CVE-2026-21509
Microsoft Office Security Feature Bypass Vulnerability
Does this matter?
Known to be exploited in the wild (CISA KEV, CISA remediation deadline 16 February 2026). Treat as an emergency change: patch or isolate now, then hunt for prior compromise.
Description
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
- CVSS 3.1
- 7.8 HIGHCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
- EPSS
- 70.80% probability · 99th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Listed 26 January 2026 · due 16 February 2026
- Weakness
- CWE-807
- Affected
- microsoft/365 apps · microsoft/office · microsoft/office long term servicing channel
- Source
- secure@microsoft.com
EPSS moved from 70.8% on 27 September 2026 to 70.8% on 26 October 2026. The dashed line is the ten per cent mark our analysts treat as “patch before the next change window”.
CISA notes
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. 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
References
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509Vendor Advisory
- https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerabilityThird Party Advisory
- https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerabilityMitigation, Third Party Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509US Government Resource
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.