Known exploited · Microsoft
CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability
What it is
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.
- Vendor
- Microsoft
- Product
- Office
- Added to KEV
- January 26, 2026
- CISA due date
- February 16, 2026
- Ransomware use
- Unknown
What CISA requires
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Forensic triage
No
What to do now
- Find every instance of Office you run, including the forgotten ones, and check its version against the vendor advisory.
- Apply the vendor's fix or mitigation, before February 16, 2026 if you are bound by CISA's deadline, sooner if it faces the internet.
- Assume it was tried against you already: look for signs of exploitation from the date it was added, not from the day you patched.
References
H2 Security
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Start freeSource: CISA Known Exploited Vulnerabilities Catalog (public domain), catalog version 2026.10.01.