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Microsoft Security Response CenterCVE-2026-72971

Windows Container Isolation FS Filter Driver (unionfs.sys) Tampering Vulnerability

Improper link resolution before file access ('link following') in Windows Container Isolation FS Filter Driver (unionfs.sys) allows an authorized attacker to perform tampering locally.

Read the vendor bulletin Get alerts for Windows
5.5CVSS out of 10High severity
Scope

What the vendor says is affected

  • Windows 11 version 26H1 for x64-based Systems
  • Windows 11 Version 26H1 for ARM64-based Systems

Versions the vendor lists as fixed

  • 10.0.28000.2704
Next step

What the vendor recommends

Install KB5121000.

Review the complete instructions on the vendor's site
Update history

What changed in later vendor updates

SecurityAlert records field-level changes from the point we begin following a bulletin. Earlier vendor changes may not have a field-by-field record.

    • The vendor changed its remediation guidance.
Timeline

When this advisory changed

  1. Published by Microsoft Security Response Center

    The publication date reported by the vendor.

  2. Added to SecurityAlert

    We collected the advisory from the official source.

  3. Confirmed at the source

    Our collector saw this advisory during a later source check.

Vulnerabilities

CVEs named in this advisory

CVE-2026-72971View CVSS, KEV, EPSS, and threat context
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