Windows: Elevation of privilege via untrusted search path in Setup Files Cleanup

Summary

The Windows Setup Files Cleanup component in several Windows 10, Windows 11 and Windows Server versions contains an untrusted search path weakness (CWE-426). A local attacker with a low-privileged account can exploit it to gain elevated privileges, with high impact on confidentiality, integrity and availability. The attack complexity is high, and no user interaction is needed. Microsoft fixed the issue in the October 2026 Patch Tuesday.

Affected

• Windows 10 Version 1607: 10.0.14393.0 to before 10.0.14393.9612
• Windows 10 Version 1809: 10.0.17763.0 to before 10.0.17763.9376
• Windows 10 Version 21H2: 10.0.19044.0 to before 10.0.19044.7807
• Windows 10 Version 22H2: 10.0.19045.0 to before 10.0.19045.7807
• Windows 11 Version 23H2: 10.0.22631.0 to before 10.0.22631.7652
• Windows 11 Version 24H2: 10.0.26100.0 to before 10.0.26100.9711
• Windows 11 Version 25H2: 10.0.26200.0 to before 10.0.26200.9711
• Windows 11 Version 26H1: 10.0.28000.0 to before 10.0.28000.3231
• Windows 11 Version 26H2: 10.0.26300.0 to before 10.0.26300.9711
• Windows Server 2012 and Server Core: 6.2.9200.0 to before 6.2.9200.26434
• Windows Server 2012 R2 and Server Core: 6.3.9600.0 to before 6.3.9600.23470
• Windows Server 2016: 10.0.14393.0 to before 10.0.14393.9612
• Note: the supplied CPE lists only Windows 10 1607; the CNA data lists the wider set above.

Fixed in

• Windows 10 Version 1607 and Windows Server 2016: 10.0.14393.9612
• Windows 10 Version 1809: 10.0.17763.9376
• Windows 10 Version 21H2: 10.0.19044.7807
• Windows 10 Version 22H2: 10.0.19045.7807
• Windows 11 Version 23H2: 10.0.22631.7652
• Windows 11 Version 24H2 and 25H2, Version 26H2: 10.0.26100.9711 and 10.0.26200.9711, 10.0.26300.9711
• Windows 11 Version 26H1: 10.0.28000.3231
• Windows Server 2012: 6.2.9200.26434
• Windows Server 2012 R2: 6.3.9600.23470
• KB article numbers: Unknown.

Workaround

• Unknown. No vendor-documented workaround was found in the sources reviewed.
• Interim measure: limit local logon to trusted users and restrict who can run local tools on shared systems. This is general hardening, not a vendor recommendation.

Exploitation

• No public proof-of-concept or in-the-wild exploitation was found in the sources reviewed.
• CISA SSVC data: Exploitation none, Automatable no, Technical Impact total.

Our recommendation

• Install the October 2026 security update on all affected Windows clients and servers, and check the Microsoft Security Update Guide for the KB numbers.
• Check the whole estate, not only Windows 10 1607, because the affected list is much wider.
• Prioritise legacy systems such as Windows 10 1607 and 1809, and verify their support status on the Microsoft lifecycle page.
• Until patched, limit local logon and local code execution to trusted users, and monitor for unexpected privilege changes.

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

• Microsoft Security Update Guide: CVE-2026-83509 msrc.microsoft.com
• Vulners: NVD record CVE-2026-83509 vulners.com
• Vulners: CVELIST record CVE-2026-83509 vulners.com
• OpenCVE: CVE-2026-83509 app.opencve.io
• Cisco Talos: Microsoft Patch Tuesday for October 2026 blog.talosintelligence.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, 08:45.