CriticalCVE-2021-34484

    Windows User Profile Service Elevation of Privilege Vulnerability (CVE-2021-34484)

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

    What is affected, what to do

    Windows User Profile Service Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 1507 to version 10.0.10240.19022, Microsoft Windows 10 Version 1607 to version 10.0.14393.4583, Microsoft Windows 10 Version 1809 to version 10.0.17763.2114. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 1507 lt 10.0.10240.0..10.0.10240.19022
    • Microsoft Windows 10 Version 1607 lt 10.0.14393.0..10.0.14393.4583
    • 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 7 lt 6.1.0..6.1.7601.25685
    • Microsoft Windows 7 Service Pack 1 lt 6.1.0..6.1.7601.25685
    • Microsoft Windows 8.1 lt 6.3.0..6.3.9600.20094
    • Microsoft Windows Server 2008 R2 Service Pack 1 lt 6.1.7601.0..6.1.7601.25685
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) lt 6.1.7601.0..6.1.7601.25685
    • Microsoft Windows Server 2008 Service Pack 2 lt 6.0.6003.0..6.0.6003.21192
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) lt 6.0.6003.0..6.0.6003.21192
    • Microsoft Windows Server 2012 lt 6.2.9200.0..6.2.9200.23435
    • Microsoft Windows Server 2012 (Server Core installation) lt 6.2.9200.0..6.2.9200.23435
    • Microsoft Windows Server 2012 R2 lt 6.3.9600.0..6.3.9600.20094
    • Microsoft Windows Server 2012 R2 (Server Core installation) lt 6.3.9600.0..6.3.9600.20094
    • Microsoft Windows Server 2016 lt 10.0.14393.0..10.0.14393.4583
    • Microsoft Windows Server 2016 (Server Core installation) lt 10.0.14393.0..10.0.14393.4583
    • 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 1507 10.0.10240.19022
    • Microsoft Windows 10 Version 1607 10.0.14393.4583
    • 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 7 6.1.7601.25685
    • Microsoft Windows 7 Service Pack 1 6.1.7601.25685
    • Microsoft Windows 8.1 6.3.9600.20094
    • Microsoft Windows Server 2008 R2 Service Pack 1 6.1.7601.25685
    • Microsoft Windows Server 2008 R2 Service Pack 1 (Server Core installation) 6.1.7601.25685
    • Microsoft Windows Server 2008 Service Pack 2 6.0.6003.21192
    • Microsoft Windows Server 2008 Service Pack 2 (Server Core installation) 6.0.6003.21192
    • Microsoft Windows Server 2012 6.2.9200.23435
    • Microsoft Windows Server 2012 (Server Core installation) 6.2.9200.23435
    • Microsoft Windows Server 2012 R2 6.3.9600.20094
    • Microsoft Windows Server 2012 R2 (Server Core installation) 6.3.9600.20094
    • Microsoft Windows Server 2016 10.0.14393.4583
    • Microsoft Windows Server 2016 (Server Core installation) 10.0.14393.4583
    • 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

    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

    21.8 %

    EPSS score

    Elevated — model estimates 10-50% exploitation likelihood.

    Source: FIRST EPSS, percentile 97.6

    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
    215.27 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
    21.8 %
    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
    97.6
    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
    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 4 February 2025

    Response and mitigation

    Why act now?

    Prioritisation rationale

    This is a low-complexity local privilege escalation requiring no user interaction and yielding full loss of confidentiality, integrity and availability — a classic post-exploitation step to go from a phished or credential-stuffed standard account to SYSTEM. Public PoCs and the widely publicised bypass of the first patch explain its very high exploitation likelihood; CISA has not flagged known ransomware use. Exposure is greatest in multi-user and terminal-server environments and in organisations still running Windows 7 or end-of-support Windows 10 builds. For KRITIS/NIS2 operators: high priority within the regular patch cycle, escalating to critical wherever legacy or shared hosts exist.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the August 2021 security update (or any later cumulative update) to all Windows clients and servers. Minimum builds per vendor data: Windows 10 1507 → 10.0.10240.19022, 1607 → 10.0.14393.4583, 1809 → 10.0.17763.2114, 1909 → 10.0.18363.1734, 2004 → 10.0.19041.1165, 20H2 → 10.0.19042.1165, 21H1 → 10.0.19043.1165, Windows 7 → 6.1.7601.25685. Look up the per-build KB number in the Microsoft advisory.
      • Do not treat the original fix as final. The August patch was publicly bypassed and Microsoft shipped follow-up fixes in later cumulative updates (including January 2022). Install the current monthly rollup, not just the August 2021 baseline.
      • Inventory legacy systems: query CMDB/Intune/SCCM for Windows 7 without ESU and out-of-support Windows 10 1507/1909/2004 builds. Isolate or migrate these as a priority.
      • Patch multi-user hosts first: terminal servers, RDS/Citrix hosts, jump hosts, kiosk and training PCs — anywhere multiple accounts can log on interactively.
      • If compromise is suspected: disconnect the host, rotate local administrator and service-account passwords (force a LAPS rotation), since SYSTEM-level access enables credential theft from LSASS.

    2. Runbook step 2

      Mitigation layers

      • IAM / least privilege: public exploits require credentials for a second local or domain account. Restrict "Allow log on locally" (SeInteractiveLogonRight) and "Allow log on through Remote Desktop Services" via GPO to the groups that genuinely need them; disable guest and stale test accounts.
      • Credential hygiene: eliminate reused passwords on standard, training and service accounts; enforce Windows LAPS so harvested local admin credentials are not reusable laterally.
      • Application control: configure WDAC or AppLocker to block unsigned binaries from user-writable paths (%USERPROFILE%, %TEMP%, C:\Users\Public, C:\ProgramData) — public PoC tooling is typically launched from there.
      • Endpoint hardening: enable Microsoft Defender ASR rules in block mode (notably "Block executable files from running unless they meet a prevalence, age, or trusted list criterion" and "Block credential stealing from LSASS"); turn on Credential Guard where supported.
      • Network segmentation: move unpatchable legacy systems (Windows 7 without ESU) to a dedicated VLAN, block RDP (TCP 3389) and SMB (TCP 445) from client networks and allow access only from designated admin jump hosts.

    3. Runbook step 3

      Detection rules

      • Microsoft-Windows-User Profile Service/Operational: spikes of Event ID 1511 (logged on with a temporary profile) or 1515 on hosts where temporary profiles are otherwise rare — the public exploits force the temporary-profile fallback.
      • Windows Security log: Event ID 4624 (logon type 2/10/11) for a second account shortly after a standard user's logon on the same host, followed by Event ID 4688/Sysmon EID 1 with IntegrityLevel=System and a parent under C:\Users\. KQL sketch: DeviceProcessEvents | where ProcessIntegrityLevel == "System" and InitiatingProcessFolderPath startswith @"C:\Users\"
      • File-system telemetry (Sysmon EID 11/23/26, EDR): deletion or re-creation of directories under C:\Users\<user>\AppData\Local\Microsoft\Windows\ or C:\Users\TEMP* by svchost.exe (ProfSvc), correlated with junction/reparse-point creation by an unprivileged process (look for fsutil reparsepoint or mklink /J on the command line).
      • Process ancestry: SYSTEM cmd.exe/powershell.exe/conhost.exe whose ancestry traces back to a low-integrity user session. Sigma shape: selection: User|contains: 'SYSTEM' and ParentImage|startswith: 'C:\Users\' → condition: selection.
      • Follow-on activity: LSASS access (Sysmon EID 10, GrantedAccess 0x1010/0x1410) or new local admins (Event ID 4720 + 4732) immediately after the indicators 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

    Windows User Profile 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 1507
    • 10.0.10240.0 – < 10.0.10240.19022
    • 10.0.10240.19022
    MicrosoftWindows 10 Version 1607
    • 10.0.14393.0 – < 10.0.14393.4583
    • 10.0.14393.4583
    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 7
    • 6.1.0 – < 6.1.7601.25685
    • 6.1.7601.25685
    MicrosoftWindows 7 Service Pack 1
    • 6.1.0 – < 6.1.7601.25685
    • 6.1.7601.25685
    MicrosoftWindows 8.1
    • 6.3.0 – < 6.3.9600.20094
    • 6.3.9600.20094
    MicrosoftWindows Server 2008 R2 Service Pack 1
    • 6.1.7601.0 – < 6.1.7601.25685
    • 6.1.7601.25685
    MicrosoftWindows Server 2008 R2 Service Pack 1 (Server Core installation)
    • 6.1.7601.0 – < 6.1.7601.25685
    • 6.1.7601.25685
    MicrosoftWindows Server 2008 Service Pack 2
    • 6.0.6003.0 – < 6.0.6003.21192
    • 6.0.6003.21192
    MicrosoftWindows Server 2008 Service Pack 2 (Server Core installation)
    • 6.0.6003.0 – < 6.0.6003.21192
    • 6.0.6003.21192
    MicrosoftWindows Server 2012
    • 6.2.9200.0 – < 6.2.9200.23435
    • 6.2.9200.23435
    MicrosoftWindows Server 2012 (Server Core installation)
    • 6.2.9200.0 – < 6.2.9200.23435
    • 6.2.9200.23435
    MicrosoftWindows Server 2012 R2
    • 6.3.9600.0 – < 6.3.9600.20094
    • 6.3.9600.20094
    MicrosoftWindows Server 2012 R2 (Server Core installation)
    • 6.3.9600.0 – < 6.3.9600.20094
    • 6.3.9600.20094
    MicrosoftWindows Server 2016
    • 10.0.14393.0 – < 10.0.14393.4583
    • 10.0.14393.4583
    MicrosoftWindows Server 2016 (Server Core installation)
    • 10.0.14393.0 – < 10.0.14393.4583
    • 10.0.14393.4583
    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_150710.0.10240.19022
    Windowsmicrosoftwindows_10_1507—
    Windowsmicrosoftwindows_10_160710.0.14393.4583
    Windowsmicrosoftwindows_10_1607—
    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_76.1.7601.25685
    Windowsmicrosoftwindows_7—
    Windowsmicrosoftwindows_8.16.3.9600.20094
    Windowsmicrosoftwindows_8.1—
    Windowsmicrosoftwindows_rt_8.16.3.9600.20094
    Windowsmicrosoftwindows_rt_8.1—
    Windowsmicrosoftwindows_server_200410.0.19041.1165
    Windowsmicrosoftwindows_server_2004—
    Windowsmicrosoftwindows_server_2008r2

    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-2021-34484 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-2021-34484
      Referenceremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34484
      —
      Reference typeremoved
      https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-34484 Types: Patch, Vendor Advisory
      —
      Affected versions (publisher)changed
      Windows 10 Version 1809, Windows Server 2019, Windows Server 2019 (Server Core installation) (+22)
      Windows 10 Version 1507, Windows 10 Version 1607, Windows 10 Version 1809 (+22)

      Source identifier: secure@microsoft.com

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    <a href="https://intel.neosec.eu/en/cve/CVE-2021-34484"><img src="https://intel.neosec.eu/badge/cve/CVE-2021-34484.svg?lang=en" alt="CVE-2021-34484 – NEOSEC Intel" height="20"></a>