Microsoft will introduce a major security upgrade to Microsoft 365 apps, starting October 2025, by disabling external workbook links to blocked file types by default. This move extends File Block Settings to include external links, marking a shift in how workbooks manage data sources. Once enforced, new references to blocked files will trigger a LOCKED error, while existing links will remain static and not refresh.
The rollout will begin in October 2025 and complete by July 2026, through phased build releases. Build 2509 will issue warning notifications, while Build 2510 will enforce the restrictions. A new Workbook Links pane will highlight failed updates to enhance transparency for users.
Organizations needing continued access can override the settings by:
- Editing the registry:
HKCU\Software\Microsoft\Office\16.0\Excel\Security\FileBlock\FileBlockExternalLinks - Updating group policy via Excel’s Trust Center > File Block Settings.
Microsoft urges enterprises to audit current workbooks ahead of the transition. For companies without configured policies, changes won’t take effect immediately, allowing time for proper planning.
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