CVE-2026-21509 on CISA KEV

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

01What CISA says about this vulnerability

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

02Required action — verbatim from CISA

Apply mitigations per vendor instructions or discontinue use of the product if mitigations are unavailable.

03Notes & references

• 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-…

04SARA's analyst layer — why this matters

This vulnerability is currently on the CISA KEV list, which CISA only adds CVEs to when there is reliable evidence of active exploitation in the wild. For federal civilian agencies, BOD 22-01 requires remediation by the due date above. For everyone else, KEV is the strongest "patch immediately" signal you can get from public threat intel.

05Affected products (summary)

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

06Detection

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