[HttpGet]
        public ActionResult MyAction(MyModel model)
        {
             //custom logic
             Sitecore.Security.Authentication.AuthenticationManager.Login("extranet\"+username,password);
        }

[HttpPost]
        public ActionResult MyAction(MyModel model, string anotherParameter)
        {
               //This always returns false
               if (Sitecore.Context.User.IsAuthenticated)
               {
                    //my custom logic.
               }

        }

<sessionState mode="InProc" cookieless="false" timeout="20" sessionIDManagerType="Sitecore.SessionManagement.ConditionalSessionIdManager">
  <providers>
    <add name="mongo" type="Sitecore.SessionProvider.MongoDB.MongoSessionStateProvider, Sitecore.SessionProvider.MongoDB" sessionType="Standard" connectionStringName="session" pollingInterval="2" compression="true" />
    <add name="mssql" type="Sitecore.SessionProvider.Sql.SqlSessionStateProvider, Sitecore.SessionProvider.Sql" sessionType="Standard" connectionStringName="session" pollingInterval="2" compression="true" />
  </providers>
</sessionState>