Microsoft Windows Win32k: Use-after-free leads to local privilege escalation
Summary
A use-after-free vulnerability in the kernel-mode Win32k component of Windows allows a local attacker to elevate privileges, potentially to SYSTEM. The attacker needs local access and low privileges, so the bug is not remotely exploitable without an existing foothold. Microsoft lists affected builds for Windows 11 24H2, 25H2, 26H1, 26H2 and Windows Server 2025.
Affected
- Microsoft Windows 11 Version 24H2: builds 10.0.26100.0 to below 10.0.26100.9711
- Microsoft Windows 11 Version 25H2: builds 10.0.26200.0 to below 10.0.26200.9711
- Microsoft Windows 11 Version 26H1: builds 10.0.28000.0 to below 10.0.28000.3231
- Microsoft Windows 11 Version 26H2: builds 10.0.26300.0 to below 10.0.26300.9711
- Microsoft Windows Server 2025 (including Server Core): builds 10.0.26100.0 to below 10.0.26100.33650
Fixed in
- Windows 11 24H2: build 10.0.26100.9711 or later
- Windows 11 25H2: build 10.0.26200.9711 or later
- Windows 11 26H1: build 10.0.28000.3231 or later
- Windows 11 26H2: build 10.0.26300.9711 or later
- Windows Server 2025: build 10.0.26100.33650 or later
- Fix provided in the October 2026 cumulative update. Confirm the KB numbers on the MSRC page.
Workaround
Unknown. No vendor workaround has been identified. Patching is the remediation path.
Exploitation
- CISA SSVC assessment: exploitation none, not automatable, technical impact total.
- Exploit code maturity is unproven (E:U).
- No public proof-of-concept or exploitation in the wild found at the time of research.
- Not listed in the CISA KEV catalogue.
Our recommendation
- Install the October 2026 cumulative update on all affected Windows 11 and Windows Server 2025 systems in the normal patch cycle.
- Prioritise workstations, terminal servers and multi-user systems where untrusted files or users are present.
- Review local accounts and restrict local logon where possible.
- Monitor for signs of privilege escalation and patch sooner if a foothold is suspected.
Information from Microsoft (Patch Tuesday October 2026)
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- Microsoft Security Update Guide: CVE-2026-83721 msrc.microsoft.com
- OpenCVE: CVE-2026-83721 app.opencve.io
- Vulners: NVD record CVE-2026-83721 vulners.com
- Vulners: CNA data CVE-2026-83721 vulners.com
- Strix: CVE-2026-83721 www.strix.ai
- Zero Day Initiative: October 2026 security update review www.zerodayinitiative.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 2 Nov 2026, 03:14.
Original description (NVD)
Use after free in Windows Win32K allows an authorized attacker to elevate privileges locally.
NVD references (1)
Sources
The tracker combines official, freely usable data sources – updated continuously:
- NVD (NIST) – CVEs, rating (CVSS), affected products
- CISA KEV – vulnerabilities confirmed by CISA as actively exploited
- CISA Vulnrichment – CISA assessment (SSVC) in the official CVE record
- FIRST EPSS – probability of exploitation
- Microsoft MSRC – Patch Tuesday: vulnerabilities, severity, updates
- PoC-in-GitHub – public proof-of-concept repositories
- Metasploit (Rapid7) – modules of the Metasploit Framework
- Exploit-DB (OffSec) – archive of public exploits (links only)
- Patch days: official vendor pages (linked per vendor) (Vendor patch days)
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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