Windows 11 26H1: Elevation of privilege in unionfs.sys driver
Summary
An off-by-one error in the Windows Container Isolation FS Filter Driver (unionfs.sys) allows an authorized local attacker to elevate privileges. According to the CVE record, Windows 11 version 26H1 builds from 10.0.28000.0 before 10.0.28000.3501 are affected. The attacker needs an existing low-privileged account on the host and no user interaction; the impact is limited to integrity (CVSS 5.5). The risk is highest on shared hosts and systems that run Windows containers.
Affected
- Microsoft Windows 11 version 26H1 (ARM64 and x64), builds 10.0.28000.0 before 10.0.28000.3501
- Other Windows branches: Unknown. The CVE record lists only 26H1; verify in the MSRC Security Update Guide.
Fixed in
- Build 10.0.28000.3501 or later, included in KB5132059 (November 2026 update for 26H1)
Workaround
- No workaround documented.
- Unknown: whether an interim mitigation exists.
Exploitation
- Microsoft rates exploit maturity as unproven (E:U).
- CISA SSVC: exploitation none, automatable no, technical impact total.
- No public proof of concept or exploitation reports found; published 10 Nov 2026.
- Not listed in CISA KEV according to a third-party aggregator; no CISA entry found.
Our recommendation
- Confirm the affected branches in the MSRC Security Update Guide before concluding that other Windows versions are not affected.
- Install the November 2026 cumulative update (KB5132059 for 26H1) on affected hosts, prioritising multi-user systems and container hosts.
- Until patched, restrict local logon and limit container-host use to trusted accounts.
- Monitor for new exploitation reports, as this CVE is very new.
Information from Microsoft (Patch Tuesday November 2026)
- Severity (Microsoft)
- Important
- Impact
- Elevation of privilege – Windows Container Isolation FS Filter Driver (unionfs.sys)
- Status on Patch Tuesday
- Neither exploited nor publicly disclosed
- Exploitability assessment
- Exploitation less likely
- Product family
- Windows
- Updates (KB)
- KB5132059
Public exploits
PoC-in-GitHub, Metasploit and Exploit-DB list nothing for this CVE.
Research sources
- MSRC Security Update Guide: CVE-2026-113771 msrc.microsoft.com
- CVE Record CVE-2026-113771 www.cve.org
- KB5132059: Windows 11 26H1 security update support.microsoft.com
- SANS ISC: Microsoft Patch Tuesday August 2026 isc.sans.edu
- Sibling unionfs.sys CVE CVE-2026-97810 (affected branches) vulners.com
- Third-party build tracker: Windows 11 build 28000.3501 windowsforum.com
- Strix: CVE-2026-113771 www.strix.ai
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 11 Nov 2026, 16:47.
Original description (NVD)
Off-by-one error in Windows Container Isolation FS Filter Driver (unionfs.sys) 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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