Actively exploited · Published Jan 26, 2026 · Assigned by microsoft
CVE-2026-21509Microsoft Office Security Feature Bypass Vulnerability
High 7.8 Exploited
In plain English
A high-severity security flaw in Microsoft 365 Apps for Enterprise from Microsoft. Attackers are actively exploiting it in the wild (listed by CISA on January 26, 2026). The chance it is exploited in the next 30 days is estimated at 70.8% (higher than 99.4% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
CISA guidance
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Federal agencies must remediate by Feb 16, 2026.
Official description
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Microsoft | Microsoft 365 Apps for Enterprise | 16.0.1 – before https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Office 2016 | 16.0.0 – before 16.0.5539.1001 |
| Microsoft | Microsoft Office 2019 | 19.0.0 – before 16.0.10417.20095 |
| Microsoft | Microsoft Office LTSC 2021 | 16.0.1 – before https://aka.ms/OfficeSecurityReleases |
| Microsoft | Microsoft Office LTSC 2024 | 16.0.0 – before https://aka.ms/OfficeSecurityReleases |
Weakness: CWE-807 · Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
References
- Microsoft Office Security Feature Bypass Vulnerability vendor-advisory, patch
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509 government-resource
- 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