CVEs / CVE-2026-21509
CVE-2026-21509: Microsoft Office Security Feature Bypass Vulnerability
- Known exploited in the CISA KEV catalog since 26 Jan 2026
- high 7.8 CVSS 3.1 (from the CNA)
- EPSS 70.7% chance of exploitation in the next 30 days, higher than 99% of CVEs (30 Oct 2026)
- Last changed at its sources
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Summary
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.
Description
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Affected and fixed versions
| Product | Affected | Fixed in |
|---|---|---|
| Microsoft Microsoft 365 Apps for Enterprise | from 16.0.1, before https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft Microsoft Office 2016 | from 16.0.0, before 16.0.5539.1001 | 16.0.5539.1001 |
| Microsoft Microsoft Office 2019 | from 19.0.0, before 16.0.10417.20095 | 16.0.10417.20095 |
| Microsoft Microsoft Office LTSC 2021 | from 16.0.1, before https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
| Microsoft Microsoft Office LTSC 2024 | from 16.0.0, before https://aka.ms/OfficeSecurityReleases | https://aka.ms/OfficeSecurityReleases |
Exploitation
| In CISA KEV since | 26 Jan 2026 |
|---|---|
| Due date for US federal agencies | 16 Feb 2026 |
| Used in ransomware campaigns | Unknown |
| 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. |
Severity
| CVSS 3.1 | 7.8 highCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:HScored by the CNA ([email protected]) |
|---|---|
| Weakness | CWE-807 |
References
- CVE-2026-21509 on NVD
- CVE-2026-21509 in the CISA KEV catalog
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509 (cited by CISA)
- https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509 Vendor Advisory
- https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability Third Party Advisory
- https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability Mitigation, Third Party Advisory
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509 US Government Resource
Sources
Exploitation status from the CISA Known Exploited Vulnerabilities catalog. EPSS scores from FIRST. Descriptions, severity and references from the National Vulnerability Database. Open-source package versions from OSV (CC-BY 4.0). Text is shown as each source states it; Stackurity does not edit it. Check the vendor's advisory before acting.
This product uses data from the NVD API but is not endorsed or certified by the NVD.