A new cumulative update (September 2026 CU) has been released for Office Online Server.
OOS Build: 16.0.10417.20204
Important before installation:
Updating Office Online Server requires removing the server from its farm and then recreating the farm or joining the updated farm. Follow the instructions in How to update Office Online Server.
Office Online Server CU – KB article describing the CU:
Office Online Server CU – CU download link:
Important notes
Note for this CU:
The release version of Microsoft Office Online Server must already be installed before applying this update.
In Excel Online, pasting may leave the source selected without changing the destination. The operation can fail without a sound or an error message. Microsoft resolves this issue in the September 22, 2026 update (KB4461632).
Included fixes and improvements
Office Online Server
This update addresses remote code execution and information disclosure vulnerabilities in Microsoft Excel. The KB lists the following security advisories: