Vulnerability in Microsoft 365 Apps
CVE-2026-21509
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.8 (High). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H.
EPSS exploit prediction
EPSS: 0.708 (99.4th percentile), scored .
High probability of exploitation in the next 30 days — patch promptly. 99th percentile — 99.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.726 · newest: 0.708 · change: -0.018
Affected products
- Microsoft 365 Apps
- Microsoft 365 Apps For Enterprise — versions 16.0.1
- Microsoft Office 2016 — versions 16.0.0
- Microsoft Office 2019 — versions 19.0.0
- Microsoft Office Ltsc 2021 — versions 16.0.1
- Microsoft Office Ltsc 2024 — versions 16.0.0
- Microsoft Office — versions 2016, 2019
- Microsoft Office Long Term Servicing Channel — versions 2021, 2024
Weakness classification (CWE)
CISA KEV (Known Exploited Vulnerabilities)
This CVE is on the CISA KEV catalog, added on . CISA KEV inclusion means CISA has confirmed in-the-wild exploitation; US federal agencies are required to remediate within a published due date.
BOD 22-01 due date: .
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.
Public proof-of-concept exploits
- gavz/CVE-2026-21509-PoC
- kimstars/Ashwesker-CVE-2026-21509
- suuhm/CVE-2026-21509-handler
- SimoesCTT/CTT-MICROSOFT-OFFICE-OLE-MANIFOLD-BYPASS-CVE-2026-21509
- decalage2/detect_CVE-2026-21509
- SimoesCTT/CTT-NFS-Vortex-RCE
- ksk-itdk/KSK-ITDK-CVE-2026-21509-Mitigation
- SimoesCTT/SCTT-2026-33-0007-The-OLE-Vortex-Laminar-Bypass-
- razureink/cve-2026-21509-office_security_bypass_reproduction
- sentinel-aidefense/CVE-2026-21509
References
- Microsoft Office Security Feature Bypass Vulnerability (vendor-advisory, patch, Vendor Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Third Party Advisory)
- af854a3a-2127-422b-91ae-364da2661108 (Mitigation, Third Party Advisory)
- 134c704f-9b21-4f2e-91b3-4a467353bcc0 (US Government Resource)
Frequently asked questions
- What is CVE-2026-21509?
- CVE-2026-21509 is a high-severity vulnerability in Microsoft 365 Apps, classified under CWE-807. CVSS score: 7.8/10. Published 2026-01-26.
- How severe is CVE-2026-21509?
- High severity. CVSS v3 base score is 7.8 out of 10.
- Is CVE-2026-21509 known to be exploited?
- Yes. CVE-2026-21509 is listed in the CISA Known Exploited Vulnerabilities catalog (added 2026-01-26), indicating it is being actively exploited. 13 public proof-of-concept repositories are indexed.