CVE-2026-21509
HIGHCISA KEVEPSS 99th pctlDescription
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
How to fix
TridentStack Control can deploy fixes like this automatically across your Windows, macOS, and Linux fleet. See how it works
Remediation is compiled from vendor and distribution security advisories. Always confirm against the linked source for your exact version and platform.
CVSS v3 Vector
Exploitability
Impact
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H/E:F/RL:O/RC:C
Exploit Intelligence
Very high risk: more likely to be exploited than 99% of all known CVEs.
Microsoft Office Security Feature Bypass Vulnerability
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.
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Remediation due: 2026-02-16
References
Related Vulnerabilities
Other CWE-807 vulnerabilities, ordered by exploit likelihood. View all
| CVE | Severity | CVSS | EPSS | Exploited | Fix |
|---|---|---|---|---|---|
| CVE-2026-34486 | High | 7.5 | 81% | KEV | Fix |
| CVE-2024-13974 | High | 8.1 | 6.7% | - | Fix |
| CVE-2021-41129 | High | 8.1 | 1.8% | - | Fix |
| CVE-2026-21514 | High | 7.8 | 1.5% | KEV | - |
| CVE-2025-49827 | Critical | 9.8 | 1.4% | - | Fix |
| CVE-2021-31999 | High | 8.8 | 1.1% | - | Fix |
Common questions
How do I fix CVE-2026-21509?
Install KB5002713 for Microsoft Office 2016.
Is CVE-2026-21509 being actively exploited?
Yes. CVE-2026-21509 is listed in the CISA Known Exploited Vulnerabilities catalog, which means exploitation has been observed in the wild. US federal agencies were required to remediate it by 2026-02-16. Treat it as a priority whatever its CVSS score says.
How severe is CVE-2026-21509?
CVE-2026-21509 has a CVSS v3 base score of 7.8, rated important. CVSS rates the technical impact if the vulnerability is exploited, not how likely that is, so weigh it alongside the exploit-prediction score when you decide what to patch first.
What does CVE-2026-21509 affect?
Published advisories record a fix for Microsoft Office 2016 (Microsoft). Only products with a sourced advisory are listed, so treat this as what we can cite rather than a complete inventory.
Embed a live status badge for CVE-2026-21509
Markdown
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This product uses NVD data but is not endorsed or certified by the NVD. EPSS scores courtesy of FIRST.org (https://www.first.org/epss). Source: CISA KEV Catalog. Data as of 2026-06-25.