How severe is CVE-2026-21509?

CVE-2026-21509 has a CVSS score of 7.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-21509?

Affected package information for CVE-2026-21509 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2026-21509?

No fixed version has been published for CVE-2026-21509 yet. Monitor the advisory for updates, apply mitigations recommended by the vendor, and use O3 Security to track when a fix becomes available in your stack.

Is CVE-2026-21509 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-21509 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-21509?

CVE-2026-21509 has an EPSS (Exploit Prediction Scoring System) score of 12.1%, placing it in the 94th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score warrants monitoring and prompt remediation.

Is CVE-2026-21509 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2026-21509 (Microsoft Office Security Feature Bypass Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2026-01-26. CISA required all US federal civilian agencies to remediate this vulnerability by 2026-02-16. The required action was: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

What type of vulnerability is CVE-2026-21509?

CVE-2026-21509 is classified as CWE-807 (CWE-807). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

Which products and versions are affected by CVE-2026-21509?

CVE-2026-21509 affects microsoft 365 apps; microsoft office (2 versions, e.g. 2016, 2019); microsoft office long term servicing channel (2 versions, e.g. 2021, 2024). Check the affected-products list on this page against your deployed version to confirm exposure.

When was CVE-2026-21509 published, and has it been updated?

CVE-2026-21509 was published on January 26, 2026 and was last updated on February 11, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CISA KEV·Added 2026-01-26 — agencies required to remediate by 2026-02-16

CVE-2026-21509

HIGH

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

Published

Jan 26, 2026

Updated

Feb 11, 2026

Affected

0 pkgs

Patched

None yet

Exploits

None indexed

EPSS Exploitation Probability

via FIRST.org ↗

12.1%probability of exploitation in next 30 days

Moderate Risk94th percentile-3.24%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Description

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

Affected Products

3 products · 10 configurations

Application

365 appsmicrosoft

all

Application

officemicrosoft

2 versions

20162019

Application

office long term servicing channelmicrosoft

2 versions

20212024

Frequently Asked Questions

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

O3 Security · Impact-Aware SCA

Is CVE-2026-21509 in your stack?

O3 detects CVE-2026-21509 across dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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