What’s Vulnerable › Vulnerabilities

CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability

• Severity: high (CVSS 7.8 v3.1)
• Exploited in the wild: yes — in CISA KEV since 2026-01-26
• EPSS: 70.8% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-01-26 · updated 2026-07-30
• Weakness: CWE-807

Description

Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.

Affected products

• Microsoft 365 Apps: all versions
• Microsoft Office: 2016, 2019
• Microsoft Office LTSC for Mac 2021: 2021, 2024
• Microsoft 365 Apps: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
• Microsoft Office LTSC 2021: from 16.0.1 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases
• Microsoft Office: from 16.0.0 before 16.0.5539.1001 — fixed in 16.0.5539.1001
• Microsoft Office LTSC 2024: from 16.0.0 before https://aka.ms/OfficeSecurityReleases — fixed in https://aka.ms/OfficeSecurityReleases

How to fix

Upgrade to a fixed version: Microsoft 365 Apps https://aka.ms/OfficeSecurityReleases; Microsoft Office LTSC 2021 https://aka.ms/OfficeSecurityReleases; Microsoft Office 16.0.5539.1001; Microsoft Office LTSC 2024 https://aka.ms/OfficeSecurityReleases. Check the vendor advisory for mitigations.

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
• https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
• https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509

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