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Generally it is not a good practice to provide modify permissions for your entire folder structure of the website. It is recommended that you use the App_Data folder and follow this tutorial
which explains how to use themes under the the protected folder. We have special mechanism for linking to items from the App_Data folder. As long as you register a folder as a theme folder under App_Data we create a special handler for it and then you can link to items without using App_Data in the path (as long as the resources are under a registered theme). It is recommended that you run with minimal permissions for the root folder and provide permissions only for specific files where you know that no sensitive data is stored.
the Telerik team