evercookie

Find the latest details, code, and implementations on github @ https://github.com/samyk/evercookie

DESCRIPTION

EXAMPLE

Cookie found: uid = currently not set Click to create an evercookie. Don't worry, the cookie is a random number between 1 and 1000, not enough for me to track you, just enough to test evercookies. Now, try deleting this "uid" cookie anywhere possible, then or

evercookie is written in JavaScript and contains portions in Java, SWF/ActionScript (Flash) and C# (Silverlight). Some backend pieces in PHP, but also available in Node.js and Django. Get the latest source from github: http://github.com/samyk/evercookie

FAQ

What is the point of evercookie? Evercookie is designed to make persistent data just that, persistent. By storing the same data in several locations that a client can access, if any of the data is ever lost (for example, by clearing cookies), the data can be recovered and then reset and reused. Simply think of it as cookies that just won't go away. PRIVACY CONCERN! How do I stop websites from doing this? Great question. So far, I've found that using Private Browsing in Safari will stop ALL evercookie methods after a browser restart. What if the user deletes their cookies? That's the great thing about evercookie. With all the methods available, currently thirteen, it only takes one cookie to remain for most, if not all, of them to be reset again. For example, if the user deletes their standard HTTP cookies, LSO data, and all HTML5 storage, the PNG cookie and history cookies will still exist. Once either of those are discovered, all of the others will come back (assuming the browser supports them). Why not use EFF's Panopticlick? Panopticlick is an awesome idea, however the uniqueness really only helps in consumer machines and typically not systems running in a business or corporation. Typically those systems are virtually identical and provide no difference in information where a home user's laptop would. Evercookie is meant to be able to store the same unique data a normal cookie would. Does this work cross-browser? If a user gets cookied on one browser and switches to another browser, as long as they still have the Flash Local Shared Object cookie, the Silverlight Isolated Storage, the Java JNLP PersistenceService or the Java CVE-2013-0422 exploit cookie, the cookie should reproduce in both browsers. Does the client have to install anything? No, the client simply uses the website without even knowing about the persistent data being set, just as they would use a website with standard HTTP cookies. Does the server have to install anything? The server must at least have access to the JavaScript evercookie file. Additionally, to use Local Shared Object (Flash Cookies) storage, the evercookie.swf file must be present, and to use the auto-generated PNG caching, standard caching and ETag storage mechanisms, PHP must be installed and evercookie_(png|etag|cache).php must be on the server. All of these are available in the download. Is evercookie open source? Yes, evercookie is open source. The code is in readable format without any obfuscation. Additionally, the PHP files are open source as is the FLA (Flash) code used to generate the SWF Flash object. You can compile the Flash object yourself or use the pre-compiled version (evercookie.swf). How does the PNG caching work? When evercookie sets a cookie, it accesses evercookie_png.php with a special HTTP cookie, different than the one used for standard session data. This special cookie is read by the PHP file, and if found, generates a PNG file where all the RGB values are set to the equivalent of the session data to be stored. Additionally, the PNG is sent back to the client browser with the request to cache the file for 20 years. When evercookie retrieves this data, it deletes the special HTTP cookie, then makes the same request to the same file without any user information. When the PHP script sees it has no information to generate a PNG with, it returns a forged HTTP response of "304 Not Modified" which forces the web browser to access its local cache. The browser then produces the cached image and then applies it to an HTML5 Canvas tag. Once applied, evercookie reads each pixel of the Canvas tag, extracting the RGB values, and thus producing the initial cookie data that was stored. How does the Web History storage work When evercookie sets a cookie, assuming the Web History caching is enabled, it Base64 encodes the data to be stored. Let's assume this data is "bcde" in Base64. Evercookie then accesses the following URLs in the background: google.com/evercookie/cache/b google.com/evercookie/cache/bc google.com/evercookie/cache/bcd google.com/evercookie/cache/bcde google.com/evercookie/cache/bcde- These URLs are now stored in history. When checking for a cookie, evercookie loops through all the possible Base64 characters on google.com/evercookie/cache/, starting with "a" and moving up, but only for a single character. Once it sees a URL that was accessed, it attempts to brute force the next letter. This is actually extremely fast because no requests are made to theserver. The history lookups are simply locally in JavaScript using the CSS History Knocker. Evercookie knows it has reached the end of the string as soon as it finds a URL that ends in "-".

USAGE

<script type="text/javascript" src="evercookie.js"></script> <script> var ec = new evercookie(); // set a cookie "id" to "12345" // usage: ec.set(key, value) ec.set("id", "12345"); // retrieve a cookie called "id" (simply) ec.get("id", function(value) { alert("Cookie value is " + value) }); // or use a more advanced callback function for getting our cookie // the cookie value is the first param // an object containing the different storage methods // and returned cookie values is the second parameter function getCookie(best_candidate, all_candidates) { alert("The retrieved cookie is: " + best_candidate + "

" + "You can see what each storage mechanism returned " + "by looping through the all_candidates object."); for (var item in all_candidates) document.write("Storage mechanism " + item + " returned: " + all_candidates[item] + "<br>"); } ec.get("id", getCookie); // we look for "candidates" based off the number of "cookies" that // come back matching since it's possible for mismatching cookies. // the best candidate is most likely the correct one </script>

SEE ALSO

BUGS

See CONTACT.

CONTACT

Questions or comments, email me: code@samy.pl. Visit http://samy.pl for more awesome stuff.

evercookie, by Samy Kamkar, 09/20/2010