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CVE-2021-31205: Windows SMB Client Security Feature Bypass Vulnerability

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 2.9% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-05-11 · updated 2026-06-17

Description

Windows SMB Client Security Feature Bypass Vulnerability

Affected products

• Microsoft Windows 10 Version 1607: 20h2, 2004
• Microsoft Windows Server 2016: 20h2, 2004
• Microsoft Windows 10 2004: from 10.0.0 before 10.0.19041.982 — fixed in 10.0.19041.982
• Microsoft Windows 10 20H2: from 10.0.0 before 10.0.19042.982 — fixed in 10.0.19042.982
• Microsoft Windows Server version 2004: from 10.0.0 before 10.0.19041.982 — fixed in 10.0.19041.982
• Microsoft Windows Server version 20H2: from 10.0.0 before 10.0.19042.982 — fixed in 10.0.19042.982

How to fix

Upgrade to a fixed version: Microsoft Windows 10 2004 10.0.19041.982; Microsoft Windows 10 20H2 10.0.19042.982; Microsoft Windows Server version 2004 10.0.19041.982; Microsoft Windows Server version 20H2 10.0.19042.982. Check the vendor advisory for mitigations.

References

• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31205
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2021-31205

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