# CVE-2026-21509

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

- Page: https://www.vulneraibility.com/cve/CVE-2026-21509
- CVSS: 7.8
- Known exploited: yes, in CISA KEV since 2026-01-26; CISA due date 2026-02-16
- EPSS: 70.80%
- Unvetted proof-of-concept code on GitHub: 13 GitHub proofs of concept (first 2026-01-27)
- CISA SSVC: exploitation active, automatable no, technical impact total (assessed 2026-01-26)
- Weakness: CWE-807
- Published: 2026-01-26

## Affected versions

- Microsoft Microsoft 365 Apps for Enterprise: affected >= 16.0.1 and < https://aka.ms/OfficeSecurityReleases
- Microsoft Microsoft Office 2016: affected >= 16.0.0 and < 16.0.5539.1001
- Microsoft Microsoft Office 2019: affected >= 19.0.0 and < 16.0.10417.20095
- Microsoft Microsoft Office LTSC 2021: affected >= 16.0.1 and < https://aka.ms/OfficeSecurityReleases
- Microsoft Microsoft Office LTSC 2024: affected >= 16.0.0 and < https://aka.ms/OfficeSecurityReleases

A version outside these ranges is not a statement that it is unaffected.

## References

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

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Summary, impact and action are written by the vulneraibility.com AI analysis pipeline from the CVE record, NVD, CISA and FIRST data; verify against the references.