CVE-2026-21509: Microsoft Office security feature bypass via untrusted inputs

CVE-2026-21509 · Severity: critical · CVSS 7.8 · Exploited in the wild · Published 2026-01-26

A security vulnerability in Microsoft Office could allow an attacker to bypass built-in security protections. This issue affects common productivity tools like Word and Excel, potentially allowing unauthorized actions on a user's computer if they are tricked into opening a malicious file. Successful exploitation could lead to a full compromise of the user's data and system access.

Technologies: Microsoft 365 Apps for Enterprise, Microsoft Office, Microsoft Office 2016, Microsoft Office 2019, Microsoft Office LTSC 2021, Microsoft Office LTSC 2024. Vendors: Microsoft.

Quick answers

Is CVE-2026-21509 exploited in the wild?
Yes. CVE-2026-21509 is known to be exploited in the wild, first recorded on 26 January 2026. Treat it as urgent.
How severe is CVE-2026-21509?
Critical severity, with a CVSS score of 7.8, published 26 January 2026.
Which products and versions are affected?
Microsoft 365 Apps for Enterprise 16.0.1 and later, x86 and x64 editions; Microsoft Office 2016 16.0.0 to 16.0.5539.1001, x86 and x64 editions; Microsoft Office 2019 19.0.0 to 16.0.10417.20095, x86 and x64 editions; Microsoft Office LTSC 2021 16.0.1 and later, x86 and x64 editions; and 1 more.
Is there a fix for CVE-2026-21509?
No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.
How is CVE-2026-21509 attacked?
It is attacked with local access, in a network appliance.
Am I affected by CVE-2026-21509?
Check whether you run Microsoft 365 Apps for Enterprise, Microsoft Office, Microsoft Office 2016 or one of the others listed at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

Technical details

A security feature bypass vulnerability exists in Microsoft Office (CWE-807) due to the application relying on untrusted inputs when making security-critical decisions. An attacker can exploit this locally by convincing a user to open a specially crafted file, leading to a bypass of intended security restrictions. The vulnerability affects multiple versions of Office, including Microsoft 365 Apps, Office 2016, 2019, and LTSC versions. Microsoft has released security updates to address this issue, and CISA has added this vulnerability to its Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation.

Affected products

  • Microsoft Microsoft 365 Apps for Enterprise 16.0.1 and later
  • Microsoft Microsoft Office 2016 16.0.0 to 16.0.5539.1001
  • Microsoft Microsoft Office 2019 19.0.0 to 16.0.10417.20095
  • Microsoft Microsoft Office LTSC 2021 16.0.1 and later
  • Microsoft Microsoft Office LTSC 2024 16.0.0 and later

Timeline

  • 2026-01-26: advisory: Initial publication by Microsoft and NVD
  • 2026-01-26: kev added: Added to CISA Known Exploited Vulnerabilities catalog

References

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