CVE-2026-21509
HighExploitedPublic exploitMicrosoft Office Security Feature Bypass Vulnerability
Published Jan 26, 2026 · Updated Jul 30, 2026
Key Facts
Patch immediately- CVE-2026-21509 is a high-severity vulnerability (CVSS 7.8 v3.1) affecting 365 Apps (Microsoft), Office (Microsoft), Office 2016 (Microsoft) and 7 more products.
- It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Jan 26, 2026. Public exploit code is available.
- EPSS estimates a 71% probability of exploitation in the next 30 days.
- Fixed versions are available, including Microsoft 365 Apps for Enterprise https://aka.ms/OfficeSecurityReleases, Office 2016 16.0.5539.1001, Office 2019 16.0.10417.20095.
- Brandefense priority: patch immediately, because it is listed as exploited in the wild.
CVSS v3.1
7.8
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
EPSS (30-day exploit likelihood)
71%
Higher than 99.3% of all scored CVEs
Exploited in the wild
Yes
4 catalog listings
Public exploit code
Available
Proof-of-concept or weaponized module
Get the full CVE-2026-21509 brief as a PDF
Every affected product and fixed release, exploitation status and a first 24 hours response checklist, emailed within minutes.
Description
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Known-Exploited Catalog Listings
| Catalog | Added | Remediation due | Ransomware use |
|---|---|---|---|
| CISA KEV | Jan 26, 2026 | Feb 16, 2026 | Not reported |
| Industry KEV feed | Jan 26, 2026 | Feb 16, 2026 | Not reported |
| CIRCL KEV | Jan 26, 2026 | Feb 16, 2026 | Not reported |
| ENISA EUVD | Jan 26, 2026 | n/a | Not reported |
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.
Affected Products
21 affected entries in total| Product | Affected versions | Fixed in |
|---|---|---|
| 365 AppsMicrosoft | See advisory | n/a |
| OfficeMicrosoft | 2016, 2019 | n/a |
| Office 2016Microsoft | >= 16.0.0 | 16.0.5539.1001 |
| Office 2019Microsoft | >= 19.0.0 | 16.0.10417.20095 |
| Office 2021Microsoft | See advisory | n/a |
Patched Releases
Microsoft 365 Apps for Enterprise
https://aka.ms/OfficeSecurityReleases
Office 2016
16.0.5539.1001
Office 2019
16.0.10417.20095
Microsoft Office LTSC 2021
https://aka.ms/OfficeSecurityReleases
Microsoft Office LTSC 2024
https://aka.ms/OfficeSecurityReleases
Timeline
Publicly disclosed
Jan 26, 2026
Added to CISA KEV
Jan 26, 2026
Added to Industry KEV feed
Jan 26, 2026
Added to CIRCL KEV
Jan 26, 2026
Added to ENISA EUVD
Jan 26, 2026
Last updated
Jul 30, 2026
Weakness Type
- CWE-807 Reliance on Untrusted Inputs in a Security Decision
References
- advisoryMicrosoft Office Security Feature Bypass Vulnerability
- advisoryhttps://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
- advisoryhttps://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
- exploitVulnCheck exploitation report
- exploitVulnCheck exploitation report
FAQ
Common Questions.
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Affected: 365 Apps (Microsoft); Office (Microsoft) versions 2016, 2019; Office 2016 (Microsoft) versions >= 16.0.0, among others. The full list is in the CVE brief.
It is exploited in the wild, listed in CISA KEV, Industry KEV feed, CIRCL KEV, ENISA EUVD since Jan 26, 2026.
Yes. It was added on Jan 26, 2026 with a federal remediation due date of Feb 16, 2026.
Public exploit code is available.
Fixed versions are available, including Microsoft 365 Apps for Enterprise https://aka.ms/OfficeSecurityReleases, Office 2016 16.0.5539.1001, Office 2019 16.0.10417.20095.
Brandefense priority: patch immediately, because it is listed as exploited in the wild.
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