CVE-2026-21509

Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.

CVSS v3 vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Weaknesses (CWE)

CWE-807

Threats tracking this CVE

• Microsoft Office Zero-Day CVE-2026-21509: OLE Security Bypass Under Active Exploitation — HIGH
• CVE-2026-21509 - Microsoft Office Security Feature Bypass (CISA KEV) — HIGH
• CVE-2026-21509: Russian Hackers Exploit Microsoft Office Vulnerability Against Ukraine — CRITICAL
• APT28 Operation Neusploit: MS Office CVE-2026-21509 Espionage Campaign — CRITICAL
• APT28 Microsoft Office Security Feature Bypass (CVE-2026-21509) — CISA KEV, Targeting Ukraine & EU via COREPER-Themed Spear-Phishing — HIGH

References

• https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
• https://www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability
• https://www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509

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