Apple's Tracking-Prevention Feature in Safari has a Privacy Bug
Last month, engineers at Google published a very curious privacy bug in Apple’s Safari web browser. Apple’s Intelligent Tracking Prevention, a feature designed to reduce user tracking, has vulnerabilities that themselves allow user tracking. Some details:
ITP detects and blocks tracking on the web. When you visit a few websites that happen to load the same third-party resource, ITP detects the domain hosting the resource as a potential tracker and from then on sanitizes web requests to that domain to limit tracking. Tracker domains are added to Safari’s internal, on-device ITP list. When future third-party requests are made to a domain on the ITP list, Safari will modify them to remove some information it believes may allow tracking the user (such as cookies).
The details should come as a surprise to everyone because it turns out that ITP could effectively be used for:
- information leaks: detecting websites visited by the user (web browsing history hijacking, stealing a list of visited sites)
- tracking the user with ITP, making the mechanism function like a cookie
- fingerprinting the user: in ways similar to the HSTS fingerprint, but perhaps a bit better
I am sure we all agree that we would not expect a privacy feature meant to protect from tracking to effectively enable tracking, and also accidentally allowing any website out there to steal its visitors’ web browsing history. But web architecture is complex, and the consequence is that this is exactly the case.
Apple fixed this vulnerability in December, a month before Google published.
If there’s any lesson here, it’s that privacy is hard—and that privacy engineering is even harder. It’s not that we shouldn’t try, but we should recognize that it’s easy to get it wrong.