    MicrosoftWindows 10 Version 21H1
    • 10.0.0 – < 10.0.19043.1237
    • 10.0.19043.1237
    MicrosoftWindows 7
    • 6.1.0 – < 6.1.7601.25712
    • 6.1.7601.25712
    MicrosoftWindows 7 Service Pack 1
    • 6.1.0 – < 6.1.7601.25712
    • 6.1.7601.25712
    MicrosoftWindows 8.1
    • 6.3.0 – < 6.3.9600.20120
    • 6.3.9600.20120
    MicrosoftWindows Server 2008 R2 Service Pack 1
    • 6.1.7601.0 – < 6.1.7601.25712
    • 6.1.7601.25712
    MicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)
    • 6.1.7601.0 – < 6.1.7601.25712
    • 6.1.7601.25712
    MicrosoftWindows Server 2008 Service Pack 2
    • 6.0.6003.0 – < 6.0.6003.21218
    • 6.0.6003.21218
    MicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)
    • 6.0.6003.0 – < 6.0.6003.21218
    • 6.0.6003.21218
    MicrosoftWindows Server 2012
    • 6.2.9200.0 – < 6.2.9200.23462
    • 6.2.9200.23462
    MicrosoftWindows Server 2012 (Server Core installation)
    • 6.2.9200.0 – < 6.2.9200.23462
    • 6.2.9200.23462
    MicrosoftWindows Server 2012 R2
    • 6.3.9600.0 – < 6.3.9600.20120
    • 6.3.9600.20120
    MicrosoftWindows Server 2012 R2 (Server Core installation)
    • 6.3.9600.0 – < 6.3.9600.20120
    • 6.3.9600.20120
    MicrosoftWindows Server 2016
    • 10.0.14393.0 – < 10.0.14393.4651
    • 10.0.14393.4651
    MicrosoftWindows Server 2016 (Server Core installation)
    • 10.0.14393.0 – < 10.0.14393.4651
    • 10.0.14393.4651
    MicrosoftWindows Server 2019
    • 10.0.17763.0 – < 10.0.17763.2183
    • 10.0.17763.2183
    MicrosoftWindows Server 2019 (Server Core installation)
    • 10.0.17763.0 – < 10.0.17763.2183
    • 10.0.17763.2183
    MicrosoftWindows Server 2022
    • 10.0.20348.0 – < 10.0.20348.230
    • 10.0.20348.230
    MicrosoftWindows Server version 2004
    • 10.0.0 – < 10.0.19041.1237
    • 10.0.19041.1237
    MicrosoftWindows Server version 20H2
    • 10.0.0 – < 10.0.19042.1237
    • 10.0.19042.1237

    Vendors and products

    Affected operating systems

    Affected operating systems
    Windowsmicrosoftwindows_10_150710.0.10240.19060
    Windowsmicrosoftwindows_10_1507—
    Windowsmicrosoftwindows_10_160710.0.14393.4651
    Windowsmicrosoftwindows_10_1607—
    Windowsmicrosoftwindows_10_180910.0.17763.2183
    Windowsmicrosoftwindows_10_180910.0.18363.1801
    Windowsmicrosoftwindows_10_180910.0.19041.1237
    Windowsmicrosoftwindows_10_1809—
    Windowsmicrosoftwindows_10_190910.0.18363.1801
    Windowsmicrosoftwindows_10_1909—
    Windowsmicrosoftwindows_10_2004—
    Windowsmicrosoftwindows_10_20H210.0.19042.1237
    Windowsmicrosoftwindows_10_20h2—
    Windowsmicrosoftwindows_10_21H110.0.19043.1237
    Windowsmicrosoftwindows_10_21h1—
    Windowsmicrosoftwindows_76.1.7601.25712
    Windowsmicrosoftwindows_7—
    Windowsmicrosoftwindows_8.16.3.9600.20120
    Windowsmicrosoftwindows_8.1—
    Windowsmicrosoftwindows_rt_8.16.3.9600.20120
    Windowsmicrosoftwindows_rt_8.1—
    Windowsmicrosoftwindows_server_200410.0.19041.1237
    Windowsmicrosoftwindows_server_2004—
    Windowsmicrosoftwindows_server_2008r2

    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

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-36955x_refsource_MISC, x_transferred
    • Microsoft Security Update GuideWindows Common Log File System Driver Elevation of Privilege VulnerabilityVendor advisory, Patch
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-36955government-resource

    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. NVD analysis modifiedNVD

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Reference typeadded
      —
      Microsoft Corporation: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36955 Types: Patch, Vendor Advisory

      Source identifier: nvd@nist.gov

    2. Record modifiedMicrosoft

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36955
      Referenceremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36955
      —
      Reference typeremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36955 Types: Patch, Vendor Advisory
      —
      Affected versions (publisher)changed
      Windows 10 Version 1809, Windows Server 2019, Windows Server 2019 (Server Core installation) (+23)
      Windows 10 Version 1507, Windows 10 Version 1607, Windows 10 Version 1809 (+23)

      Source identifier: secure@microsoft.com

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