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CVE-2021-40456: Windows AD FS Security Feature Bypass Vulnerability

• Severity: medium (CVSS 5.0 v2.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-10-13 · updated 2026-06-17
• Weakness: CWE-863

Description

Windows AD FS Security Feature Bypass Vulnerability

Affected products

• Microsoft Windows Server: 20h2
• Microsoft Windows Server 2019: all versions
• Microsoft Windows Server 2022: all versions
• Microsoft Windows Server 2019: from 10.0.0 before 10.0.17763.2237 — fixed in 10.0.17763.2237
• Microsoft Windows Server 2022: from 10.0.0 before 10.0.20348.288 — fixed in 10.0.20348.288
• Microsoft Windows Server version 2004: from 10.0.0 before 10.0.19041.1288 — fixed in 10.0.19041.1288
• Microsoft Windows Server version 20H2: from 10.0.0 before 10.0.19042.1288 — fixed in 10.0.19042.1288

How to fix

Upgrade to a fixed version: Microsoft Windows Server 2019 10.0.17763.2237; Microsoft Windows Server 2022 10.0.20348.288; Microsoft Windows Server version 2004 10.0.19041.1288; Microsoft Windows Server version 20H2 10.0.19042.1288. Check the vendor advisory for mitigations.

References

• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40456
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-40456

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