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High 1 exploited CVE MSRC-CVE-2026-21509 1 CVE

Microsoft Office Security Feature Bypass Vulnerability

Published Jan 26, 2026 · Updated Jan 29, 2026 · Revision 7

Vendor: Microsoft

Summary

Impact: Security Feature Bypass. Microsoft rating: Important. Exploited according to Microsoft — Exploitation Detected. Customer action: Required.

Details

According to the CVSS metric, user interaction is required (UI:R). What interaction would the user have to do?
An attacker must send a user a malicious Office file and convince them to open it.

Are the updates for Microsoft Office 2016 and 2019 currently available?
Yes. As of January 26, 2026, the security update for Microsoft Office 2016 and 2019 is available. Customers running Microsoft Office 2016 and 2019 should ensure the update is installed to be protected from this vulnerability.

How do I know what version of Office 2016 and 2019 I am running?
On January 26 2026, Microsoft released build numbers for Office 2016 16.0.5539.1001 and Office 2019 16.0.10417.20095 to address this vulnerability.
To see what version you have installed:
In a document click the File tab.
Click Account in the left hand pane.
Click About . The top line of the About dialog box will display the Build number.

What kind of security feature could be bypassed by successfully exploiting this vulnerability?
This update addresses a vulnerability that bypasses OLE mitigations in Microsoft 365 and Microsoft Office which protect users from vulnerable COM/OLE controls.

Is the Preview Pane an attack vector for this vulnerability?
No, the Preview Pane is not an attack vector.

Security updates
16.0.10417.20095 — https://learn.microsoft.com/en-us/officeupdates/update-history-office-2019
https — https://docs.microsoft.com/en-us/officeupdates/office365-proplus-security-updates
16.0.5539.1001 — https://support.microsoft.com/help/5002713

Vulnerabilities 1

Revision history

  1. #7

    Updated mitigation. This is an informational change only.

  2. #6

    Acknowledgement added. This is an informational change only.

  3. #5

    Updated FAQ information. This is an informational change only.

  4. #1

    Information published.

  5. #2

    Corrected CVSS score. This is an informational change only.

  6. #3

    The following revisions have been made: 1) Microsoft is announcing the availability of the security updates for Microsoft Office 2016 and 2019. Customers running these versions of Office should install the update for their product to be protected from this vulnerability. Customers whose systems are configured to receive automatic updates do not need to take any further action. 2) Updated FAQ and Mitigations.

  7. #4

    The following revisions have been made: 1) Added the build numbers for Office 2019. 2) Added an FAQ describing how to view Office 2019 build numbers. 3) Updated Publicly Disclosed to No. 4) Added acknowledgement. These are informational changes only.

References