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CVE-2021-33779: Windows AD FS Security Feature Bypass Vulnerability

• Severity: medium (CVSS 5.5 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-07-14 · updated 2026-08-10

Description

Windows AD FS Security Feature Bypass Vulnerability

Affected products

• Microsoft Windows Server 2016: 20h2, 2004
• Microsoft Windows Server 2019: all versions
• Microsoft Windows Server 2016: from 10.0.14393.0 before 10.0.14393.4530 — fixed in 10.0.14393.4530
• Microsoft Windows Server 2019: from 10.0.17763.0 before 10.0.17763.2061 — fixed in 10.0.17763.2061
• Microsoft Windows Server version 2004: from 10.0.0 before 10.0.19041.1110 — fixed in 10.0.19041.1110
• Microsoft Windows Server version 20H2: from 10.0.0 before 10.0.19042.1110 — fixed in 10.0.19042.1110

How to fix

Upgrade to a fixed version: Microsoft Windows Server 2016 10.0.14393.4530; Microsoft Windows Server 2019 10.0.17763.2061; Microsoft Windows Server version 2004 10.0.19041.1110; Microsoft Windows Server version 20H2 10.0.19042.1110. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2021-33779
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-33779

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