CVE-2026-21509: in the CISA KEV catalogue
Microsoft Office Security Feature Bypass Vulnerability. CISA added CVE-2026-21509 to the Known Exploited Vulnerabilities catalogue on 2026-01-26 with a default due date of 2026-02-16. Known ransomware campaign use: Unknown. Forensic triage flag: No.
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. (CISA description)
- Vendor / product
- Microsoft Office
- CWE
- CWE-807
- CISA required action
- Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- EPSS
- 71%, percentile 99 (scored 2026-11-03)
EPSS puts about 71 in 100 odds (71%) on exploitation activity being observed in the next 30 days. Its score is higher than or equal to that of 99% of scored CVEs. This is a model estimate about exploitation anywhere, not about your environment.
Vendor and reference links (from CISA)
Your own deadline under CISA BOD 26-04 depends on exposure, automatability and technical impact: work it out with the BOD 26-04 calculator.
Frequently asked questions
Is CVE-2026-21509 in the CISA KEV catalogue?
Yes. CISA added CVE-2026-21509 (Microsoft Office Security Feature Bypass Vulnerability) to the Known Exploited Vulnerabilities catalogue on 2026-01-26. CISA's default due date for it is 2026-02-16.
Is CVE-2026-21509 used in ransomware attacks?
CISA marks known ransomware campaign use as Unknown for CVE-2026-21509. That means CISA lacks confirmation, not that ransomware groups do not use it.
What should I do about CVE-2026-21509?
CISA's required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Sources
- CISA, Known Exploited Vulnerabilities Catalog (accessed 2026-11-03)
- FIRST, Exploit Prediction Scoring System (EPSS) (EPSS by FIRST; scores generated by Empirical Security)
- NVD record for CVE-2026-21509