Sunday’s New York Times included a story about how the presidential campaigns are making extensive use of third-party web trackers. In response to privacy concerns, “[o]fficials with both campaigns emphasize[d] that [tracking] data collection is ‘anonymous.’”1
The campaigns are wrong: tracking data is very often identified or identifiable. Arvind Narayanan has previously written a comprehensive and accessible explanation of why web tracking is hardly anonymous; my survey paper on web tracking provides more extensive discussion.
One of the ways in which web tracking data can become identified or identifiable is “leakage”—data flowing to trackers from the websites that users interact with. Leakage most commonly occurs when a website includes identifying information in a page URL or title. Embedded third parties receive the identifying information if they receive the URL (e.g. referrer headers) or the title (e.g.
document.title). Even a little identifying information leakage thoroughly undermines the privacy properties of web tracking: once a user’s identity leaks to a tracker, all of the tracker’s past, present, and future data about the user becomes identifiable.
Web services frequently fail to account for information leakage in their design and testing; a study I conducted last year found that over half of popular websites were leaking identifying information.2 More than a few website operators have made inaccurate representations about the information they share with third parties; in just the past year the Federal Trade Commission settled deception claims against both Facebook and Myspace for falsely disclaiming identifying information leakage.
The Times coverage piqued my curiosity: Are the campaigns identifying their supporters to third-party trackers? Are they directly undermining the anonymity properties that they are so quick to invoke?
Yes, they are. I tested the two leading candidate websites using the methodology from my prior study of identifying information leakage. Both leak. The following sections describe my observations from the Barack Obama and Mitt Romney campaign websites.