using System.Web; 
using System; 
using System.Configuration; 
using System.Diagnostics; 
using System.Reflection; 
using System.Web.SessionState;   

namespace WebLibrary 
{
     public class CrossDomainCookie : IHttpModule
     {
         private string m_RootDomain = string.Empty;

         #region IHttpModule Members
         public void Dispose() 
         {  
         } 
         public void Init(HttpApplication context)
         {
             m_RootDomain = ConfigurationManager.AppSettings["RootDomain"];
             Type stateServerSessionProvider = typeof(HttpSessionState).Assembly.GetType("System.Web.SessionState.OutOfProcSessionStateStore");
             FieldInfo uriField = stateServerSessionProvider.GetField("s_uribase", BindingFlags.Static | BindingFlags.NonPublic); 

             if (uriField == null)
                 throw new ArgumentException("UriField was not found");

             uriField.SetValue(null, m_RootDomain); 

             context.EndRequest += new System.EventHandler(context_EndRequest);
         }
         void context_EndRequest(object sender, System.EventArgs e)
         {
             HttpApplication app = sender as HttpApplication;
             for (int i = 0; i < app.Context.Response.Cookies.Count; i++)
             {
                 app.Context.Response.Cookies[i].Domain = m_RootDomain; 
             }
         }
         #endregion
     }
 } 

<add key="RootDomain" value=".itivy.com"/> 

<httpModules>
    <add name="CrossDomainCookieModule" type="WebLibrary.CrossDomainCookie, WebLibrary"/> 
</httpModules>

<sessionState mode="StateServer" stateConnectionString="tcpip=127.0.0.1:42424" timeout="30" /> 

<modules>
    <add name="CrossDomainCookieModule" preCondition="managedHandler" type="WebLibrary.CrossDomainCookie, WebLibrary"/> 
</modules>