Data spills in banner ads

Richard M. Smith (
Feb. 14, 2000

Most people who use the Internet probably do not realize that banner ads that they are seeing on Web pages are also sending information about them back to Internet marketing companies. In this write-up, I have put together examples of how one of these marketing companies, DoubleClick (, is receiving a great deal of sensitive information about people as they surf the Web. I chose to focus on DoubleClick because they are largest provider of banner ads on the Internet. Their servers currently send out more than a billion banner ads every day according to a recent company press release.

I have been tracking over the last couple of months, what information is being sent from my own computer to DoubleClick ad servers. I used a packet sniffer to do the monitoring. I found more than a dozen examples from different Web sites of information being transmitted to DoubleClick that most people would consider rather sensitive. All this information can be tied to me, because all transmissions to the DoubleClick ad servers also include the same unique ID number in a DoubleClick cookie. I found both personally identifiable information and transactional data being sent to DoubleClick servers.

Personal data I saw being sent to DoubleClick servers included:

Transactional data that was sent to DoubleClick included: In some cases, this information was explicitly being transmitted by Web sites to DoubleClick encoded in the URLs of banner ads. In other cases, the data is encoded in the URLs of the Web page themselves. The Web page URLs are sent to DoubleClick servers as referring URLs when banner ads are fetched.

Except for one banner ad from LifeMinders, all of the data is sent to DoubleClick when I viewed the Web pages. It was not necessary for me to click on the banner ads for information to be sent to DoubleClick servers.

At some Web sites, I found that personal data is accidentally being leaked in referring URLs. I reported these problems to the sites and they have fixed the leaks either by removing the banner ads from Web pages or removing the personal data from URLs.

The following tables provide details of the information I saw going to DoubleClick. Personal data and transactional data is color-coded in the URLs.

Personal identifable data sent to DoubleClick

Transaction information sent to DoubleClick