CVE-2026-21509 — Reliance on untrusted inputs in a security decision in Microsoft Office allows a

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

01What is this vulnerability?

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

02Affected products

No structured affected-product list available — see references below for vendor advisories.

03Active exploitation status

Yes — actively exploited. Added to the CISA KEV catalog on 2026-01-26. Ransomware use: Unknown.

04Recommended remediation

• Patch to a fixed version listed in the vendor advisory (see references below).
• Mitigate with WAF rules, network egress filters, or feature flags where the patch is not yet available.
• Hunt historical logs for exploitation indicators — see Detection signatures below.

05Technical details

For the full vendor write-up, exploit chains, and reference implementations, see the references list in section 09.

06Detection signatures

Open the Sigma generator with a pre-filled prompt for this CVE to draft a starting detection in your stack of choice:

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07Related CVEs

No directly-cited follow-up CVEs in the KB record for this advisory. The references list in section 09 carries the vendor cross-references.

08Timeline

• Published: 2026-01-26T18:16:38.540
• Last modified: 2026-06-25T05:16:53.167
• Added to CISA KEV: 2026-01-26

09References

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