CVE-2026-21509

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

Score

CVSS 3.1 7.8 High

Status

Analyzed - secure@microsoft.com

Published

2026-01-26

Last modified

2026-07-30

Known exploited (CISA KEV)

since 2026-01-26, action due 2026-02-16: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

Weaknesses

CWE-807

Affected products

• 365 Apps (Microsoft)
• Office (Microsoft)
• Office 2016 (Microsoft)
• Office 2019 (Microsoft)
• Office 2021 (Microsoft)
• Office 2024 (Microsoft)
• Office Long Term Servicing Channel (Microsoft)

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-vulnera…
• https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulner…
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509