System Security SecurityException

private void GetRSS()
 {
     try
     {
         WebRequest rssReq = WebRequest.Create("http://www.aljazeera.net/AljazeeraRss/845674de-e247-4149-b2c4-432d70b0076b/acefc2c1-1a68-4977-91c3-969026916497");

         //Create a Proxy
         WebProxy px = new WebProxy("http://www.aljazeera.net/AljazeeraRss/845674de-e247-4149-b2c4-432d70b0076b/acefc2c1-1a68-4977-91c3-969026916497", true);
         //   XPath="/rss/channel/item";
         //Assign the proxy to the WebRequest
         rssReq.Proxy = px;

         //Set the timeout in Seconds for the WebRequest
         rssReq.Timeout = 5000;
         try
         {
             //Get the WebResponse
             WebResponse rep = rssReq.GetResponse();

             //Read the Response in a XMLTextReader
             XmlTextReader xtr = new XmlTextReader(rep.GetResponseStream());

             //Create a new DataSet
             DataSet ds = new DataSet();
             //Read the Response into the DataSet
             ds.ReadXml(xtr);
             //Bind the Results to the Repeater
             Repeater1.DataSource = ds.Tables[3]; ;
             Repeater1.DataBind();
         }
         catch(Exception EX)
         {
             throw EX;
         }
         //ProcessRSSItem("http://www.bbc.co.uk/arabic/index.xml");

     }
     catch (Exception EX)
     {
         throw EX;
     }
 }