Readers of this blog will probably already know that, up till the
middle of last year, it was possible to
venerable tradition of drawing hyperlinks to already-visited URLs in
purple instead of blue. Last year, though, David Baron came up with a defense
against history sniffing which has now been adopted by every major
browser except Opera. One fewer thing to worry about when visiting the
internets, hooray? Not so fast.
Imagine for a moment that the next time you visited an unfamiliar website and you wanted to leave a comment without creating an account, instead of one of those illegibly distorted codes that you have to type back in, you saw this:
Please click on all the chess pawns.
As you click on the pawns, they turn green. Nifty, innit? Much easier than an illegibly distorted code. Also easy for a spambot equipped with image processing software, but it turns out the distorted codes are not that hard for spambots anymore either and probably no one’s written the necessary image processing code for this one yet. Possibly also easier on people with poor eyesight, and there could still be a link to an audio challenge for people with no eyesight.
… What’s this got to do with history sniffing? That chessboard isn’t
really a CAPTCHA. All the squares have pawns on them. But each
one is a hyperlink, and the pawns linked to sites you haven’t visited
are being drawn in the same color as the square, so they’re invisible.
You only click on the pawns you can see, of course, and so you reveal to
the site which of those URLs you have visited. A little technical
cleverness is required—the pawns have to be Unicode dingbats, not
images; all the normal interactive behavior of hyperlinks has to be
suppressed; etcetera—but nothing too difficult. I and three other
researchers with CMU
Silicon Valley’s Web Security Group have tested this and a few other
such fake CAPTCHAs on 300 people. We found them to be practical,
although you have to be careful not to make the task too hard; for
details please see our
paper (to be presented at the IEEE Symposium on
Security and Privacy, aka
An attacker obviously can’t use an
interactive sniffing attack
like this one to find out which sites out of the entire Alexa 10K your
victim has visited—nobody’s going to work through that many
chessboards—and for the same reason, deanonymization
attacks that require the attacker to probe hundreds of thousands of URLs
are out of reach. However, an attacker could reasonably probe a couple
hundred URLs with an interactive attack, and according to Dongseok Jang’s study
of actual history sniffing (paper),
that’s about how many URLs real attackers want to sniff. It seems that
the main thing real attackers want to know about your browsing history
is which of their competitors you patronize, and that’s never going to
need more than a few dozen URLs.
On the other hand, CAPTCHAs are such a hassle for users that they cause 10% to 33% attrition in conversion rates. And users don’t expect to see them on every visit to a site—just the first, usually, or each time they submit an anonymous comment. Even websites that were sniffing history when it was possible to do so automatically, and want to keep doing it, may consider that too high a price. But we can imagine similar attacks on higher-value information, where even a tiny success rate would be worth it. For instance, a malicious site could ask you to type a string of gibberish to continue—which happens to be your Amazon Web Services secret access key, IFRAMEd in from their management console. Amazon has taken steps to make this precise scenario difficult, but I’m not prepared to swear that it’s impossible, and other cloud services providers may have been less cautious.
Going forward, we also need to think carefully about how new
web-platform capabilities might enable attackers to make similar
end-runs around the browser’s security policies. In the aforementioned
research project, we were also able to sniff history without
user interaction by using a webcam to detect the color of the light
reflecting off the user’s face; even with our remarkably crude
image processing code this worked great as long as the user held still.
It’s not terribly practical, because the user has to grant access to
their webcam, and it involves putting an annoying
flashing box on the screen, but it demonstrates the problem. We are
particularly concerned about WebGL
right now, since its
shader programs can perform arbitrary
computations and have access to cross-domain content that page
element’s information leakage rules. Right now it’s not possible to
put the rendering of a web page into a GL texture, so this couldn’t be
used to snoop on browsing history, but there’s legitimate reasons to
want to do that, so it might become possible in the future.