Microsoft to Block External Workbook Links to Unsafe File Types by Default – Computer Emergency Response Team (Mauritius)

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 error while 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.