Ajax Authentication without DoS attack possible
public bool ValidateUser(string username, string password,bool rememberMe)
    {
        if(Membership.ValidateUser(username, password))
        {
            FormsAuthentication.SetAuthCookie(username, rememberMe);
            return true;
        }
        return false;
    }

function validateUser() {

            var username = $('#<%=UserName.ClientID %>').val();
            var pass =$('#<%=Password.ClientID %>').val();
                            $.ajax({
                type: "POST",
                url: "Services/LoginService.asmx/ValidateUser",
                data: "{'username ':'" + username + "','password':'" + pass + "','rememberMe':'" + $('#<%=chbRememberMe.ClientID %>').attr('checked') + "'}",
                contentType: "application/json; charset=utf-8",
                dataType: "json",
                success: function(data, status) { OnSuccessLogin(data, status); },
                error: OnErrorLogin
            });
        }