CVE-2026-21509 Microsoft Office Security Feature Bypass Vulnerability
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.
From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.
Should You Worry?
- CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 26 Jan 2026.
- Rated High severity (CVSS 7.8, scored by Vendor).
- It has been in the catalogue for 284 days. Systems still unpatched now have almost certainly been scanned for it.
What It Is
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Affected Versions
| Product | Versions |
|---|---|
| Microsoft Microsoft 365 Apps for Enterprise | 16.0.1 to before https://aka.ms/OfficeSecurityReleases |
| Microsoft Microsoft Office 2016 | 16.0.0 to before 16.0.5539.1001 |
| Microsoft Microsoft Office 2019 | 19.0.0 to before 16.0.10417.20095 |
| Microsoft Microsoft Office LTSC 2021 | 16.0.1 to before https://aka.ms/OfficeSecurityReleases |
| Microsoft Microsoft Office LTSC 2024 | 16.0.0 to before https://aka.ms/OfficeSecurityReleases |
From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.
What To Do Now
- 1
Find it
Search your asset inventory and external attack surface for Microsoft Office. Include cloud images, appliances and anything a supplier runs for you.
Attack Surface Management - 2
Fix it
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Vulnerability Management - 3
Hunt for compromise
If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.
Incident Response - 4
Prove it is fixed
Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.
Penetration Testing
Vendor Advisories
The Compliance Clock
General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.
Not Sure If You Are Exposed?
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