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Three Ways Sites Can Track Visitors Without Cookies, Part 2

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In part 1, I wrote about the EFF’s Panopticlick project and the implications for anonymity. I’ve got two more methods up my sleeve.

2. Use the cache.

Cookies aren’t the only thing your browser downloads and keeps around, and for good reason. Logos and other images with stable filenames don’t tend to change very often, so instead of re-downloading them each time you revisit a site your browser caches them on disk. Other external files like Javascript can also be cached. This makes surfing the web a lot faster for everyone.

Any time someone is able to send you a file that sticks around, though, they’ve got a way to figure out if you’ve been there before. And as Josh Duck outlined in his blog post, Abusing the Cache: Tracking Users without Cookies, it’s not too tough to embed a tracking code to track your user sessions whether or not you clear your cookies.

This isn’t too terrible – users can always clean their cache, and this is generally most useful for tracking individual users visits to a single site. If you could convince enough site owners to add your widget to their site, though, you might be able to get more interesting data.

3. Check which links the user has visited.

This isn’t a new technique, at least by web standards, you can see examples as early as 2006 by Jeremiah Grossman. CSS gives you the ability to set up custom styles for links – the default style, the style when the user hovers or clicks, and most importantly for this hack the style after the user has visited the link. Browsers styled visited links differently even back in the ancient days of the web, turning blue link text to purple to help you navigate.

Any site can create a list of links to other sites and, with a bit of Javascript, tell if you’ve visited those sites in the recent past. The list of links can be hidden from the users view, so they might not even notice what’s going on. Spyjax is one example implementation with source code.

This is limited since you have to explicitly check for each potentially-visited site. So you might be able to check to see if they’ve been to Facebook, but not get the list of every social networking site they’ve ever been to. On the other hand with browsers like Chrome and Firefox getting faster all the time, checking lots of links by brute force is more possible. Users can always limit or clear their browsing history to make this technique less effective.

Should I panic yet?

Not quite, but it’s always a good idea to be on the lookout for things that undermine the assumptions of privacy and anonymity that people tend to have while surfing the web.

We’ve looked at clever ways to track a user from visit to visit, from site to site, and to get information about other sites they’ve visited. But each can be defeated, so if you want more anonymity you can still have it. To be honest I worry more about malware stealing passwords, phishing sites tricking people into giving away bank account info, and companies that have lots of sensitive info being hacked or ordered to divulge info by government. None of those problems rely on new Javascript hacks or can be fixed by clearing the browser cache.

Found a new clever hack for tracking users? Got even more important privacy concerns that I missed? Please post in the comments below.

Three Ways Sites Can Track Visitors Without Cookies

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There’s an old joke about the Internet that’s important for two reasons. First the joke:

On the internet, nobody knows you're a dog

It’s important because it illustrates a key cultural and technological underpinning of the Internet: anonymity. The second reason it’s important is that it’s so old, printed in the New Yorker in 1993, which is basically old testament times in Internet years. So for decades, the web has allowed people to browse without telling or proving who they are. Though many sites would love if you created an account and logged in, the vast majority are perfectly happy to serve up pages to you without even knowing if you’re a person or a dog.

But there are many reasons to want to track a user from page to page or from site to site, and there are various ways to do it. The most common way involves cookies. Web developers need a way to create user sessions or else things users like (shopping carts, preferences, the ability to update your profile picture) are impossible to implement.

Cookies are pretty well understood, and users can turn them off or clear them out if they really want. Google Chrome, for example, has “Incognito Mode” which allows you to surf without saving cookies, history, etc. from session to session. Even with cookies off, though, maintaining a user session within a particular site by passing around a session id isn’t too hard. It’s trivial to do in PHP for example.

Most users are pretty comfortable with this state of affairs – Facebook knows who I am because I logged in, but I trust them. Amazon knows who I am but that’s cool because I’m shopping. Some other site doesn’t know who I am, but it knows that I’m the same person who clicked on the widget to change the language a couple minutes ago.

People start getting uncomfortable when you start tracking them across sites. People become even more uncomfortable when they no longer have control over their anonymity. Three recent techniques violate both of those comfort zones in limited ways.

1. The EFF’s Panopticlick project.

Follow the link above and click the “test me” button. Is your browser silently betraying you? This is a very clever hack based on the fact that browsers almost always send some information to web servers in http headers (the user agent, what type of content the browser is willing to accept, etc.). People have been misusing user agent headers to try to get Javascript working in multiple browsers for years. Panopticlick also checks for available plugins and fonts. Adding all this data up there’s enough variability from one browser to the next that you can apparently reliably identify individuals. The EFF has a great post on the information theory behind the project.

This doesn’t mean sites will know who you are, but they could use this information to know that you visited web page A, B and C whether or not you want them too. An ad network could use this info to track you across many sites. An unscrupulous site could sell this info, giving your browsing history away for cash, and if you log into a site that has personally-identifying info about you (email, shipping addresses, etc.) the history could potentially be tied back to a person.

Next post, I’ll talk about another way to track users without cookies and a way for a site to tell if you’ve visited other sites in the past. I’ll also tell you why you shouldn’t panic, though I admit a better writer would have told you that first.

Issues to examine in rape reporting

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A response to Taking Sides – Clashing Views in Mass Media and Society – Issue 5

In his memo to NBC news staff, Michael Gartner gives his rational for printing the name of the alleged rape victim in the William Kennedy Smith case and argues such names should be printed in most cases.  Katha Pollitt, on the other hand, says there are no good reasons to print a victim’s name.

Gartner has four main points to make.  First, the job of the news media is to disseminate news, not to cover it up or leave out facts that are important to the story.  Second, giving the victim the decision takes it out of the editors’ hands and rape is the only case in which this is even considered.  Third, not naming rape victims plays in to the stereotype that there is something shameful about being raped, where the rapist is the only one who should feel any shame.  And fourth, that news media consistently report the names of rape suspects, even if they haven’t been formally charged, and fairness dictates the same be done with the accuser.

Pollitt disagrees.  She says that the media often cover up things and leave out salient for other reasons, that anonymity for accusers is standard practice in America and not unfair, and that the press is uneven with its use of anonymity-demanding it for sources but denying it to rape victims.  Pollitt says that printing the name along with information about the accuser does not treat rape like other crimes because it call into question if the accuser was asking for it and that naming does nothing to dispel the stigma of being raped.

Personally I’m perfectly willing to keep a rape victim’s name anonymous.  As a rule I’m wary of anonymous sources and leaving names out of a story, but I think there are good reasons to, and the emotional pain that would come from publicity of rape is one of them.  Related to all of this is how the news media cover rape in general.  Working on the Transcript, we’ve come under fire for reporting on rape at all.  There are plenty of women (and probably some men) who don’t think the news media should even report a rape occurred on a campus this small, because people might be able to figure out who it was.  Pollitt made a point I think is telling when she asked where the media is at the thousands of Take Back the Night demonstrations-most likely, they’ve been specifically excluded.  Every year, dozens of date rapes and worse happen on this campus, yet no one ever hears about it.  Going to Take Back the Night amazes people-so many women have been violated, but no one ever brings the issue up.  That’s exactly the problem-because of the emotional distress and shame surrounding rape, many victims never even report it to the school, let along the police or the Transcript, who would happily plead their case and leave their name out of it.  The over-arching silence is not helping anybody and leads people to believe it’s not a problem.  It is a problem, but the media cannot expose it if no one is talking.