WCF Service Exception

I'm currently working on an Silverlight 3 project, I'm using 2 machines to test it.

"harbinger" is the web server running Win7 + IIS . I've deployed the webpage and the WCF webservice to that machine.

I've entered the following url's in my browser :

http://harbinger:43011/UserService.svc
http://harbinger:43011/UserService.svc?wsdl

and got pages load expected contents for both

Next I've decided to check if I can call the webservice from my machine, I've added the ServiceReference, executed a call to one of the methods and .... BOOM :

System.ServiceModel.CommunicationException was unhandled by user code
  Message="An error occurred while trying to make a request to URI 'http://harbinger:43011/UserService.svc'. This could be due to attempting to access a service in a cross-domain way without a proper cross-domain policy in place, or a policy that is unsuitable for SOAP services. You may need to contact the owner of the service to publish a cross-domain policy file and to ensure it allows SOAP-related HTTP headers to be sent. This error may also be caused by using internal types in the web service proxy without using the InternalsVisibleToAttribute attribute. Please see the inner exception for more details."
  StackTrace:
       at System.ServiceModel.AsyncResult.End[TAsyncResult](IAsyncResult result)
       at System.ServiceModel.Channels.ServiceChannel.SendAsyncResult.End(SendAsyncResult result)
       at System.ServiceModel.Channels.ServiceChannel.EndCall(String action, Object[] outs, IAsyncResult result)
       at System.ServiceModel.ClientBase`1.ChannelBase`1.EndInvoke(String methodName, Object[] args, IAsyncResult result)
       at Energy.USR.UserServiceClient.UserServiceClientChannel.EndGetAllUsers(IAsyncResult result)
       at Energy.USR.UserServiceClient.Energy.USR.UserService.EndGetAllUsers(IAsyncResult result)
       at Energy.USR.UserServiceClient.OnEndGetAllUsers(IAsyncResult result)
       at System.ServiceModel.ClientBase`1.OnA开发者_如何学CsyncCallCompleted(IAsyncResult result)
  InnerException: System.Security.SecurityException
       Message=""
       StackTrace:
            at System.Net.Browser.AsyncHelper.BeginOnUI(SendOrPostCallback beginMethod, Object state)
            at System.Net.Browser.BrowserHttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
            at System.ServiceModel.Channels.HttpChannelFactory.HttpRequestChannel.HttpChannelAsyncRequest.CompleteGetResponse(IAsyncResult result)
       InnerException: System.Security.SecurityException
            Message="Security error."
            StackTrace:
                 at System.Net.Browser.BrowserHttpWebRequest.InternalEndGetResponse(IAsyncResult asyncResult)
                 at System.Net.Browser.BrowserHttpWebRequest.<>c__DisplayClass5.<EndGetResponse>b__4(Object sendState)
                 at System.Net.Browser.AsyncHelper.<>c__DisplayClass2.<BeginOnUI>b__0(Object sendState)
            InnerException: 

Can someone explain what just happened? What do I need to do to avoid this?

See this blog post - I think that's the root cause of your problem:

Enabling cross-domain calls for Silverlight apps on self-hosted web services

or this one here:

WCF and Silverlight with cross-domain issue

Without any special policy files, you won't be able to call cross-domain in Silverlight.