CriticalCVE-2021-42287

    Active Directory Domain Services Elevation of Privilege Vulnerability (CVE-2021-42287)

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

    What is affected, what to do

    Active Directory Domain Services 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 Server 2008 R2 Service Pack 1 to version 6.1.7601.25769, Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) to version 6.1.7601.25769, Microsoft Windows Server 2008 Service Pack 2 to version 6.0.6003.21282. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.7601.0..6.1.7601.25769
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.1.7601.0..6.1.7601.25769
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.6003.0..6.0.6003.21282
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.6003.0..6.0.6003.21282
    • Microsoft Windows Server 2012 lt 6.2.9200.0..6.2.9200.23517
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.9200.0..6.2.9200.23517
    • Microsoft Windows Server 2012 R2 lt 6.3.9600.0..6.3.9600.20174
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.9600.0..6.3.9600.20174
    • Microsoft Windows Server 2016 lt 10.0.14393.0..10.0.14393.4770
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.14393.0..10.0.14393.4770
    • Microsoft Windows Server 2019 lt 10.0.17763.0..10.0.17763.2300
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.17763.0..10.0.17763.2300
    • Microsoft Windows Server 2022 lt 10.0.20348.0..10.0.20348.350
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19041.1348
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19041.1348
    Fixed in
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25769
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25769
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21282
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21282
    • Microsoft Windows Server 2012 6.2.9200.23517
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23517
    • Microsoft Windows Server 2012 R2 6.3.9600.20174
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20174
    • Microsoft Windows Server 2016 10.0.14393.4770
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4770
    • Microsoft Windows Server 2019 10.0.17763.2300
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.2300
    • Microsoft Windows Server 2022 10.0.20348.350
    • Microsoft Windows Server version 2004 10.0.19041.1348
    • Microsoft Windows Server version 20H2 10.0.19041.1348

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

    Metrics

    7.5

    Priority

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

    Source: NVD, CVSS v3

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: CVE.org

    77.2 %

    EPSS score

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

    Source: FIRST EPSS, percentile 99.55

    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
    241.78 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
    7.5 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
    7.5
    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
    77.2 %
    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
    99.55
    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 19 August 2025

    Response and mitigation

    Why act now?

    Prioritisation rationale

    Chained with CVE-2021-42278, this flaw lets any authenticated domain user with default rights take over the whole domain in minutes. Public tooling automates every step, so the "high complexity" in the vector reflects only the chaining requirement and offers no real protection. CISA flags known ransomware campaign use: operators use it after initial access via phishing or VPN to escalate quickly before deploying encryptors, and exploitation likelihood is very high. Any AD estate with even one unpatched DC, or a MachineAccountQuota still at the default, should treat this as critical with a 24-hour remediation target. For NIS2 entities, a retro-hunt hit is likely a reportable incident.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Patch every domain controller (DC): Microsoft's November 2021 security updates fix CVE-2021-42287 (KDC PAC validation). In practice it is exploited together with CVE-2021-42278 (sAMAccountName spoofing), known as "noPac". Look up the exact KB per server version in the Microsoft advisory. Every DC, RODCs included, must be updated, because one unpatched DC keeps the path open.
      • Verify builds against inventory: Reference builds from the advisory data are Server 2016 at 10.0.14393.4770 or later, Server 2019 at 10.0.17763.2300 or later, and Server 2022 at 10.0.20348.350 or later. Sweep with Get-ADDomainController -Filter * | % { Get-HotFix -ComputerName $_.HostName }.
      • Set ms-DS-MachineAccountQuota to 0: This stops standard users from creating computer accounts and breaks the common attack chain immediately: Set-ADDomain -Identity <domain> -Replace @{"ms-DS-MachineAccountQuota"="0"}.
      • Enforce PAC requestor validation: Set HKLM\SYSTEM\CurrentControlSet\Services\Kdc\PacRequestorEnforcement to 2, or confirm that later cumulative updates have made enforcement mandatory. Make sure nobody has set it back to 0.
      • Retro-hunt for compromise: Search DC Security logs back to at least November 2021 for the rename patterns below. A hit means you should treat the domain as compromised: reset krbtgt twice, rotate privileged credentials and invoke the AD incident-response plan.

    2. Runbook step 2

      Mitigation layers

      • IAM / least privilege: Use GPO to remove "Add workstations to domain" (SeMachineAccountPrivilege) from Authenticated Users and grant it only to a dedicated domain-join group. Audit write access to sAMAccountName/servicePrincipalName on computer objects with BloodHound or ADACLScanner.
      • Network segmentation: Allow Kerberos (88/TCP+UDP), LDAP/LDAPS (389, 636, 3268/3269), SMB (445) and RPC (135 plus the dynamic range) to DCs only from sanctioned client and server subnets. Guest, OT and DMZ segments get no direct DC access.
      • Tiering: Enforce Tier 0: administer DCs only from PAWs. Put privileged accounts in Protected Users and flag them "sensitive and cannot be delegated" to blunt S4U2self abuse.
      • IPS/NDR: Alert on a Kerberos AS-REQ for a principal without $ that matches a DC name, followed by an S4U2self TGS-REQ (Zeek kerberos.log, Suricata). Microsoft Defender for Identity flags this natively as "Suspected SAM name spoofing".
      • Configuration hygiene: Find computer objects created by non-admins (populated mS-DS-CreatorSID) and remove any that are not authorised.

    3. Runbook step 3

      Detection rules

      • Event ID 4781 (account name changed): Flag a computer account renamed to a name without a trailing $, especially one matching a DC hostname. KQL: SecurityEvent | where EventID == 4781 and OldTargetUserName endswith "$" and NewTargetUserName !endswith "$"
      • Kerberos sequence on DCs: 4741 (computer created by a non-admin), then 4781, then 4768 (TGT for the DC name without $), then 4781 again (rename back), then 4769 (S4U2self service ticket), all within 10 minutes. Build this as a Sigma correlation rule.
      • Post-patch KDC events (System log, source Microsoft-Windows-Kerberos-Key-Distribution-Center): IDs 35, 36, 37 and 38 (PAC missing or requestor mismatch) indicate attempted exploitation or legacy clients. Directory-Services-SAM 16990/16991 show blocked sAMAccountName changes.
      • EDR / process telemetry: Watch for noPac.exe, Rubeus.exe s4u /self, Python running Impacket (addcomputer.py, renameMachine.py, getST.py -self -impersonate) and Powermad's Set-MachineAccountAttribute. Sources are Sysmon EID 1 and PowerShell 4104.
      • Follow-on activity: DCSync, seen as Event ID 4662 with the replication GUIDs 1131f6aa-…/1131f6ad-… from a non-DC source, shortly after any of the chains above.

    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

    Active Directory Domain Services Elevation of Privilege Vulnerability

    Source: cvelistv5, CISA KEV Catalog, NVD (NIST), 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 Server 2008 R2 Service Pack 1
    • 6.1.7601.0 – < 6.1.7601.25769
    • 6.1.7601.25769
    MicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)
    • 6.1.7601.0 – < 6.1.7601.25769
    • 6.1.7601.25769
    MicrosoftWindows Server 2008 Service Pack 2
    • 6.0.6003.0 – < 6.0.6003.21282
    • 6.0.6003.21282
    MicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)
    • 6.0.6003.0 – < 6.0.6003.21282
    • 6.0.6003.21282
    MicrosoftWindows Server 2012
    • 6.2.9200.0 – < 6.2.9200.23517
    • 6.2.9200.23517
    MicrosoftWindows Server 2012 (Server Core installation)
    • 6.2.9200.0 – < 6.2.9200.23517
    • 6.2.9200.23517
    MicrosoftWindows Server 2012 R2
    • 6.3.9600.0 – < 6.3.9600.20174
    • 6.3.9600.20174
    MicrosoftWindows Server 2012 R2 (Server Core installation)
    • 6.3.9600.0 – < 6.3.9600.20174
    • 6.3.9600.20174
    MicrosoftWindows Server 2016
    • 10.0.14393.0 – < 10.0.14393.4770
    • 10.0.14393.4770
    MicrosoftWindows Server 2016 (Server Core installation)
    • 10.0.14393.0 – < 10.0.14393.4770
    • 10.0.14393.4770
    MicrosoftWindows Server 2019
    • 10.0.17763.0 – < 10.0.17763.2300
    • 10.0.17763.2300
    MicrosoftWindows Server 2019 (Server Core installation)
    • 10.0.17763.0 – < 10.0.17763.2300
    • 10.0.17763.2300
    MicrosoftWindows Server 2022
    • 10.0.20348.0 – < 10.0.20348.350
    • 10.0.20348.350
    MicrosoftWindows Server version 2004
    • 10.0.0 – < 10.0.19041.1348
    • 10.0.19041.1348
    MicrosoftWindows Server version 20H2
    • 10.0.0 – < 10.0.19041.1348
    • 10.0.19041.1348

    Vendors and products

    Affected operating systems

    Affected operating systems
    Windowsmicrosoftwindows_server_200410.0.19041.1348
    Windowsmicrosoftwindows_server_2004—
    Windowsmicrosoftwindows_server_2008r2
    Windowsmicrosoftwindows_server_2008—
    Windowsmicrosoftwindows_server_2008_R26.1.7601.25769
    Windowsmicrosoftwindows_server_2008_sp26.0.6003.21282
    Windowsmicrosoftwindows_server_20126.2.9200.23517
    Windowsmicrosoftwindows_server_2012r2
    Windowsmicrosoftwindows_server_2012—
    Windowsmicrosoftwindows_server_2012_R26.3.9600.20174
    Windowsmicrosoftwindows_server_201610.0.14393.4770
    Windowsmicrosoftwindows_server_20162004
    Windowsmicrosoftwindows_server_2016—
    Windowsmicrosoftwindows_server_201910.0.17763.2300
    Windowsmicrosoftwindows_server_2019—
    Windowsmicrosoftwindows_server_202210.0.20348.350
    Windowsmicrosoftwindows_server_2022—
    Windowsmicrosoftwindows_server_20H210.0.19041.1348
    Windowsmicrosoftwindows_server_20h2—

    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

    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-2021-42287
      Referenceremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42287
      —
      Reference typeremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-42287 Types: Patch, Vendor Advisory
      —
      Affected versions (publisher)changed
      Windows Server 2019, Windows Server 2019 (Server Core installation), Windows Server 2022 (+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

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: active, automatable: no

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      SSVC assessmentchanged
      {"id":"CVE-2021-42287","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2025-12-20T04:56:06.847529Z"}
      {"id":"CVE-2021-42287","role":"CISA Coordinator","options":[{"exploitation":"active"},{"automatable":"no"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2025-08-19T00:00:00+00:00"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

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    Badge preview for CVE-2021-42287
    <a href="https://intel.neosec.eu/en/cve/CVE-2021-42287"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-42287.svg?lang=en" alt="CVE-2021-42287 – NEOSEC Intel" height="20"></a>