Microsoft to disable Excel workbook links to blocked file types

Microsoft will begin disabling external workbook links to file types that are blocked by the Trust Center by default between October 2025 and July 2026, introducing a new FileBlockExternalLinks policy (and an HKCU registry key) so admins can re-enable links if needed. The change will cause workbooks referencing blocked types to show a #BLOCKED error or fail to refresh; Microsoft is warning admins and recommending they review workbooks and communicate impacts. This policy is one of multiple Office/Windows hardening steps Microsoft is taking to reduce abuse vectors (macros, ActiveX, blocked attachment types).