The JavaScript function getComputedStyle() and its related functions are fast and can be used to guess visitedness at hundreds of thousands of links per minute. These are particularly worrisome since they enable not only very focused attacks, but also the widespread brute-force attacks that are, in general, more useful to a variety of attackers (potentially including fingerprinting).
The biggest threats here are the high-bandwidth techniques, or those that extract lots of information from users’ browsers quickly. We think this represents the best solution to the problem, and we’ll be delighted if other browsers approach this the same way.
David Baron has implemented a way to help keep users’ data private while minimizing the effect on the web, and we are deploying it to protect our users. The most obvious fix is to disable different styles for visited versus unvisted links, but this would be employed at the expense of utility: while sites can no longer figure out which links you’ve clicked, neither can you. So this is a pretty old problem, and resurfaces every once in a while to generate more paranoid netizens. Originally specified as a useful feature for the Web, visited link styling has been part of the web for… well, forever. The result: not only can you see where you’ve been, but so can the web site! The problem is that appearance can be detected by the page showing you links, cluing the page into which of the presented pages you’ve been to.
This is just one of the many features web designers use to make the web the best it can be, and for the most part that’s a good thing. You’ve probably seen this before: in some cases, visited links are purple instead of blue. Links can look different on web sites based on whether or not you’ve visited the page they reference. It’s a tough problem to fix, though, so I’d like to describe how we ended up with this approach. We’re really excited about this fix, we hope other browsers will follow suit. We’re close to landing some changes in the Firefox development tree that will fix a privacy leak that browsers have been struggling with for some time.
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