Improper access control in Microsoft Windows and Windows Server - CVE-2026-20839

Vulnerability identifier: #VU121329

CSH Severity: Low

CVSS v4: 6.8 [CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N]

CVE-ID: CVE-2026-20839

CWE-ID: CWE-284

Exploitation vector: Local access

Exploit availability: No public exploit available

The vulnerability allows a local user to gain unauthorized access to otherwise restricted functionality.

The vulnerability exists due to improper access restrictions in Windows Client-Side Caching (CSC) Service. A local user can bypass implemented security restrictions and gain unauthorized access to sensitive information on the system.

Microsoft Windows
Windows Server

Install updates from vendor's website.

Microsoft Windows - addressed in versions 10 21H2 10.0.19044.6809, 10 22H2 10.0.19045.6809, 10 1607 10.0.14393.8783, 10 1809 10.0.17763.8276, 11 23H2 10.0.22631.6491, 11 24H2 10.0.26100.7623
Windows Server - addressed in versions 2008 R2 6.1.7601.28117, 2012 R2 6.3.9600.22968, 2012 6.2.9200.25868, 2016 10.0.14393.8783, 2019 10.0.17763.8276, 2022 23H2 10.0.25398.2092, 2022 10.0.20348.4648, 2025 10.0.26100.7623, 2025 10.0.26100.32230

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20839

• Information disclosure in Microsoft Windows Client-Side Caching (CSC) Service