The ability to run unsecure Office web add-ins and Catalogs must be disabled.

This policy setting allows users to run unsecure web add-in, which are add-ins that have web page or catalog locations that are not SSL-secured (https://), and are not in users' Internet zones. If you enable this policy setting, users can run unsecure apps. To enable specific unsecure web add-ins, you must also configure the Trusted Web add-in Catalog policy settings to trust the catalogs that contains those Add-ins. If you disable or do not configure this policy setting, unsecure web add-ins are not allowed.