Microsoft Office Security Feature Bypass Vulnerability
Microsoft / Office
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.
- CVE identifier
- CVE-2026-21509
- Added to KEV
- Jan 26, 2026
- CISA catalog dueDate
- Feb 16, 2026
The dates above are catalog fields. The addition date does not establish when the vulnerability was discovered or first exploited. Confirm which requirements apply to your organisation separately.
Publisher'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.
This wording is attributed to the CISA snapshot. Verify current vendor guidance, affected versions and your deployment before applying changes.
Keep the signals separate.
- Ransomware use, as reported by CISA
- Unknown
- CVSS severity
- Not supplied by KEV
- EPSS probability
- 70.80%
EPSS score date: 2026-09-27T12:00:21Z. Percentile: 99.38%. Model version: v2026.06.15.
A dated estimate of exploitation probability over the following 30 days, not a severity score or a probability of a breach in your environment.
Inspect the EPSS source ↗“Unknown” ransomware use is an unknown source field, not proof that ransomware activity has not occurred.
Source notes
Please adhere to Microsoft’s recommended guidelines to address this vulnerability. Implement all final mitigations provided by the vendor for Office 2021, and apply the interim corresponding mitigations for Office 2016 and Office 2019 until the final patch becomes available. For more information please see: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509 ; https://nvd.nist.gov/vuln/detail/CVE-2026-21509
Source & freshness
Source updated: Sep 27, 2026, 9:30 PM UTC
Successfully checked: Sep 27, 2026, 11:50 PM UTC
This snapshot is older than 36 hours. Check current publisher guidance.
Automated source record, not an editorial advisory or an independent discovery. No affected or fixed version is inferred, and no organisation-specific risk rating is generated.