In a network comprising number of web sites and at least one simulator web page calls are simulated or monitored and the response to the calls, including content, re-directed calls and cookies are examined in order to identify whether the content of cookies has been written or modified illegally. Illegal modification of content of cookies is referred to as writing of data by a Buyer in cookies of a User that was directed to the Buyer by a Data Publisher, without the consent of the Data Publisher. In some embodiments when illegal cookies are identified an alert may be issued to a user.