Making a Service Available Across Domain Boundaries

Using for cross-domain communication requires guarding against several types of security vulnerability that can be used to exploit Web applications. Cross-site forgery is a class of exploits that becomes a threat when allowing cross-domain calls. This exploit involves a malicious Silverlight control transmitting unauthorized commands to a third-party service, without the user's knowledge. To prevent cross-site request forgery, Silverlight only allows site-of-origin communication by default for all requests other than images and media. For example, a Silverlight control hosted at http://contoso.com/mycontrol.aspx can only access services on that same domain by default – for example http://contoso.com/service.svc, but not a service at http://fabrikam.com/service.svc. This prevents a malicious Silverlight control hosted on the http://contoso.com domain from calling unauthorized operations on a service hosted on the http://fabrikam.com domain.

To enable a Silverlight control to access a service in another domain, the service must explicitly opt-in to allow cross-domain access. By opting-in, a service states that the operations it exposes can safely be invoked by a Silverlight control, without potentially damaging consequences to the data the service stores.

Silverlight 2 supports two different mechanisms for services to opt-in to cross-domain access:

• Place a clientaccesspolicy.xml file at the root of the domain where the service is hosted to configure the service to allow cross-domain-access.


• Place a valid crossdomain.xml file at the root of the domain where the service is hosted. The file must mark the entire domain public. Silverlight supports a subset of the crossdomain.xml schema.

To use a clientaccesspolicy.xml file to allow cross-domain access

1. Build a service than enables access by a Silverlight client. For more information about how to do this, see How to: Build a Service for Silverlight Clients.


2. Create a clientaccesspolicy.xml file that allows access to the service. The following configuration allows access from any other domain to all resources on the current domain.
   
   
   

   
   

   <?xml version="1.0" encoding="utf-8"?>
   <access-policy>
     <cross-domain-access>
       <policy>
         <allow-from http-request-headers="*">
           <domain uri="*"/>
         </allow-from>
         <grant-to>
           <resource path="/" include-subpaths="true"/>
         </grant-to>
       </policy>
     </cross-domain-access>
   </access-policy>

   
   

   
   


3. Save the clientaccesspolicy.xml file to the root of the domain where the service is hosted. If, for example, the service is hosted in http://fabrikam.com then the file must be located at http://fabrikam.com/clientaccesspolicy.xml.


4. The valid values for the headers attribute are the wildcard (“*”) which allows all headers that have not been blacklisted and a comma separated list of allowed headers. These allowed headers may use a wildcard suffix, for example, “X-CUSTOM-*”.


5. Alternatively, if you want to allow access form only one other domain, such as http://contoso.com, the clientaccesspolicy.xml should contain the following configuration.
   
   
   

   
   

   <?xml version="1.0" encoding="utf-8"?>
   <access-policy>
     <cross-domain-access>
       <policy>
         <allow-from http-request-headers="*">
           <domain uri="http://contoso.com"/>
         </allow-from>
         <grant-to>
           <resource path="/" include-subpaths="true"/>
         </grant-to>
       </policy>
     </cross-domain-access>
   </access-policy>

   
   

   
   


6. Test that the access is enabled by invoking the service from the other domain.

To use a crossdomain.xml file to allow cross-domain access

1. Build a service than enables access by a Silverlight client. For more information about how to do this, see How to: Build a Service for Silverlight Clients.


2. Create a crossdomain.xml file that contains the following configuration. The file must be configured to allow access to the service from any other domain, or it is not recognized by Silverlight 2.
   
   
   

   
   

   <?xml version="1.0"?>
   <!DOCTYPE cross-domain-policy SYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">
   <cross-domain-policy>
     <allow-http-request-headers-from domain="*" headers="*"/>
   </cross-domain-policy>

   
   

   
   


3. Save the crossdomain.xml file to the root of the domain where the service is hosted. If, for example, the service is hosted in http://fabrikam.com then the file must be located at http://fabrikam.com/crossdomain.xml.


4. Test that the service is enabled by invoking the service from the other domain.

* This article is collected from MSDN.