Microsoft to Block External Workbook Links to Unsafe File Types by Default

Microsoft announced a significant security enhancement for Excel users, revealing plans to block external workbook links to unsafe file types by default starting in October 2025.

This major change aims to strengthen workbook security by preventing potential security vulnerabilities that could arise from external links to blocked file formats.

Enhanced Security Measures Take Effect

The technology giant will implement a new group policy that expands File Block Settings to include external workbook links, giving administrators greater control over which file types can be accessed through external references.

When the policy takes effect, users attempting to create new references to blocked file types will encounter a **#BLOCKED errorwhile existing links will display their last successfully refreshed values but will no longer update with new data.

This security enhancement represents Microsoft’s continued commitment to protecting users from potential threats that could exploit external file connections.

The company has identified that external workbook links to certain file types pose security risks that warrant blocking by default.

Microsoft plans to begin the rollout in early October 2025, with the process continuing through late July 2026.

The implementation will occur in phases to ensure smooth transition for organizations worldwide.

Starting with Build 2509, users will receive warning notifications through a business bar when opening workbooks containing external links to blocked file types, alerting them to the upcoming changes.

By Build 2510, if administrators have not configured the policy, users will lose the ability to refresh existing external links or create new references to blocked file types.

The Workbook Links pane will clearly indicate which workbooks failed to refresh due to the blocking mechanism.

Organizations requiring continued access to external links for blocked file types can maintain previous functionality through specific configuration changes.

Administrators have two primary options: they can modify the registry setting by setting HKCUSoftwareMicrosoftOffice16.0ExcelSecurityFileBlockFileBlockExternalLinks to 0, or utilize Group Policy settings by disabling “File Block includes external link files” under Excel Group Policy Template.

Microsoft strongly recommends that administrators review existing workbooks within their organizations and communicate these changes to users who depend on external links for their daily workflows.

This proactive approach will help ensure business continuity and prevent unexpected disruptions.

The company advises organizations to assess their current usage of external workbook links and identify any dependencies on blocked file types.

This preparation period allows IT departments to determine whether they need to implement override settings or can safely adopt the enhanced security measures.

This change aligns with Microsoft’s broader security initiatives, including previous measures like blocking internet macros by default in Office applications, demonstrating the company’s ongoing efforts to create a more secure computing environment for enterprise and individual users alike.

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