Server Write permissions

Hi,

I’m developing an application that gives a user group the ability to update templates by imputing data into an HTML form. The form submits to an ASP page that writes new data or appends existing data contained in an XML file.

My question involves security issues and server permissions related to write permissions for this type of application. Will third party web hosting services allow write permissions of this kind? Or will I have to do the updates via a database?