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Microsoft Windows Hyper-V: Elevation of privilege via TOCTOU race condition

CVE-2026-83839High 7.0

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Summary

A time-of-check time-of-use race condition (CWE-367) in the Windows Hyper-V hypervisor allows an authorized local attacker to elevate privileges. A successful attack could compromise confidentiality, integrity and availability of the affected host. Exploitation requires local access with low privileges and a race that is hard to win reliably.

Affected

  • Windows 10 Version 1809 and Windows Server 2019 (including Server Core): builds before 10.0.17763.9376
  • Windows 10 Version 21H2: builds before 10.0.19044.7807
  • Windows 10 Version 22H2: builds before 10.0.19045.7807
  • Windows 11 Version 23H2: builds before 10.0.22631.7652
  • Windows 11 Version 24H2 and Windows Server 2025 (including Server Core): builds before 10.0.26100.9711 and 10.0.26100.33650 respectively
  • Windows 11 Version 25H2: builds before 10.0.26200.9711
  • Windows 11 Version 26H1: builds before 10.0.28000.3231
  • Windows 11 Version 26H2: builds before 10.0.26300.9711
  • Windows Server 2022: builds before 10.0.20348.5761

Fixed in

  • Windows 10 1809 / Server 2019: 10.0.17763.9376 (KB5126651, to be verified on MSRC)
  • Windows 10 21H2: 10.0.19044.7807
  • Windows 10 22H2: 10.0.19045.7807
  • Windows 11 23H2: 10.0.22631.7652
  • Windows 11 24H2: 10.0.26100.9711
  • Windows 11 25H2: 10.0.26200.9711
  • Windows 11 26H1: 10.0.28000.3231
  • Windows 11 26H2: 10.0.26300.9711
  • Windows Server 2022: 10.0.20348.5761
  • Windows Server 2025: 10.0.26100.33650 (KB5126646, to be verified on MSRC)

Workaround

Unknown.

Exploitation

  • CISA SSVC: exploitation none, automatable no, technical impact total
  • Not listed in the CISA KEV catalog
  • VulDB states no exploit exists
  • EPSS below 1%
  • No public proof of concept or exploitation in the wild found at the time of research

Our recommendation

  • Patch affected Windows hosts to the builds listed under fixed at the next maintenance window, prioritising Hyper-V hosts with multiple tenants or local user access
  • Inventory all Windows 10, 11 and Server systems, not only Windows 10 1809, and check whether Hyper-V is enabled
  • Check whether Windows 10 1809 is LTSC 2019 or an out-of-support edition and plan an upgrade if needed
  • Verify build numbers and KB articles on the MSRC page and re-check the CISA KEV catalog before closing the item

Information from Microsoft (Patch Tuesday October 2026)

Severity (Microsoft)
Important
Impact
Elevation of privilege – Windows Hyper-V
Status on Patch Tuesday
Neither exploited nor publicly disclosed
Exploitability assessment
Exploitation less likely
Product family
Windows, Windows ESU (Extended Security Updates)
Updates (KB)
KB5126651, KB5126655, KB5126657, KB5126646, KB5127648, KB5126653

Entry in the Microsoft Security Update Guide

Public exploits

PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.

Research sources

Researched and written up in English and German by an AI (Claude by Anthropic) based on NVD, CISA, EPSS and the sources listed. The vendor information is authoritative. Researched on 4 Nov 2026, 08:08.

Original description (NVD)

Time-of-check time-of-use (toctou) race condition in Windows Hyper-V allows an authorized attacker to elevate privileges locally.

NVD references (1)

Sources

The tracker combines official, freely usable data sources – updated continuously:

This product uses data from the NVD API but is not endorsed or certified by the NVD. EPSS data © FIRST.org.

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