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What is CVE-2026-21509?
CVE-2026-21509 is a security feature bypass vulnerability in Microsoft Office. Microsoft’s description, reproduced in the NIST National Vulnerability Database (NVD) record, says that reliance on untrusted inputs in a security decision allows an unauthorized attacker to bypass a security feature locally.
The attack described by CSA requires user interaction: a person must be tricked into opening a malicious Office file. The cited advisory does not describe a no-click remote compromise. CSA’s alert, dated January 28, 2026, said emergency out-of-band updates had been released and that the vulnerability was reportedly being exploited in the wild.
Which Office versions are listed as affected?
The CSA advisory lists these product families. NVD’s affected configurations include 32-bit and x64 editions in the applicable families.
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- Microsoft Office 2016 and Office 2019
- Office LTSC 2021 and Office LTSC 2024
- Microsoft 365 Apps for Enterprise
This list identifies product families, not a universal fixed build. Microsoft’s release notes are organized by product and servicing channel, so use the Microsoft Office security update release notes and the current Microsoft Security Response Center (MSRC) record to check the build and instructions for your installation. The exact fixed build is not stated here because it varies by channel and current Microsoft guidance should take precedence.
What should you do?
| Office product or version | CSA guidance | What to verify |
|---|---|---|
| Office 2021 and later, including applicable LTSC editions | Restart Office applications; CSA says this automatically secures them. | Confirm your exact edition, platform, servicing channel, and current Microsoft instructions. |
| Office 2016 and Office 2019 | Update to the latest version immediately. | Check Microsoft’s current update and lifecycle guidance. Office 2019 support ended October 14, 2025. |
| Microsoft 365 Apps for Enterprise | The advisory includes this product family; follow the current instructions for its servicing channel. | Check the applicable Microsoft release notes and your organization’s deployment status. |
The restart-versus-update summary comes from CSA’s January 28, 2026 alert, which was updated April 30, 2026. For managed devices, administrators should identify the installed edition and channel, deploy the applicable update through their normal management process, and verify that deployment completed. Microsoft’s release notes show channel- and date-specific entries; a build number from one entry should not be treated as the universal fix.
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Why does the advisory call it serious?
Microsoft, as the CVE Numbering Authority (CNA), assigned a CVSS 3.1 score of 7.8 High. The vector is AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H: it records a local attack vector, low attack complexity, no required privileges, and required user interaction, with high potential confidentiality, integrity, and availability impacts. This is Microsoft’s rating recorded by NVD; NVD had not provided its own assessment in the cited record.
NVD also records the CVE’s inclusion in CISA’s Known Exploited Vulnerabilities catalog, with an addition date of January 26, 2026, and a federal remediation due date of February 16, 2026. These dates, along with CSA’s January alert, place the emergency-patch and exploitation reporting in January 2026—not October 2026.
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How to handle mixed Office installations
If an organization has several Office editions or servicing channels, do not apply a single action or build number to every device. Inventory product family, edition, platform, and channel, then match each installation to Microsoft’s current instructions. Prioritize completing the applicable restart or update and confirming its status; the CSA’s generation-level guidance is a useful starting point, not a substitute for channel-specific verification.
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