Microsoft Windows Win32k: Use-after-free allows local privilege escalation

Summary

A use-after-free vulnerability (CWE-416) in the Win32k kernel driver of Windows 10 version 1607 and Windows Server 2016 allows an authorized local user to elevate privileges. An attacker who can already run code with low privileges on the system can gain SYSTEM rights. Exploitation requires local access, so the risk is highest on multi-user systems and terminal servers.

Affected

• Windows 10 Version 1607 (builds before 10.0.14393.9612)
• Windows Server 2016 and Server Core installation (builds before 10.0.14393.9612)
• Windows 10 Version 1809 and Windows Server 2019 (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 Server 2022 (builds before 10.0.20348.5761)
• Windows 11 23H2 (builds before 10.0.22631.7652)
• Windows 11 24H2 (builds before 10.0.26100.9711)
• Windows 11 25H2 and 26H2 (builds before 10.0.26200.9711 and 10.0.26300.9711)
• Windows 11 26H1 (builds before 10.0.28000.3231)

Fixed in

• Windows 10 1607 and Server 2016: 10.0.14393.9612 or later
• Windows 10 1809 and Server 2019: 10.0.17763.9376 or later
• Windows 10 21H2: 10.0.19044.7807 or later
• Windows 10 22H2: 10.0.19045.7807 or later
• Windows Server 2022: 10.0.20348.5761 or later
• Windows 11 23H2: 10.0.22631.7652 or later
• Windows 11 24H2: 10.0.26100.9711 or later
• Windows 11 25H2 and 26H2: 10.0.26200.9711 and 10.0.26300.9711 or later
• Windows 11 26H1: 10.0.28000.3231 or later
• Fix released with the October 13, 2026 security update (for Windows 10 1607, KB5126868 according to the Microsoft KB page; the CVE mapping should be confirmed in the MSRC table)

Workaround

Unknown.

• No vendor-documented workaround or mitigation was found.

Exploitation

• Microsoft: not publicly disclosed, not exploited, exploitation less likely.
• CISA SSVC: exploitation none, automatable no, technical impact total.
• Not listed in CISA KEV; EPSS below 1 percent.
• No public proof of concept or exploit code was found in the research.

Our recommendation

• Identify all Windows 10 1607 and Windows Server 2016 hosts, especially terminal servers and shared workstations.
• Install the October 2026 update (build 10.0.14393.9612 or later) and confirm the build with winver or your inventory tool.
• Verify the end-of-support date for Windows 10 1607 and Server 2016 on Microsoft's lifecycle page and plan an upgrade if support ends.
• Isolate or replace hosts that cannot be patched.
• Monitor for unexpected SYSTEM processes and kernel crashes, and recheck the exploitation status periodically.

Public exploits iAttack code is publicly available (PoC on GitHub, Metasploit or Exploit-DB).

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

Research sources

• MSRC Security Update Guide: CVE-2026-83866 msrc.microsoft.com
• NVD mirror and affected version ranges (Vulners) vulners.com
• Microsoft KB5126868 for Windows 10 1607 support.microsoft.com
• OpenCVE: CVE-2026-83866 app.opencve.io
• VulDB: CVE-2026-83866 vuldb.com

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, 08:05.