Method to detect cloned software being used on a client user unit. An initialization phase comprises: defining a tag value as being equal to an initial random value, opening a new record storing the tag value and introducing the tag value into the client user unit. An operating phase comprises: preparing a client message comprising the request and a value depending on the tag value; sending the client message to the server; and checking if the tag value of the client message is correct with respect to the stored tag value. If they do not match, the requested service is denied. If they do match, the method sends a server message to the user unit; updates the tag value with a new tag value; and stores the new tag value on the server and user unit.