CriticalCVE-2021-36948

    Windows Update Medic Service Elevation of Privilege Vulnerability (CVE-2021-36948)

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

    What is affected, what to do

    Windows Update Medic Service Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1809 to version 10.0.17763.2114, Microsoft Windows 10 Version 1909 to version 10.0.18363.1734, Microsoft Windows 10 Version 2004 to version 10.0.19041.1165. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1809 lt 10.0.17763.0..10.0.17763.2114
    • Microsoft Windows 10 Version 1909 lt 10.0.0..10.0.18363.1734
    • Microsoft Windows 10 Version 2004 lt 10.0.0..10.0.19041.1165
    • Microsoft Windows 10 Version 20H2 lt 10.0.0..10.0.19042.1165
    • Microsoft Windows 10 Version 21H1 lt 10.0.0..10.0.19043.1165
    • Microsoft Windows Server 2019 lt 10.0.17763.0..10.0.17763.2114
    • Microsoft Windows Server 2019 (Server Core installation) lt 10.0.17763.0..10.0.17763.2114
    • Microsoft Windows Server version 2004 lt 10.0.0..10.0.19041.1165
    • Microsoft Windows Server version 20H2 lt 10.0.0..10.0.19042.1165
    Fixed in
    • Microsoft Windows 10 Version 1809 10.0.17763.2114
    • Microsoft Windows 10 Version 1909 10.0.18363.1734
    • Microsoft Windows 10 Version 2004 10.0.19041.1165
    • Microsoft Windows 10 Version 20H2 10.0.19042.1165
    • Microsoft Windows 10 Version 21H1 10.0.19043.1165
    • Microsoft Windows Server 2019 10.0.17763.2114
    • Microsoft Windows Server 2019 (Server Core installation) 10.0.17763.2114
    • Microsoft Windows Server version 2004 10.0.19041.1165
    • Microsoft Windows Server version 20H2 10.0.19042.1165

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    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

    23.3 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 97.8

    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
    215.47 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
    7.8 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
    23.3 %
    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
    97.8
    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

    Response and mitigation

    Why act now?

    Prioritisation rationale

    Exploitation needs local access with low privileges but no user interaction, and yields full compromise of confidentiality, integrity and availability — attackers use bugs like this as the second stage after phishing, stolen VPN credentials or a webshell to go from user context to SYSTEM. Exploitation likelihood is rated very high, and while CISA does not flag known ransomware use, that does not diminish its value as a chain component. For fleets on current cumulative updates the residual risk is low; it becomes pressing on terminal servers, VDI estates and any lingering out-of-support Windows 10 builds, which should be scored high and scheduled for replacement. Under NIS2, a fix available since 2021 still missing in production is a vulnerability-management finding in its own right, regardless of the individual host risk.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Inventory build levels: Use Intune, SCCM/MECM or the UBR value (HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\UBR) to find every host below the fixed builds: Windows 10 1809 / Server 2019 (incl. Server Core) < 10.0.17763.2114, Windows 10 1909 < 10.0.18363.1734, Windows 10 2004 / Server 2004 < 10.0.19041.1165, 20H2 < 10.0.19042.1165, 21H1 < 10.0.19043.1165.
      • Deploy the cumulative security update that brings each host to at least those builds. The patch is available from Microsoft — look up the KB number per version in the Microsoft Security Update Guide entry for CVE-2021-36948. Any later cumulative update also contains the fix.
      • Prioritise multi-user and Tier-0/1 hosts: RDS/terminal servers, jump hosts, Citrix VDAs and Server 2019 boxes with interactive logons go first, since low-privileged users already execute code there.
      • Flag out-of-support builds: Windows 10 1809 (non-LTSC), 1909, 2004, 20H2, 21H1 and Windows Server 2004 no longer receive security updates. Track them as legacy debt with an upgrade ticket and treat them as compromisable until replaced (see segmentation below).
      • If compromise is suspected: Isolate the host via EDR, rotate LAPS-managed local admin passwords and any privileged credentials cached on that machine — SYSTEM-level access means LSASS is within reach.

    2. Runbook step 2

      Mitigation layers

      • Configuration: WaaSMedicSvc (Windows Update Medic Service, svchost.exe -k wusvcs) is a protected service and cannot be disabled in a supported way. Forcing it off via ACL tampering is not recommended because it degrades update health — patching is the only real fix.
      • Endpoint hardening / application control: Configure WDAC or AppLocker to block execution and DLL loading from user-writable paths (%TEMP%, %APPDATA%, %LOCALAPPDATA%, C:\Users\Public, C:\ProgramData), making it harder to stage exploit tooling after initial access.
      • Defender ASR rules in block mode, especially "Block credential stealing from LSASS" (9e6c4e1f-7d60-472f-ba1a-a39ef669e4b2) and "Block executable files unless they meet a prevalence, age, or trusted list criterion", to contain post-escalation activity. Enable Credential Guard and LSA Protection (RunAsPPL) as well.
      • IAM / least privilege: Restrict "Allow log on locally" on servers via GPO to the groups that genuinely need it, keep users off local admin, and enforce LAPS so a local SYSTEM compromise yields no reusable admin credential.
      • Network segmentation: Move unpatchable legacy hosts into a dedicated VLAN and block SMB (445/tcp), RPC (135/tcp), WinRM (5985/5986/tcp) and RDP (3389/tcp) from that segment towards server and admin networks. As the bug is local, this mainly limits lateral spread after successful exploitation.

    3. Runbook step 3

      Detection rules

      • Sysmon EID 1 / Security EID 4688 — anomalous children: Processes whose parent is WaaSMedicAgent.exe or svchost.exe with -k wusvcs spawning shells or LOLBins. KQL: DeviceProcessEvents | where InitiatingProcessFileName =~ "WaaSMedicAgent.exe" or InitiatingProcessCommandLine has "wusvcs" | where FileName in~ ("cmd.exe","powershell.exe","pwsh.exe","rundll32.exe","regsvr32.exe","mshta.exe")
      • Sysmon EID 7 — suspicious image loads: Non-Microsoft-signed DLLs, or DLLs from user-writable paths, loaded into WaaSMedicAgent.exe or the wusvcs svchost. Sigma shape: Image|endswith: '\WaaSMedicAgent.exe' AND Signed: 'false' OR ImageLoaded|contains: ['\Users\', '\ProgramData\', '\Temp\'].
      • Sysmon EID 12/13 — registry tampering: Writes to HKLM\SYSTEM\CurrentControlSet\Services\WaaSMedicSvc\* or ...\Services\wuauserv\* by anything other than TrustedInstaller.exe, svchost.exe or MoUsoCoreWorker.exe.
      • Privilege jump in process ancestry: A process running as NT AUTHORITY\SYSTEM whose EDR ancestry traces back to a standard user session. Splunk: index=sysmon EventCode=1 User="NT AUTHORITY\\SYSTEM" ParentUser!="NT AUTHORITY\\SYSTEM" | stats count by host, ParentImage, Image.
      • Follow-on activity: Correlate Security EID 7045/4697 (new service), EID 4720/4732 (new local account / added to Administrators) and Sysmon EID 10 with TargetImage lsass.exe occurring shortly after any hit above on the same host.

    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 Update Medic Service 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 10 Version 1809
    • 10.0.17763.0 – < 10.0.17763.2114
    • 10.0.17763.2114
    MicrosoftWindows 10 Version 1909
    • 10.0.0 – < 10.0.18363.1734
    • 10.0.18363.1734
    MicrosoftWindows 10 Version 2004
    • 10.0.0 – < 10.0.19041.1165
    • 10.0.19041.1165
    MicrosoftWindows 10 Version 20H2
    • 10.0.0 – < 10.0.19042.1165
    • 10.0.19042.1165
    MicrosoftWindows 10 Version 21H1
    • 10.0.0 – < 10.0.19043.1165
    • 10.0.19043.1165
    MicrosoftWindows Server 2019
    • 10.0.17763.0 – < 10.0.17763.2114
    • 10.0.17763.2114
    MicrosoftWindows Server 2019 (Server Core installation)
    • 10.0.17763.0 – < 10.0.17763.2114
    • 10.0.17763.2114
    MicrosoftWindows Server version 2004
    • 10.0.0 – < 10.0.19041.1165
    • 10.0.19041.1165
    MicrosoftWindows Server version 20H2
    • 10.0.0 – < 10.0.19042.1165
    • 10.0.19042.1165

    Vendors and products

    Affected operating systems

    Affected operating systems
    Windowsmicrosoftwindows_10_180910.0.17763.2114
    Windowsmicrosoftwindows_10_180910.0.18363.1734
    Windowsmicrosoftwindows_10_180910.0.19041.1165
    Windowsmicrosoftwindows_10_1809—
    Windowsmicrosoftwindows_10_190910.0.18363.1734
    Windowsmicrosoftwindows_10_1909—
    Windowsmicrosoftwindows_10_2004—
    Windowsmicrosoftwindows_10_20H210.0.19042.1165
    Windowsmicrosoftwindows_10_20h2—
    Windowsmicrosoftwindows_10_21H110.0.19043.1165
    Windowsmicrosoftwindows_10_21h1—
    Windowsmicrosoftwindows_server_200410.0.19041.1165
    Windowsmicrosoftwindows_server_2004—
    Windowsmicrosoftwindows_server_201910.0.17763.2114
    Windowsmicrosoftwindows_server_2019—
    Windowsmicrosoftwindows_server_20H210.0.19042.1165
    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. NVD analysis modifiedNVD

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Reference typeadded
      —
      Microsoft Corporation: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-36948 Types: Patch, Vendor Advisory

      Source identifier: nvd@nist.gov

    2. 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-2021-36948
      Referenceremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36948
      —
      Reference typeremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-36948 Types: Patch, Vendor Advisory
      —
      Affected versions (publisher)changed
      Windows 10 Version 1809, Windows Server 2019, Windows Server 2019 (Server Core installation) (+6)
      Windows 10 Version 1809, Windows 10 Version 1809, Windows 10 Version 1909 (+7)

      Source identifier: secure@microsoft.com

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