Microsoft Office Security Feature Bypass Vulnerability
Microsoft — Microsoft 365 Apps for Enterprise
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
7.8
72.6%
Vector breakdown
- Attack vector
- Local
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
- Published
- Jan 26, 2026
- Modified
- Jul 30, 2026
- Added to KEV
- Jan 26, 2026
- Federal patch due
- Feb 16, 2026
CISA 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.
Weakness classification
Proof-of-concept & exploitation references
Weaponization
Public repositories whose name or description references CVE-2026-21509, found via a live GitHub search at request time. These are community-sourced signals, not a verified working exploit — cross-check each one before relying on it, and treat higher star counts and recent activity as (weak) corroboration, not proof.
No public GitHub repositories referencing CVE-2026-21509 were found at last check. Absence here is not proof no exploit exists — see the references below for vendor advisories and write-ups.
References
Frequently asked questions
What is CVE-2026-21509?
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
How severe is CVE-2026-21509?
CVE-2026-21509 has a CVSS base score of 7.8 out of 10 (CVSS 3.1).
Is CVE-2026-21509 actively exploited in the wild?
Yes. CVE-2026-21509 is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, added Jan 26, 2026, meaning CISA has confirmed evidence of active exploitation.
What is the EPSS score for CVE-2026-21509?
72.6% - meaning FIRST.org's EPSS model estimates a 72.6% probability this vulnerability will be exploited in the wild within 30 days (99% percentile among all scored CVEs).
What type of vulnerability is CVE-2026-21509?
CVE-2026-21509 is classified under CWE-807 (CWE-807: Reliance on Untrusted Inputs in a Security Decision).
Cross-checked against
Impactr finds and proves whether CVE-2026-21509 — or flaws like it — are actually reachable in your own web apps and APIs, with a reproducible exploit as evidence.