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Windows Win32k: Heap-based buffer overflow allows local privilege escalation

CVE-2026-70285High 7.8

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Summary

A heap-based buffer overflow in the Windows Win32k kernel subsystem allows an authorized local attacker to elevate privileges. An attacker who already has a low-privileged foothold on a system, for example after a phishing click or malware on a workstation, could gain SYSTEM rights. Microsoft rates the flaw as Important with a CVSS 3.1 base score of 7.8 and does not consider it exploited or publicly disclosed.

Affected

  • Windows 10 Version 1607 (builds below 10.0.14393.9612)
  • Windows 10 Version 1809 (builds below 10.0.17763.9376)
  • Windows 10 Version 21H2 (builds below 10.0.19044.7807)
  • Windows 10 Version 22H2 (builds below 10.0.19045.7807)
  • Windows 11 Version 23H2 (builds below 10.0.22631.7652)
  • Windows 11 Version 24H2 (builds below 10.0.26100.9711)
  • Windows 11 Version 25H2 (builds below 10.0.26200.9711)
  • Windows 11 Version 26H1 (builds below 10.0.28000.3231)
  • Windows 11 Version 26H2 (builds below 10.0.26300.9711)
  • Windows Server 2016 (builds below 10.0.14393.9612)
  • Windows Server 2019 (builds below 10.0.17763.9376)
  • Windows Server 2022 (builds below 10.0.20348.5761)

Fixed in

  • Install the October 2026 security update for your Windows version, which contains the fixed builds listed above.
  • Windows 10 1607 and Server 2016: build 10.0.14393.9612 or later. KB number: Unknown.
  • Windows 10 1809 and Server 2019: KB5126651 according to third-party plugin titles, to be confirmed on MSRC.
  • Windows Server 2025: KB5126646 according to third-party plugin titles, to be confirmed on MSRC.

Workaround

  • Unknown. Microsoft documents no workaround.
  • Reduce local privileges by keeping users out of built-in administrative groups unless absolutely required. This lowers the impact but does not fix the flaw.

Exploitation

  • Microsoft reports no public disclosure and no known exploitation.
  • Exploitability assessment: Exploitation Less Likely.
  • CISA SSVC: exploitation none, automatable no, technical impact total.
  • No public proof-of-concept code found in the sources reviewed.

Our recommendation

  • Install the October 2026 security update on all affected Windows systems, starting with servers, terminal servers and any Windows 10 1607 or Server 2016 hosts.
  • Confirm the KB number for each build on the MSRC page before rollout.
  • Restrict local administrator rights and keep endpoint protection active.
  • Monitor for signs of privilege escalation and review the status of the CVE regularly.

Information from Microsoft (Patch Tuesday October 2026)

Severity (Microsoft)
Important
Impact
Elevation of privilege – Windows Win32 Kernel Subsystem
Status on Patch Tuesday
Neither exploited nor publicly disclosed
Exploitability assessment
Exploitation less likely
Product family
Windows, Windows ESU (Extended Security Updates)
Updates (KB)
KB5127648, KB5126651, KB5126655, KB5126657, KB5126646, 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 2 Nov 2026, 03:51.

Original description (NVD)

Heap-based buffer overflow in Windows Win32 Kernel Subsystem 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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