CVE-2026-21509

What the flaw is

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.

Description as published by CISA. We have not independently reproduced it.

What CISA requires

Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.

This deadline is binding on US federal civilian agencies under BOD 22-01. It is not a legal deadline for anyone else, but it is a useful urgency signal — CISA sets it by how actively the flaw is being used, not by CVSS.

Public exploit code

We hold no public exploit references for this CVE. That means our dataset has none — it is not evidence that no exploit exists. CISA has confirmed this vulnerability is being exploited in the wild regardless.

Vendor advisories

• msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509
• nvd.nist.gov/vuln/detail/CVE-2026-21509

Primary sources

• NVD entry — CVSS score, CPE list and references
• CISA KEV catalog — the source for this page
• CVE Record at cve.org