CriticalCVE-2025-21334

    Windows Hyper-V NT Kernel Integration VSP Elevation of Privilege Vulnerability (CVE-2025-21334)

    Published In CISA KEV since Sources: pwn.watch, Msrc Update Guide, CVE.org

    What is affected, what to do

    Windows Hyper-V NT Kernel Integration VSP Elevation of Privilege Vulnerability

    The vulnerability is being actively exploited; treat it as a priority. Update Microsoft Windows 10 Version 21H2 to version 10.0.19044.5371, Microsoft Windows 10 Version 22H2 to version 10.0.19045.5371, Microsoft Windows 11 version 22H2 to version 10.0.22621.4751. Immediate actions, mitigation and detection rules are in the runbook below.

    Affected
    • Microsoft Windows 10 Version 21H2 lt 10.0.19044.0..10.0.19044.5371
    • Microsoft Windows 10 Version 22H2 lt 10.0.19045.0..10.0.19045.5371
    • Microsoft Windows 11 version 22H2 lt 10.0.22621.0..10.0.22621.4751
    • Microsoft Windows 11 version 22H3 lt 10.0.22631.0..10.0.22631.4751
    • Microsoft Windows 11 Version 23H2 lt 10.0.22631.0..10.0.22631.4751
    • Microsoft Windows 11 Version 24H2 lt 10.0.26100.0..10.0.26100.2894
    • Microsoft Windows Server 2022, 23H2 Edition (Server Core installation) lt 10.0.25398.0..10.0.25398.1369
    • Microsoft Windows Server 2025 lt 10.0.26100.0..10.0.26100.2894
    • Microsoft Windows Server 2025 (Server Core installation) lt 10.0.26100.0..10.0.26100.2894
    Fixed in
    • Microsoft Windows 10 Version 21H2 10.0.19044.5371
    • Microsoft Windows 10 Version 22H2 10.0.19045.5371
    • Microsoft Windows 11 version 22H2 10.0.22621.4751
    • Microsoft Windows 11 version 22H3 10.0.22631.4751
    • Microsoft Windows 11 Version 23H2 10.0.22631.4751
    • Microsoft Windows 11 Version 24H2 10.0.26100.2894
    • Microsoft Windows Server 2022, 23H2 Edition (Server Core installation) 10.0.25398.1369
    • Microsoft Windows Server 2025 10.0.26100.2894
    • Microsoft Windows Server 2025 (Server Core installation) 10.0.26100.2894

    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: pwn.watch, Msrc Update Guide, CVE.org

    1.6 %

    EPSS score

    Moderate — model estimates 1-10% exploitation likelihood.

    Source: FIRST EPSS, percentile 75.3

    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
    191.87 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: VulnCheck KEV (opens in a new tab) 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
    1.6 %
    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
    75.3
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    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
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-416

    Response and mitigation

    Why act now?

    Prioritisation rationale

    CISA added this flaw to KEV on the same day Microsoft shipped the fix, meaning it was exploited as a zero-day before a patch existed. It is a low-complexity local privilege escalation that needs only a low-privileged account and no user interaction, yet yields full SYSTEM-level compromise of confidentiality, integrity and availability — attackers typically chain it after initial access (phishing, stolen credentials) to gain SYSTEM, kill EDR and harvest credentials. CISA does not flag known ransomware use, and the model-based exploitation likelihood is only moderate, but Hyper-V hosts, VDI and RDS estates and anything close to Tier 0 should still be patched at top priority. Standalone endpoints without Hyper-V components can follow in an accelerated regular patch cycle.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Deploy the January 2025 Patch Tuesday security update and verify the resulting build: Windows 10 21H2 ≥ 10.0.19044.5371, Windows 10 22H2 ≥ 10.0.19045.5371, Windows 11 22H2 ≥ 10.0.22621.4751, Windows 11 22H3/23H2 ≥ 10.0.22631.4751, Windows 11 24H2 and Windows Server 2025 ≥ 10.0.26100.2894, Windows Server 2022 23H2 (Server Core) ≥ 10.0.25398.1369. Look up the per-platform KB number in the vendor advisory. Build check: (Get-ItemProperty 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion').UBR.
      • Patch in order of blast radius: Hyper-V hosts (virtualisation clusters, VDI hosts) first, then multi-user systems (RDS/terminal servers, jump hosts), then endpoints with Hyper-V, WSL2, Windows Sandbox or VBS enabled.
      • Inventory where Hyper-V is active: Get-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V (client) or Get-WindowsFeature Hyper-V (server), and reconcile against patch status from WSUS/Intune/SCCM.
      • On any host with existing compromise indicators (malware detections, suspicious logons), assume SYSTEM was obtained: isolate the host, rotate local admin passwords (LAPS), service-account credentials and any cached domain credentials.
      • Reboot Hyper-V hosts after patching and confirm the reboot completed — the fix is not effective until then.

    2. Runbook step 2

      Mitigation layers

      • Configuration change: The context documents no vendor workaround. Where Hyper-V is not a business requirement, remove the role (Disable-WindowsOptionalFeature -Online -FeatureName Microsoft-Hyper-V-All). Weigh the trade-off: WSL2, Windows Sandbox, VBS/HVCI and Credential Guard all depend on the hypervisor — disabling VBS weakens other protections and is only acceptable as a short-term measure.
      • IAM/least privilege: Restrict interactive and RDP logon on Hyper-V hosts to dedicated Tier-0 admin accounts via GPO ("Allow log on locally" / "Allow log on through Remote Desktop Services"). No standard users, no service accounts with interactive logon rights.
      • Endpoint hardening: Enforce application control (WDAC or AppLocker) for user profiles, %TEMP% and Downloads — a local exploit binary has to execute first. Enable Microsoft Defender ASR rules, notably blocking untrusted/unsigned executables.
      • Network segmentation: Place Hyper-V host management interfaces in a dedicated management VLAN; permit RDP (3389/TCP), WinRM (5985/5986/TCP) and SMB (445/TCP) only from PAWs/jump hosts. This raises the bar for the initial foothold that a local privilege escalation requires.
      • EDR configuration: Enforce tamper protection so that an attacker with SYSTEM cannot silently disable the sensor.

    3. Runbook step 3

      Detection rules

      • EDR/Sysmon EID 1 or Security 4688 — integrity jump: process running as SYSTEM whose parent runs as a standard user at Medium/Low integrity and is not a service-control process. KQL: DeviceProcessEvents | where AccountSid == "S-1-5-18" and InitiatingProcessIntegrityLevel in ("Medium","Low") and InitiatingProcessFileName !in~ ("services.exe","svchost.exe","msiexec.exe").
      • Process ancestry: cmd.exe, powershell.exe or rundll32.exe holding a SYSTEM token with an unsigned ancestor in C:\Users\*\AppData\*, C:\Users\Public\* or %TEMP%. Sigma shape: selection: User|contains: 'SYSTEM'; ParentImage|contains: ['\AppData\','\Temp\','\Users\Public\']; filter: ParentImage signed.
      • Kernel instability as a side effect: use-after-free exploitation frequently crashes the box. Correlate System log Event ID 1001 (BugCheck) and Event ID 41 (Kernel-Power) on Hyper-V hosts and VBS-enabled endpoints, especially clusters from a single host or shortly after a standard user logs on.
      • Post-escalation activity: Security 4697/System 7045 (new service), Sysmon EID 10 targeting lsass.exe, Defender events 5001/5007 (real-time protection disabled / configuration changed) shortly after a suspicious SYSTEM process.
      • Hyper-V telemetry: monitor Microsoft-Windows-Hyper-V-VMMS-Admin and Microsoft-Windows-Hyper-V-Worker-Admin for unexpected errors and vmwp.exe crashes.

    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 Hyper-V NT Kernel Integration VSP Elevation of Privilege Vulnerability

    Source: pwn.watch, Microsoft Security Update Guide, cvelistv5, NVD (NIST), CISA KEV Catalog, 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 21H2
    • 10.0.19044.0 – < 10.0.19044.5371
    • 10.0.19044.5371
    MicrosoftWindows 10 Version 22H2
    • 10.0.19045.0 – < 10.0.19045.5371
    • 10.0.19045.5371
    MicrosoftWindows 11 version 22H2
    • 10.0.22621.0 – < 10.0.22621.4751
    • 10.0.22621.4751
    MicrosoftWindows 11 version 22H3
    • 10.0.22631.0 – < 10.0.22631.4751
    • 10.0.22631.4751
    MicrosoftWindows 11 Version 23H2
    • 10.0.22631.0 – < 10.0.22631.4751
    • 10.0.22631.4751
    MicrosoftWindows 11 Version 24H2
    • 10.0.26100.0 – < 10.0.26100.2894
    • 10.0.26100.2894
    MicrosoftWindows Server 2022, 23H2 Edition (Server Core installation)
    • 10.0.25398.0 – < 10.0.25398.1369
    • 10.0.25398.1369
    MicrosoftWindows Server 2025
    • 10.0.26100.0 – < 10.0.26100.2894
    • 10.0.26100.2894
    MicrosoftWindows Server 2025 (Server Core installation)
    • 10.0.26100.0 – < 10.0.26100.2894
    • 10.0.26100.2894

    Vendors and products

    Affected operating systems

    Affected operating systems
    Windowsmicrosoftwindows_10_21h2—
    Windowsmicrosoftwindows_10_22h2—
    Windowsmicrosoftwindows_11_22h2—
    Windowsmicrosoftwindows_11_23h2—
    Windowsmicrosoftwindows_11_24h2—
    Windowsmicrosoftwindows_server_2022_23h2—
    Windowsmicrosoftwindows_server_2025—

    Weakness classes (CWE)

    • CWE-416Use After Free

      Variant

      The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.

      Entry at MITRE CWE-416

    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

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    <a href="https://intel.neosec.eu/en/cve/CVE-2025-21334"><img src="https://intel.neosec.eu/badge/cve/CVE-2025-21334.svg?lang=en" alt="CVE-2025-21334 – NEOSEC Intel" height="20"></a>