Stanford Center for Internet and Society

Click the local Home Depot ad and your email address gets handed to a dozen companies monitoring you. Your web browsing, past, present, and future, is now associated with your identity. Swap photos with friends on Photobucket and clue a couple dozen more into your username. Keep tabs on your favorite teams with Bleacher Report and you pass your full name to a dozen again. This isn't a 1984-esque scaremongering hypothetical. This is what's happening today.

A number of technologies have been touted to offer consumers control over third-party web tracking. This post reviews the tools that are available and presents empirical evidence on their effectiveness. Here are the key takeaways:

1. Most desktop browsers currently do not support effective self-help tools. Mobile users are almost completely out of luck.
2. Self-help tools vary substantially in performance.
3. The most effective self-help tools block third-party advertising.

Following the usage model in the FTC staff's 2010 preliminary online privacy report, this post is oriented towards the user who wants a simple, persistent, comprehensive solution such that with high confidence no third party collects her browsing history. We assume that some third-party trackers will use non-cookie tracking methods including supercookies and fingerprinting (e.g. Microsoft, KISSmetrics, Epic Marketplace, BlueCava, Interclick, Quantcast).

Thanks to Jovanni Hernandez and Akshay Jagadeesh for assisting with data collection, and to Arvind Narayanan and Peter Eckersley for input on drafts.

Despite all the attention they've received in the debates around online privacy, cookies are far from the only way to track a user. Broadly speaking, a website can either stash a unique identifier anyplace in the browser ("tagging")¹ or explore features of the browser until it becomes unique ("fingerprinting").² Tracking technologies that do not rely on cookies are often referred to as "supercookies," and they are widely viewed as unsavory in the computer security community because they continue tracking even when a user clears her cookies to preserve privacy. Sometimes a site will use a supercookie to "respawn" its original identifier cookie, creating a "zombie cookie" — the basis of several lawsuits.

In one of our recent FourthParty web measurement crawls we included a cookie clearing step to emulate a user's privacy choice. We observed that after clearing the browser's cookies an identifier cookie (named "MUID" for "machine unique identifier") respawned on live.com, a Microsoft domain. We dug into Microsoft's cross-domain cookie syncing code and discovered two independent supercookie mechanisms, one of which was respawning cookies. We contacted Microsoft with our observations, and we have collaborated to assist in rectifying the issues we uncovered. Here is what we know.

Thanks, once again, to Jovanni Hernandez and Akshay Jagadeesh for their indispensable research assistance.

(Jovanni Hernandez and Akshay Jagadeesh are the first authors of this study.)

Last week marked the twentieth anniversary of the public World Wide Web, and there is much to celebrate. The early web consisted of a few text pages linked together; the modern web supports audio, video, interactivity, complex storage, and even native applications. Both Microsoft and Google are now developing entire operating systems around web technologies.

Tools for measuring the web have not kept pace. Many studies still rely on HTTP header logging and static analysis of HTML, CSS, and JavaScript. Researchers who want to go beyond these simple tools are often forced to develop purpose-built software from scratch.

Today we're releasing FourthParty, an open-source platform for web measurement. FourthParty is built on Mozilla Firefox and the Add-on SDK, making it fast, modular, easy to use, multi-platform, and up-to-date with the latest web technologies. And FourthParty is already generating research results: it's the tool we've been using in our Tracking the Trackers studies (1, 2). To learn more and get started, visit fourthparty.info.

Last week we reported some early results from the Stanford Security Lab's new web measurement platform on how advertising networks respond to opt outs and Do Not Track. This week we're back with a new discovery in the online advertising ecosystem: Epic Marketplace,¹ a member of the self-regulatory Network Advertising Initiative (NAI), is history stealing.

Many thanks once again to research assistants Akshay Jagadeesh and Jovanni Hernandez.

Over the past several months researchers at the Stanford Security Lab have been developing a platform for measuring dynamic web content. One of our chief applications is a system for automated enforcement of Do Not Track by detecting the myriad forms of third-party tracking, including cookies, HTML5 storage, fingerprinting, and much more. While the software isn't quite polished enough for public release, we're eager to share some unexpected early results on the advertising ecosystem. Please bear in mind that these are preliminary findings from experimental software; our primary aims at this stage are developing the platform and validating the approach to third-party tracking detection. Many thanks to Jovanni Hernandez and Akshay Jagadeesh for their invaluable research assistance.

Late last week FTC Commissioner Rosch penned a column in which he repeated a number of hackneyed criticisms of Do Not Track. Senators McCaskill and Pryor articulated similar concerns at a recent hearing. This piece sequentially deconstructs Rosch's column and replies to each of his substantive critiques.

Do Not Track is on its way to becoming an Internet standard. In collaboration with Sid Stamm at Mozilla we've submitted an Internet-Draft to the IETF, specifying both the HTTP header syntax and the requirements for compliance.

This is just the beginning of the IETF's process and the evolution of the draft. But it's a transformative moment for web privacy: Do Not Track is now a formal standards proposal. Every browser, advertising network, analytics service, and social plug-in provider has a clear instruction manual on how to implement Do Not Track.

We owe a tremendous debt of gratitude to the colleagues and friends whose efforts have made Do Not Track a reality: Alissa Cooper, Peter Eckersley, Alex Fowler, John Mitchell, Ashkan Soltani, Lee Tien, and Harlan Yu. And we particularly thank Chris Soghoian, Do Not Track's unflagging champion for nearly two years.

Last Friday we submitted a comment to the FTC articulating our vision for Do Not Track. We expanded on a number of views already expressed on this blog: Do Not Track is about much more than behavioral advertising, an HTTP header is the right implementation, and Do Not Track is no threat to ad-supported businesses. Here are the new highlights. (For a fuller exposition of each, please see our comment.)