CriticalCVE-2021-31956

    Windows NTFS Elevation of Privilege Vulnerability (CVE-2021-31956)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Windows NTFS Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1809 to version 10.0.17763.1999, Microsoft Windows Server 2019 to version 10.0.17763.1999, Microsoft Windows Server 2019 (Server Core installation) to version 10.0.17763.1999. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1809 lt 10.0.0..10.0.17763.1999
    • Microsoft Windows Server 2019 lt 10.0.0..10.0.17763.1999
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.0..10.0.17763.1999
    • Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1621
    • Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1052
    • Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1052
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19041.1052
    • Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1052
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19042.1052
    • Microsoft Windows 10 Version 1507 lt 10.0.0..10.0.10240.18967
    • Microsoft Windows 10 Version 1607 lt 10.0.0..10.0.14393.4467
    • Microsoft Windows Server 2016 lt 10.0.0..10.0.14393.4467
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.0..10.0.14393.4467
    • Microsoft Windows 7 lt 6.1.0..6.1.7601.25632
    • Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25632
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20045
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20044
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.0..6.0.6003.21137
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.0..6.0.6003.21137
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.0..6.1.7601.25632
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.0.0..6.1.7601.25632
    • Microsoft Windows Server 2012 lt 6.2.0..6.2.9200.23372
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.0..6.2.9200.23372
    • Microsoft Windows Server 2012 R2 lt 6.3.0..6.3.9600.20045
    • Microsoft Windows Server 2012 R2 lt 6.3.0..6.3.9600.20044
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.0..6.3.9600.20045
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.0..6.3.9600.20044
    Fixed in
    • Microsoft Windows 10 Version 1809 10.0.17763.1999
    • Microsoft Windows Server 2019 10.0.17763.1999
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.1999
    • Microsoft Windows 10 Version 1909 10.0.18363.1621
    • Microsoft Windows 10 Version 21H1 10.0.19043.1052
    • Microsoft Windows 10 Version 2004 10.0.19041.1052
    • Microsoft Windows Server version 2004 10.0.19041.1052
    • Microsoft Windows 10 Version 20H2 10.0.19042.1052
    • Microsoft Windows Server version 20H2 10.0.19042.1052
    • Microsoft Windows 10 Version 1507 10.0.10240.18967
    • Microsoft Windows 10 Version 1607 10.0.14393.4467
    • Microsoft Windows Server 2016 10.0.14393.4467
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4467
    • Microsoft Windows 7 6.1.7601.25632
    • Microsoft Windows 7 Service Pack 1 6.1.7601.25632
    • Microsoft Windows 8.1 6.3.9600.20045
    • Microsoft Windows 8.1 6.3.9600.20044
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21137
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21137
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25632
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25632
    • Microsoft Windows Server 2012 6.2.9200.23372
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23372
    • Microsoft Windows Server 2012 R2 6.3.9600.20045
    • Microsoft Windows Server 2012 R2 6.3.9600.20044
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20045
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20044

    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

    22.3 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 97.7

    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
    215.32 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 exploitedSource: ANSSI / CERT-FR How we classify
    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
    22.3 %
    Elevated — model estimates 10-50% 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
    97.7
    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 26 July 2024
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-191

    Response and mitigation

    Why act now?

    Prioritisation rationale

    CVE-2021-31956 is a heap overflow in the ntfs.sys kernel driver. It gives any low-privileged local user a reliable path to SYSTEM without further user interaction. It was used as a zero-day chained with a browser exploit to escape the sandbox and escalate privileges, and publicly documented exploitation techniques have since made it accessible to a broad range of attackers. Although it is not network-reachable on its own and CISA does not flag ransomware use, its exploitation likelihood is very high. Treat it as high priority on workstations, terminal servers and VDI estates where many users can run code. Any host still unpatched more than three years after release also points to a systemic patch-management gap worth escalating.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the June 2021 cumulative security update to all affected Windows systems and verify the resulting build. Minimum builds: Windows 10 1507 → 10.0.10240.18967, 1607 → 10.0.14393.4467, 1809 → 10.0.17763.1999, 1909 → 10.0.18363.1621, 2004 → 10.0.19041.1052, 20H2 → 10.0.19042.1052, 21H1 → 10.0.19043.1052, Windows 7 → 6.1.7601.25632. Per-edition KB IDs are listed in the Microsoft Security Update Guide entry for CVE-2021-31956.
      • Validate patch compliance from inventory by reading CurrentBuild + UBR under HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion via Intune, SCCM/MECM or Defender Vulnerability Management. Raise a ticket for every host below the minimum build.
      • Server editions (Windows Server 2008 R2 through 2019, including Server Core) are not listed in the supplied context. Confirm their exposure in the vendor advisory and patch them at the same priority. Start with RDS, Citrix and other multi-user hosts.
      • Windows 7 machines without an ESU subscription cannot receive this fix. Locate them and either disconnect them or move them into an isolated segment, then schedule their replacement.
      • Isolate via EDR any endpoint showing signs of prior code execution, such as a browser exploit or phishing payload. This bug is typically the second stage of an attack chain, used once the attacker already has a foothold.

    2. Runbook step 2

      Mitigation layers

      • Configuration: Microsoft lists no workaround, and NTFS cannot be disabled. Patching is the only real fix; everything below only reduces exploitability or limits the blast radius.
      • Application control: Run WDAC or AppLocker in enforce mode and block unsigned binaries from user-writable paths (%TEMP%, %APPDATA%, %LOCALAPPDATA%, Downloads, C:\Users\Public). Exploitation requires running a crafted local application.
      • Initial-access reduction: Keep Chrome and Edge current and never disable the browser sandbox. Enable the ASR rules "Block executable files unless they meet prevalence, age or trusted list criterion" and "Block Office applications from creating child processes".
      • IAM / least privilege: Remove standing local admin rights and manage local admin accounts with LAPS. Enable Credential Guard so that a SYSTEM-level escalation cannot easily yield reusable domain credentials from LSASS.
      • Host hardening: Restrict interactive and RDP logon to servers via GPO ("Allow log on locally / through Remote Desktop Services"). Enable HVCI/Memory Integrity; it raises the cost of some kernel exploitation techniques but is not a substitute for the patch.
      • Network segmentation: Move unpatchable legacy hosts into a dedicated VLAN that allows only the flows they strictly need. Block outbound SMB, RDP and WinRM from that VLAN to contain lateral movement after an escalation.

    3. Runbook step 3

      Detection rules

      • EDR/MDE – integrity jump: Look for SYSTEM-integrity processes spawned by a medium- or low-integrity, non-service parent. KQL: DeviceProcessEvents | where ProcessIntegrityLevel == "System" and InitiatingProcessIntegrityLevel in ("Medium","Low") and InitiatingProcessFileName !in~ ("services.exe","svchost.exe")
      • Sysmon EID 1 / Security 4688: Look for cmd.exe, powershell.exe or unknown binaries running as NT AUTHORITY\SYSTEM where ParentImage is a browser (chrome.exe, msedge.exe), an Office process, or a path under \Users\. Sigma shape: selection: User|contains: 'SYSTEM' AND ParentImage|contains: '\Users\'.
      • Kernel instability: Failed exploitation attempts often corrupt the kernel pool. Watch for System Event ID 1001 (BugCheck) and Event ID 41 (Kernel-Power) with bugcheck codes such as 0x19 BAD_POOL_HEADER or 0x13A KERNEL_MODE_HEAP_CORRUPTION. Correlate and investigate clusters of BSODs on a single host.
      • Post-escalation activity: Watch for Sysmon EID 10 (access to lsass.exe with GrantedAccess 0x1010/0x1FFFFF), Event 7045/4697 (new service) and Event 4720/4732 (new account or addition to local Administrators). Treat these as high-signal when they occur within minutes of a suspicious process launched from a user path.
      • API telemetry (where your EDR exposes it): Flag anomalous NtSetEaFile/NtQueryEaFile calls (NTFS extended attributes) from unsigned processes in user-writable locations. These calls are central to publicly documented exploitation of this bug.

    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 NTFS Elevation of Privilege Vulnerability

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

    Vendors and products

    Affected operating systems

    Affected operating systems
    WindowsMicrosoftWindows 10 Version 150710.0.10240.18967
    WindowsMicrosoftWindows 10 Version 160710.0.14393.4467
    WindowsMicrosoftWindows 10 Version 180910.0.17763.1999
    WindowsMicrosoftWindows 10 Version 190910.0.18363.1621
    WindowsMicrosoftWindows 10 Version 200410.0.19041.1052
    WindowsMicrosoftWindows 10 Version 20H210.0.19042.1052
    WindowsMicrosoftWindows 10 Version 21H110.0.19043.1052
    WindowsMicrosoftWindows 76.1.7601.25632
    WindowsMicrosoftWindows 7 Service Pack 16.1.7601.25632
    WindowsMicrosoftWindows 8.16.3.9600.20045
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 16.1.7601.25632
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)6.1.7601.25632
    WindowsMicrosoftWindows Server 2008 Service Pack 26.0.6003.21137
    WindowsMicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)6.0.6003.21137
    WindowsMicrosoftWindows Server 20126.2.9200.23372
    WindowsMicrosoftWindows Server 2012 (Server Core installation)6.2.9200.23372
    WindowsMicrosoftWindows Server 2012 R26.3.9600.20045
    WindowsMicrosoftWindows Server 2012 R2 (Server Core installation)6.3.9600.20045
    WindowsMicrosoftWindows Server 201610.0.14393.4467
    WindowsMicrosoftWindows Server 2016 (Server Core installation)10.0.14393.4467
    WindowsMicrosoftWindows Server 201910.0.17763.1999
    WindowsMicrosoftWindows Server 2019 (Server Core installation)10.0.17763.1999
    WindowsMicrosoftWindows Server version 200410.0.19041.1052
    WindowsMicrosoftWindows Server version 20H210.0.19042.1052

    Weakness classes (CWE)

    • CWE-191Integer Underflow (Wrap or Wraparound)

      Base

      The product subtracts one value from another, such that the result is less than the minimum allowable integer value, which produces a value that is not equal to the correct result.

      Entry at MITRE CWE-191

    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
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-012 – Multiples vulnérabilités dans Microsoft Windows (clôturée 2021-10-19) (opens in a new tab)cert.cyber.gouv.fractively exploited
    PoC-in-GitHub (nomi-sec)UAC-Elevation-CVE (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-31956x_refsource_MISC, Patch, Vendor advisory
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-31956government-resource, US government resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-012 – Multiples vulnérabilités dans Microsoft Windows (clôturée 2021-10-19)cert-fr
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    Badge preview for CVE-2021-31956
    <a href="https://intel.neosec.eu/en/cve/CVE-2021-31956"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-31956.svg?lang=en" alt="CVE-2021-31956 – NEOSEC Intel" height="20"></a>