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CVE-2021-26889: Windows Update Stack Elevation of Privilege Vulnerability

• Severity: medium (CVSS 4.6 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-11 · updated 2026-08-19
• Weakness: CWE-59

Description

Windows Update Stack Elevation of Privilege Vulnerability

Affected products

• Microsoft Windows 10 Version 1607: 20h2, 1803, 1809, 1909, 2004
• Microsoft Windows Server 2016: 20h2, 1909, 2004
• Microsoft Windows Server 2019: all versions
• Microsoft Windows 10 1803: from 10.0.0 before 10.0.17134.2087 — fixed in 10.0.17134.2087
• Microsoft Windows 10: from 10.0.17763.0 before 10.0.17763.1817 — fixed in 10.0.17763.1817
• Microsoft Windows 10 1909: from 10.0.0 before 10.0.18363.1440 — fixed in 10.0.18363.1440
• Microsoft Windows 10 2004: from 10.0.0 before 10.0.19043.867 — fixed in 10.0.19043.867
• Microsoft Windows 10 20H2: from 10.0.0 before 10.0.19043.867 — fixed in 10.0.19043.867
• Microsoft Windows Server 2019: from 10.0.17763.0 before 10.0.17763.1817 — fixed in 10.0.17763.1817
• Microsoft Windows Server, version 1909 (Server Core installation): from 10.0.0 before 10.0.18363.1440 — fixed in 10.0.18363.1440
• Microsoft Windows Server version 2004: from 10.0.0 before 10.0.19043.867 — fixed in 10.0.19043.867
• Microsoft Windows Server version 20H2: from 10.0.0 before 10.0.19043.867 — fixed in 10.0.19043.867

How to fix

Upgrade to a fixed version: Microsoft Windows 10 1803 10.0.17134.2087; Microsoft Windows 10 10.0.17763.1817; Microsoft Windows 10 1909 10.0.18363.1440; Microsoft Windows 10 2004 10.0.19043.867; Microsoft Windows 10 20H2 10.0.19043.867; Microsoft Windows Server 2019 10.0.17763.1817; Microsoft Windows Server, version 1909 (Server Core installation) 10.0.18363.1440; Microsoft Windows Server version 2004 10.0.19043.867; Microsoft Windows Server version 20H2 10.0.19043.867. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-26889
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-26889
• https://www.zerodayinitiative.com/advisories/ZDI-21-328/

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