CriticalCVE-2021-1675

    Windows Print Spooler Remote Code Execution Vulnerability (CVE-2021-1675)

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

    Windows Print Spooler Remote Code Execution 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 10.0.17763.1999, 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

    85.3 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.71

    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
    243.54 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
    85.3 %
    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
    99.71
    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 29 June 2021

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This bug yields SYSTEM-level code execution through a service that runs by default on nearly every Windows machine. Public exploits have circulated since mid-2021, are built into common offensive frameworks, and its exploitation likelihood sits in the very top tier. CISA flags known ransomware campaign use: operators leverage it post-initial-access for privilege escalation and lateral movement, with domain controllers as the preferred target. One unpatched DC with the Spooler running effectively means full domain compromise. Despite the formally local attack vector, treat it as critical in any Active Directory environment and remediate immediately in NIS2/KRITIS scope.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Domain controllers first: Stop and disable the Print Spooler on every DC and Tier-0 asset (ADCS, ADFS, backup servers): Stop-Service -Name Spooler -Force; Set-Service -Name Spooler -StartupType Disabled. DCs almost never need it.
      • Deploy the security update: Roll out the Windows cumulative update from June 2021 or later to all Windows 10 and Windows Server hosts. Builds up to and including 10.0.10240.18967, 10.0.14393.4467, 10.0.17763.1999, 10.0.18363.1621 and 10.0.19041/19042/19043.1052 are vulnerable. Look up the exact KB per build in the Microsoft advisory. Windows 7 is only covered under ESU — treat non-ESU machines as unpatched.
      • Verify patch level: Use your inventory tooling (SCCM/Intune/Defender TVM) to compare OS builds against the thresholds above. Track any lagging host on an exception list.
      • Hosts with no print requirement: Disable the Spooler via GPO (Computer Configuration → Windows Settings → Security Settings → System Services → Print Spooler = Disabled).
      • Cover the sibling bug: The June update alone does not fully close the closely related CVE-2021-34527 ("PrintNightmare"). Patch to the latest cumulative level, not just June 2021.

    2. Runbook step 2

      Mitigation layers

      • Configuration (inbound connections): Where the Spooler is needed locally, set the GPO "Allow Print Spooler to accept client connections" to Disabled (Administrative Templates → Printers). This blocks the remote attack path while local printing keeps working. Restart the service afterwards.
      • Configuration (Point and Print): Under HKLM\Software\Policies\Microsoft\Windows NT\Printers\PointAndPrint, ensure NoWarningNoElevationOnInstall=0 and UpdatePromptSettings=0 (or leave them absent). Also set RestrictDriverInstallationToAdministrators=1 so that only administrators can install printer drivers.
      • Network segmentation: Allow SMB (TCP 445), the RPC endpoint mapper (TCP 135) and dynamic RPC ports to workstations and DCs only from admin and print-server segments. Consolidate printing onto dedicated, hardened print servers.
      • Endpoint hardening: Use a WDAC or AppLocker DLL rule to block unsigned DLLs loading from C:\Windows\System32\spool\drivers\. Enable Defender for Endpoint ASR rules.
      • IAM / least privilege: Remove local admin rights and minimise the accounts allowed to install drivers. Exploitation typically starts from a low-privileged authenticated domain account.

    3. Runbook step 3

      Detection rules

      • PrintService logs: Monitor Microsoft-Windows-PrintService/Admin Event ID 808 (failed to load plug-in module) and Microsoft-Windows-PrintService/Operational Event ID 316 (printer driver added). The Operational log is off by default, so enable it via GPO.
      • Sysmon EID 11 / 7: Watch for spoolsv.exe writing *.dll files under C:\Windows\System32\spool\drivers\x64\3\ (including the Old\ subfolder), and for unsigned image loads from that path. Sigma shape: Image|endswith: '\spoolsv.exe' AND TargetFilename|contains: '\spool\drivers\' AND TargetFilename|endswith: '.dll'.
      • EDR process ancestry: spoolsv.exe spawning shells or LOLBins is a high-fidelity signal. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "spoolsv.exe" and FileName in~ ("cmd.exe","powershell.exe","rundll32.exe","regsvr32.exe").
      • Network telemetry: In Zeek dce_rpc.log, hunt for RpcAddPrinterDriverEx (MS-RPRN) and RpcAsyncAddPrinterDriver (MS-PAR) calls originating from non-admin hosts, especially when they target DCs. Correlate with SMB access to \\*\print$ or foreign UNC shares that reference DLL paths.
      • Windows Security log: Review Event ID 4688 and 4697/7045 (new service) shortly after a Spooler event. These point to persistence following SYSTEM-level code execution.

    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 Print Spooler Remote Code Execution Vulnerability

    Source: CISA KEV Catalog, cvelistv5, NVD (NIST)

    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

    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-014 – [MaJ] Multiples vulnérabilités dans Microsoft Windows (clôturée 2022-01-05) (opens in a new tab)cert.cyber.gouv.fractively exploited
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-013 – [MaJ] Vulnérabilité dans Microsoft Windows (clôturée 2021-07-02) (opens in a new tab)cert.cyber.gouv.fractively exploited
    Packet StormMicrosoft PrintNightmare Proof Of Concept (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormPrintNightmare Windows Spooler Service Remote Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    Packet StormPrint Spooler Remote DLL Injection (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—
    PoC-in-GitHub (nomi-sec)CVE-2021-1675 (opens in a new tab)github.comPoC (publicly reported)reported, not verified

    References & sources

    • portal.msrc.microsoft.comen-US/security-guidance/advisory/CVE-2021-1675x_refsource_MISC
    • kb.cert.orgVU#383432Third-party advisory, x_refsource_CERT-VN
    • packetstormsecurity.comfiles/163349/Microsoft-PrintNightmare-Proof-Of-Concept.htmlx_refsource_MISC
    • packetstormsecurity.comfiles/163351/PrintNightmare-Windows-Spooler-Service-Remote-Code…x_refsource_MISC
    • packetstormsecurity.comfiles/167261/Print-Spooler-Remote-DLL-Injection.htmlx_refsource_MISC
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-1675government-resource
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-013 – [MaJ] Vulnérabilité dans Microsoft Windows (clôturée 2021-07-02)cert-fr
    • cert.cyber.gouv.frANSSI / CERT-FR: CERTFR-2021-ALE-014 – [MaJ] Multiples vulnérabilités dans Microsoft Windows (clôturée 2022-01-05)cert-fr
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    <a href="https://intel.neosec.eu/en/cve/CVE-2021-1675"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-1675.svg?lang=en" alt="CVE-2021-1675 – NEOSEC Intel" height="20"></a>