Known exploited vulnerability
CVE-2026-21509
The short answer
CVE-2026-21509 affects Microsoft Microsoft 365 Apps for Enterprise and is rated high (CVSS 7.8). CISA added it to the Known Exploited Vulnerabilities catalog on 2026-01-26: attackers are using it, so patch it first.
What it is
Reliance on untrusted inputs in a security decision in Microsoft Office allows an unauthorized attacker to bypass a security feature locally.
Weakness: CWE-807.
Severity
7.8 high
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Exploited (CISA KEV)
Yes, since Jan 26, 2026
Federal due date Feb 16, 2026
EPSS, next 30 days
71%
Higher than 99% of scored CVEs
How it can be exploited
- Attack vector
- Local
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality impact
- High
- Integrity impact
- High
- Availability impact
- High
What CISA says to do
Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
Affected products
Microsoft 365 Apps for Enterprise, Microsoft Office 2016, Microsoft Office 2019, Microsoft Office LTSC 2021, Microsoft Office LTSC 2024. NVD's record lists the exact affected versions.
References
- msrc.microsoft.com/update-guide/vulnerability/CVE-2026-21509 Vendor Advisory
- www.vicarius.io/vsociety/posts/cve-2026-21509-detection-script-microsoft-office-security-feature-bypass-vulnerability Third Party Advisory
- www.vicarius.io/vsociety/posts/cve-2026-21509-mitigation-script-microsoft-office-security-feature-bypass-vulnerability MitigationThird Party Advisory
- www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-21509 US Government Resource
Questions
Is CVE-2026-21509 being exploited?
Yes. CISA added CVE-2026-21509 to its Known Exploited Vulnerabilities catalog on 2026-01-26, with a federal remediation due date of 2026-02-16.
How do I fix CVE-2026-21509?
CISA's required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
From NVD, CISA KEV, FIRST EPSS and OSV.dev, checked . Status on NVD: Analyzed. The description is NVD's text, quoted as published.
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