CVE-2026-21509 — 365 Apps
HIGHCVE-2026-21509 is a high-severity (CVSS 7.8) CWE-807 vulnerability in microsoft 365 apps. It is in CISA's Known Exploited Vulnerabilities catalog (added 2026-01-26) — treat it as actively exploited and patch now. No vendor fix is recorded yet; mitigation options are listed below.
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Exploitation Status
Actively exploited in the wild
- Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2026-01-26. Federal agencies were required to remediate by 2026-02-16.
- A successful exploit gives an attacker total control of the affected component, not partial access.
Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2026-21509.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-21509 by exploitation likelihood (EPSS) against impact (CVSS). In the shaded patch-first corner (EPSS 50%+, CVSS 7.0+).
Where this sits among everything scored
Of 395,249 CVEs with a current EPSS score, this one falls in the 50–90% band (highlighted). Counts from FIRST.org, log-scaled.
Description
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Affected Products
365 appsmicrosoftofficemicrosoftoffice long term servicing channelmicrosoftResearch use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
gavz/CVE-2026-21509-PoC
Educational PoC for CVE‑2026‑21509 (Microsoft Office security feature by
kimstars/Ashwesker-CVE-2026-21509
CVE-2026-21509
suuhm/CVE-2026-21509-handler
PowerShell script to check, apply, and test the Kill-Bit protection for
SimoesCTT/CTT-NFS-Vortex-RCE
New Physics Disclosure This repository contains a full weaponized exploi
SimoesCTT/CTT-MICROSOFT-OFFICE-OLE-MANIFOLD-BYPASS-CVE-2026-21509
CVE-2026-21509 is a critical bypass in the Microsoft Office OLE (Object
decalage2/detect_CVE-2026-21509
YARA rule and python script to detect potential exploits for the CVE-202
Detection & mitigation playbook
Vendor / applianceDetect
Inventory every microsoft 365 apps deployment and check each version against the affected-products list above.
Remediation status
No patch has shipped for CVE-2026-21509 yet — track the microsoft 365 apps advisory for a fixed release and apply the workarounds below in the meantime.
Mitigate without a patch
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands. Alongside that, restrict the management interface to trusted networks and apply the vendor's recommended configuration mitigations.
Frequently Asked Questions
Is CVE-2026-21509 being exploited in your environment?
Detect and block the exploit chain at execution, on systems you cannot patch yet.