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

High severity, CVSS 7.8. Actively exploited: in CISA KEV since 2026-01-26. EPSS: 70.8% chance of exploitation in the next 30 days.

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: affected versions not specified
• Microsoft Office: version 2016 only; version 2019 only
• Microsoft Office Long Term Servicing Channel: version 2021 only; version 2024 only

Published 2026-01-26. Last modified 2026-06-25.