CriticalCVE-2021-26894

    Windows DNS Server Remote Code Execution Vulnerability (CVE-2021-26894)

    Published Last modified Sources: CVE.org

    What is affected, what to do

    Windows DNS Server Remote Code Execution Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows Server 2008 R2 Service Pack 1 to version 6.1.7601.24566, Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) to version 6.1.7601.24566, Microsoft Windows Server 2008 Service Pack 2 to version 6.0.6003.21070.

    Affected
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.7601.0..6.1.7601.24566
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.1.7601.0..6.1.7601.24566
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.6003.0..6.0.6003.21070
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.6003.0..6.0.6003.21070
    • Microsoft Windows Server 2012 lt 6.2.9200.0..6.2.9200.23298
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.9200.0..6.2.9200.23298
    • Microsoft Windows Server 2012 R2 lt 6.3.9600.0..6.3.9600.19968
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.9600.0..6.3.9600.19968
    • Microsoft Windows Server 2016 lt 10.0.14393.0..10.0.14393.4283
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.14393.0..10.0.14393.4283
    • Microsoft Windows Server 2019 lt 10.0.17763.0..10.0.17763.1817
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.17763.0..10.0.17763.1817
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19043.867
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19043.867
    • Microsoft Windows Server, version 1909 (Server Core installation) lt 10.0.0..10.0.18363.1440
    Fixed in
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.24566
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.24566
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21070
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21070
    • Microsoft Windows Server 2012 6.2.9200.23298
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23298
    • Microsoft Windows Server 2012 R2 6.3.9600.19968
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.19968
    • Microsoft Windows Server 2016 10.0.14393.4283
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4283
    • Microsoft Windows Server 2019 10.0.17763.1817
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.1817
    • Microsoft Windows Server version 2004 10.0.19043.867
    • Microsoft Windows Server version 20H2 10.0.19043.867
    • Microsoft Windows Server, version 1909 (Server Core installation) 10.0.18363.1440

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

    Metrics

    9.8

    Priority

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

    Source: Vendor (CNA), CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    7.8 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 94.6

    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
    120.65 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
    9.8 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
    9.8
    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
    7.8 %
    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
    94.6
    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
    —
    Published

    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 DNS Server Remote Code Execution Vulnerability

    Source: cvelistv5, NVD (NIST)

    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 Server 2008 R2 Service Pack 1
    • 6.1.7601.0 – < 6.1.7601.24566
    • 6.1.7601.24566
    MicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)
    • 6.1.7601.0 – < 6.1.7601.24566
    • 6.1.7601.24566
    MicrosoftWindows Server 2008 Service Pack 2
    • 6.0.6003.0 – < 6.0.6003.21070
    • 6.0.6003.21070
    MicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)
    • 6.0.6003.0 – < 6.0.6003.21070
    • 6.0.6003.21070
    MicrosoftWindows Server 2012
    • 6.2.9200.0 – < 6.2.9200.23298
    • 6.2.9200.23298
    MicrosoftWindows Server 2012 (Server Core installation)
    • 6.2.9200.0 – < 6.2.9200.23298
    • 6.2.9200.23298
    MicrosoftWindows Server 2012 R2
    • 6.3.9600.0 – < 6.3.9600.19968
    • 6.3.9600.19968
    MicrosoftWindows Server 2012 R2 (Server Core installation)
    • 6.3.9600.0 – < 6.3.9600.19968
    • 6.3.9600.19968
    MicrosoftWindows Server 2016
    • 10.0.14393.0 – < 10.0.14393.4283
    • 10.0.14393.4283
    MicrosoftWindows Server 2016 (Server Core installation)
    • 10.0.14393.0 – < 10.0.14393.4283
    • 10.0.14393.4283
    MicrosoftWindows Server 2019
    • 10.0.17763.0 – < 10.0.17763.1817
    • 10.0.17763.1817
    MicrosoftWindows Server 2019 (Server Core installation)
    • 10.0.17763.0 – < 10.0.17763.1817
    • 10.0.17763.1817
    MicrosoftWindows Server version 2004
    • 10.0.0 – < 10.0.19043.867
    • 10.0.19043.867
    MicrosoftWindows Server version 20H2
    • 10.0.0 – < 10.0.19043.867
    • 10.0.19043.867
    MicrosoftWindows Server, version 1909 (Server Core installation)
    • 10.0.0 – < 10.0.18363.1440
    • 10.0.18363.1440

    Vendors and products

    Affected operating systems

    Affected operating systems
    Windowsmicrosoftwindows_server_2008r2
    Windowsmicrosoftwindows_server_2008sp2
    Windowsmicrosoftwindows_server_2012r2
    Windowsmicrosoftwindows_server_2012—
    Windowsmicrosoftwindows_server_20161909
    Windowsmicrosoftwindows_server_20162004
    Windowsmicrosoftwindows_server_201620h2
    Windowsmicrosoftwindows_server_2016—
    Windowsmicrosoftwindows_server_2019—

    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
    ANSSI / CERT-FRLicence Ouverte 2.0, as of CERTFR-2021-ALE-005 – Multiples vulnérabilités dans Microsoft DNS server (clôturée 2021-05-12) (opens in a new tab)cert.cyber.gouv.fractively 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 modifiedMicrosoft

      Show 4 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26894
      Referenceremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26894
      —
      Reference typeremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26894 Types: Patch, Vendor Advisory
      —
      Affected versions (publisher)changed
      Windows Server 2019, Windows Server 2019 (Server Core installation), Windows Server, version 1909 (Server Core installation) (+13)
      Windows Server 2008 R2 Service Pack 1, Windows Server 2008 R2 Service Pack 1 (Server Core installation), Windows Server 2008 Service Pack 2 (+12)

      Source identifier: secure@microsoft.com

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