Microsoft Office Security Feature Bypass Vulnerability

[ Why it matters ]Priority signals

• Exploited in the wild: on CISA's Known Exploited Vulnerabilities list since 2026-01-26
• EPSS: 71% chance of exploitation in the next 30 days (higher than 99% of CVEs)

[ Description ]Official description

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

CISA: Microsoft Office contains a security feature bypass vulnerability in which reliance on untrusted inputs in a security decision in Microsoft Office could allow an unauthorized attacker to bypass a security feature locally. Some of the impacted product(s) could be end-of-life (EoL) and/or end-of-service (EoS). Users are advised to discontinue use and/or transition to a supported version.

[ Fixes ]Fixes and patches

CISA required action (US federal deadline 16 Feb 2026): Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ References ]References

• msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509Vendor Advisory
• www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-securit…Third Party Advisory
• www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-securi…MitigationThird Party Advisory
• www.cisa.gov/known-exploited-vulnerabilities-catalogUS Government Resource