CriticalCVE-2021-40449

    Win32k Elevation of Privilege Vulnerability (CVE-2021-40449)

    Published In CISA KEV since Sources: SingCERT, CVE.org

    What is affected, what to do

    Win32k 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 10.0.17763.2237, Microsoft Windows Server 2019 to version 10.0.17763.2237, Microsoft Windows Server 2019 (Server Core installation) to version 10.0.17763.2237. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1809 lt 10.0.0..10.0.17763.2237
    • Microsoft Windows Server 2019 lt 10.0.0..10.0.17763.2237
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.0..10.0.17763.2237
    • Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1854
    • Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19041.1288
    • Microsoft Windows Server 2022 lt 10.0.0..10.0.20348.288
    • Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1288
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19041.1288
    • Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19041.1288
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19042.1288
    • Microsoft Windows 11 version 21H2 lt 10.0.0..10.0.22000.258
    • Microsoft Windows 10 Version 1507 lt 10.0.0..10.0.10240.19086
    • Microsoft Windows 10 Version 1607 lt 10.0.0..10.0.14393.4704
    • Microsoft Windows Server 2016 lt 10.0.0..10.0.14393.4704
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.0..10.0.14393.4704
    • Microsoft Windows 7 lt 6.1.0..6.1.7601.25740
    • Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25740
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20144
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.0..6.0.6003.21251
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.0..6.0.6003.21251
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.0..6.1.7601.25740
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.0.0..6.1.7601.25740
    • Microsoft Windows Server 2012 lt 6.2.0..6.2.9200.23490
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.0..6.2.9200.23490
    • Microsoft Windows Server 2012 R2 lt 6.3.0..6.3.9600.20144
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.0..6.3.9600.20144
    Fixed in
    • Microsoft Windows 10 Version 1809 10.0.17763.2237
    • Microsoft Windows Server 2019 10.0.17763.2237
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.2237
    • Microsoft Windows 10 Version 1909 10.0.18363.1854
    • Microsoft Windows 10 Version 21H1 10.0.19041.1288
    • Microsoft Windows Server 2022 10.0.20348.288
    • Microsoft Windows 10 Version 2004 10.0.19041.1288
    • Microsoft Windows Server version 2004 10.0.19041.1288
    • Microsoft Windows 10 Version 20H2 10.0.19041.1288
    • Microsoft Windows Server version 20H2 10.0.19042.1288
    • Microsoft Windows 11 version 21H2 10.0.22000.258
    • Microsoft Windows 10 Version 1507 10.0.10240.19086
    • Microsoft Windows 10 Version 1607 10.0.14393.4704
    • Microsoft Windows Server 2016 10.0.14393.4704
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4704
    • Microsoft Windows 7 6.1.7601.25740
    • Microsoft Windows 7 Service Pack 1 6.1.7601.25740
    • Microsoft Windows 8.1 6.3.9600.20144
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21251
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21251
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25740
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25740
    • Microsoft Windows Server 2012 6.2.9200.23490
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23490
    • Microsoft Windows Server 2012 R2 6.3.9600.20144
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20144

    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: SingCERT, CVE.org

    81.9 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.65

    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
    243.44 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: CSA/SingCERT 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
    81.9 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    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
    99.65
    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 7 February 2025
    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-416

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This is a low-complexity local privilege escalation. It needs only a low-privileged account, no user interaction, and yields full compromise of confidentiality, integrity and availability on the host. Attackers chain it after phishing or webshell access to reach SYSTEM, blind EDR and harvest credentials. Exploitation likelihood is very high and ransomware use is confirmed. The fix has been available since October 2021, so any remaining unpatched host signals a patch-management failure. Under NIS2, treat such hosts as critical findings, with terminal servers and admin workstations first.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the October 2021 Patch Tuesday cumulative update, or any later cumulative update, to all Windows clients. Fixed builds start at 10.0.10240.19086 (1507), 10.0.14393.4704 (1607), 10.0.17763.2237 (1809), 10.0.18363.1854 (1909) and 10.0.19041.1288 (2004/20H2/21H1). Update Windows 11 as well. Look up the exact KB per build in the MSRC advisory for CVE-2021-40449, and check Windows Server SKUs there too.
      • Build a patch-gap inventory. Use MECM, Intune or WSUS reporting, or Get-ComputerInfo | select OsBuildNumber, to list endpoints below those builds. Start with terminal servers, VDI and jump hosts that carry many interactive users.
      • Handle multi-user hosts first. On Citrix, RDS and AVD hosts, a local privilege escalation exposes every logged-on session and every cached credential. Drain unpatched hosts from the pool until they are updated.
      • Assume escalation on hosts with initial-access alerts. For any endpoint already flagged for a phishing loader, webshell or RAT, assume SYSTEM was reached. Isolate the host, rotate local and cached domain credentials, and reset local admin passwords via LAPS.
      • Account for the ransomware link. CISA flags known ransomware campaign use. Verify offline backups and have the recovery runbook ready.

    2. Runbook step 2

      Mitigation layers

      • Endpoint hardening / application control: Enforce WDAC or AppLocker and block unsigned EXE/DLL execution from %TEMP%, %APPDATA%, %PUBLIC% and Downloads. The exploit ships as a dropped binary and needs user-level code execution to run.
      • EDR protections: Enable ASR rules, especially "Block executable files unless they meet a prevalence, age, or trusted list criterion" and "Block credential stealing from LSASS". Turn on Tamper Protection and enforce Exploit Protection (CFG, ACG) on exposed processes.
      • IAM / least privilege: Remove standing local admin rights and enforce admin tiering. Apply Credential Guard and the Protected Users group to privileged accounts. This limits the blast radius after a SYSTEM escalation: credential theft and lateral movement.
      • Network segmentation: Block client-to-client SMB (445/tcp), RPC (135/tcp), WinRM (5985/5986/tcp) and RDP (3389/tcp) through host firewall policy or private VLANs, so an escalated workstation cannot be used as a pivot.
      • Restrict printer functionality: The bug is a use-after-free in NtGdiResetDC, triggered through a user-mode printer-driver callback. Restrict printer driver installation to administrators via Point-and-Print GPO. This is attack-surface reduction, not a substitute for the patch.

    3. Runbook step 3

      Detection rules

      • Sysmon EID 1 / Security 4688 — integrity jump: Flag a child process running as SYSTEM whose parent runs at Medium integrity under a standard user and is not a service, installer or updater. KQL: DeviceProcessEvents | where ProcessIntegrityLevel=="System" and InitiatingProcessIntegrityLevel=="Medium" and InitiatingProcessAccountName !in~ ("system","local service","network service")
      • EDR process ancestry: Flag a freshly written binary in a user-writable path that spawns cmd.exe, powershell.exe or rundll32.exe as SYSTEM. Sigma shape: ParentImage|contains: '\AppData\' OR '\Temp\' AND User|contains: 'SYSTEM'.
      • Token manipulation and kernel instability: Watch EDR alerts for token theft or token swapping ("Suspicious token manipulation", "Kernel exploit"). Also watch for crashes in win32kbase.sys/win32kfull.sys, visible as System log Event 1001 (BugCheck) and WER entries. Failed exploit attempts often cause blue screens.
      • Post-escalation activity: Security 4720/4732 (local account created or added to Administrators), 7045/4697 (new service), Sysmon EID 10 with TargetImage ending in lsass.exe and GrantedAccess 0x1010/0x1FFFFF, and shadow-copy deletion (vssadmin delete shadows, wmic shadowcopy delete).
      • Correlation: Raise any initial-access alert followed by an integrity jump on the same host within 60 minutes to top severity.

    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

    Win32k Elevation of Privilege Vulnerability

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

    Vendors and products

    Affected operating systems

    Affected operating systems
    WindowsMicrosoftWindows 10 Version 150710.0.10240.19086
    WindowsMicrosoftWindows 10 Version 160710.0.14393.4704
    WindowsMicrosoftWindows 10 Version 180910.0.17763.2237
    WindowsMicrosoftWindows 10 Version 190910.0.18363.1854
    WindowsMicrosoftWindows 10 Version 200410.0.19041.1288
    WindowsMicrosoftWindows 10 Version 20H210.0.19041.1288
    WindowsMicrosoftWindows 10 Version 21H110.0.19041.1288
    WindowsMicrosoftWindows 11 version 21H210.0.22000.258
    WindowsMicrosoftWindows 76.1.7601.25740
    WindowsMicrosoftWindows 7 Service Pack 16.1.7601.25740
    WindowsMicrosoftWindows 8.16.3.9600.20144
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 16.1.7601.25740
    WindowsMicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)6.1.7601.25740
    WindowsMicrosoftWindows Server 2008 Service Pack 26.0.6003.21251
    WindowsMicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)6.0.6003.21251
    WindowsMicrosoftWindows Server 20126.2.9200.23490
    WindowsMicrosoftWindows Server 2012 (Server Core installation)6.2.9200.23490
    WindowsMicrosoftWindows Server 2012 R26.3.9600.20144
    WindowsMicrosoftWindows Server 2012 R2 (Server Core installation)6.3.9600.20144
    WindowsMicrosoftWindows Server 201610.0.14393.4704
    WindowsMicrosoftWindows Server 2016 (Server Core installation)10.0.14393.4704
    WindowsMicrosoftWindows Server 201910.0.17763.2237
    WindowsMicrosoftWindows Server 2019 (Server Core installation)10.0.17763.2237
    WindowsMicrosoftWindows Server 202210.0.20348.288

    Weakness classes (CWE)

    • CWE-416Use After Free

      Variant

      The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

      Entry at MITRE CWE-416

    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
    CSA/SingCERTOctober 2021 Monthly Patch Release (opens in a new tab)CSA/SingCERTactively exploited
    Packet StormWin32k NtGdiResetDC Use After Free Local Privilege Escalation (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-40449x_refsource_MISC, Patch, Vendor advisory
    • packetstormsecurity.comfiles/164926/Win32k-NtGdiResetDC-Use-After-Free-Local-Privilege…x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40449government-resource, US government resource
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    <a href="https://intel.neosec.eu/en/cve/CVE-2021-40449"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-40449.svg?lang=en" alt="CVE-2021-40449 – NEOSEC Intel" height="20"></a>