Sitefinity is not coming with web service providers by default. I am not quite sure that you will be able to achieve this easy. Basically Sitefinity use OpenAccess as abstraction over the database. We have abstraction over OpenAccess - DataProviders which abstracts the way how the data is loaded. So basically what you have to do is to implement DataProviders for web services for every persistent type which you will use. As you can imagine this is not an easy task.
Are you sure that the variant where you have Proxy -> Firewall -> Sitefinity -> Database won't be suitable for you? I mean to use a proxy to redirect the calls to the Sitefinity and put a firewall between the proxy and the Sitefinity.
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