A post on comp.risks described a web adverstising service called "doubleclick". As described in its web pages at <URL: http://ad.doubleclick.net >, this service provides targetted advertising on the web. Participating web sites include links to doubleclick to show graphic images. Advertisers sign up with doubleclick and specify profiles for where and when they want their ads to appear. Doubleclick then selects an ad for each user who visits a participating site. Participating sites get paid for each such hit, and advertisers pay based on how many hits are expected. Apparently this is being used quite a bit. But what I found disturbing was the scope of the information being collected by doubleclick. The various parameters that advertisers can use in setting up their profiles for where their ads should appear are described at <URL: http://ad.doubleclick.net/advertising/adreghelp.htm > and include: TARGET BY WEB PAGE/SITE CATEGORY TARGET BY SERVICE PROVIDER (SP) TARGET BY GEOGRAPHIC LOCATION We determine a person's geographic location through the physical location of their network or through user registration. We have created an extensive map of both organization and Internet Service Provider (ISP) networks. TARGET BY USER'S OPERATING SYSTEM TARGET BY USER'S BROWSER TYPE TARGET BY HIGH-LEVEL INTERNET DOMAIN TYPE TARGET BY ORGANIZATION TYPE (SIC CODE) TARGET BY ORGANIZATION SIZE OR REVENUE TARGET BY PERSONAL INTERESTS doubleclick.net continues to add to an extensive database of user interests from activity on doubleclick.net member web sites as well as from publicly available sources like netnews. User interests are kept strictly confidential and will not be released to advertisers. We do allow advertisers to target ad banners based on user's interests. The more your ad banner is targeted at specific user interests, the more likely you are to generate a response. Personal interest categories include: Arts and Literature Business, Finance, and Economy Computers, Software and Internet Culture, Religion, and Society Education and InstructionalEntertainment Government, Politics, and Military Health and Medicine News Recreation and LeisureScience and Technology Social Science Sports Travel This last category is the really worrisome one. doubleclick monitors the web browsing habits of users whenever they hit a doubleclick- participating site, and builds up databases about users from that, as well as from usenet posts. This is exactly what people have been talking about as an abuse of privacy on the net. One question is whether enough information to uniquely identify users is routinely provided by widely used browsers like Netscape. I have refrained from telling my Netscape browser my name and email address out of fear that it would reveal this information; as a result, I can't use mailto: links, which is annoying (and also suspicious; lynx allows me to do mailto: without permanently entering an email address). This points out the need for browser providers to be sensitive to the privacy needs of their users and to clearly explain when and under what circumstances private information is revealed. It also suggests that services like www.anonymizer.com will be increasingly important for people to protect their privacy while browsing. Hal