Myjson Crawler

What is myjson.com

Myjson describes itself as “a simple JSON store for your web or mobile app”. You can save JSON data using an interface on the site itself  (example) or programmatically via an API  (example). Each saved piece of JSON can be accessed via a randomly-generated URL, but unfortunately the random part of the URL is very short: it seems to be three or four characters drawn from a limited alphabet. This means that it’s easy to guess valid URLs, or even to enumerate all of them. OWASP describe this class of problem as an “insecure direct object reference”, and list this at fourth place in their 2013 Top 10 list of security risks. It’s not fair to criticise Myjson for this as they never advertised their system as secure, but I think it is fair to take a look at what people are storing.

Crawler implementation

The most obvious way (to me, at least) to implement a crawler is as follows:

  1. Generate a list of URLs using scripting language du jour
  2. Use xargs and cURL to crawl.

This has a couple of advantages:

  • It’s really simple
  • xargs has a handy multi-threaded mode allowing us to crawl several pages in parallel.

Unfortunately that would be too easy, so I decided to use JavaScript.

How it works

Full implementation available on Github

We’re going to need a function to output results. I decided to output HTML of this form:

<dl>
<dt>aaa</dt><dd>{ "json" : "that came from", "url" : "myjson.com/aaa" }</dd>
<!-- ... -->
</dl>

Here’s a tiny utility function to create this output:

/**
* Adds a row to the list of results with the query and the response
* @param {string} query
* @param {string} response
*/
var outputResult = function (bin, contents) {
    'use strict';

    var binElement = document.createElement('dt'),
    contentsElement = document.createElement('dd');

    binElement.textContent = bin || '';
    contentsElement.textContent = contents || '';

    document.getElementById('results').appendChild(binElement);
    document.getElementById('results').appendChild(contentsElement);
};

We’ll also need a function to crawl the site. XMLHttpRequest is the obvious tool. We write output as each response comes back using an anonymous function which
closes over the current bin name.

/**
* Looks up the contents of a myjson bin and adds them to the list of results
* @param {string} bin
*/
var lookupBin = function (bin) {
    'use strict';

    var xhr = new XMLHttpRequest();

    xhr.open('GET', 'http://api.myjson.com/bins/' + bin);
    xhr.onload = function () {
        if (this.status === 200) {
            outputResult(bin, this.responseText);
        }
    };
    xhr.send();
};

Finally we need to iterate over the possible bin names. Some nested loops would handle this well enough, but it’s probably tidier to encapsulate this functionality. Here’s a function to iterate over an alphabet of characters:

/**
* Iterates over a list of characters
* @param {string} alphabet List to iterate across
* @param {string} prefix String to prepend before calling CALLBACK
* @param {function} callback Callback function, called with current string
*/
var iterateCharacters = function (alphabet, prefix, callback) {
    'use strict';
    var i;
    for (i = 0; i &lt; alphabet.length; i = i + 1) {
        callback(prefix + alphabet[i]);
    }
};

For each character in the alphabet we prepend an existing string and then pass the result on to the defined callback. Iterating over all three-character bin names is then simple. This example calls our output function directly without crawling each bin:

var alphabet = 'abcdefghijklmnopqrstuvwxyz0123456789';

var run = function () {
    'use strict';

    iterateCharacters(alphabet, '', function (string) {
        iterateCharacters(alphabet, string, function (string) {
            iterateCharacters(alphabet, string, outputResult);
        });
    });
};

run();

Finally we need an HTML document to host all this:

<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <dl id="results"></dl>
    <script type="text/javascript" src="script.js"></script>
  </body>
</html>

Browsers try hard to download data as quickly as possible, and our crawl runs in parallel across several threads without extra effort.

Results

Scope

I restricted the crawl to include three-character names beginning with the letters ‘a’ to ‘d’. The above code will crawl all combinations of three-character bin names, and can also be easily extended to crawl four- and five-character names. It’s sensible to reduce the scope, however:

  • Crawling lots of bins takes lots of time.
  • It isn’t nice to load myjson.com heavily.
  • Thoughts of Weev make me nervous.
  • Brief Analysis

    The sample includes 1,637 rows. The top ten JSON strings are as follows:

    String Count
    “{}” 226
    “{“key”:”value”}” 92
    “{“foo”:”bar”}” 42
    “{“hello”:”world”}” 34
    “{“key”:”value”,”key2″:”value2″}” 30
    “{“glossary”:{“title”:”example glossary”,… 29
    “{“key_updated”:”value_updated”}” 26
    “[]“ 23
    “{“test”:”test”}” 17
    “{“key”:”updated value”,”key2″:”updated value2″}” 16

    We can therefore estimate that around 14% of bins contain only the empty object. Many of the examples above seem likely to have been created to test the service, and 69% of the extracting strings contain only 50 characters or fewer.

    It will be interesting to run a similar scrape in the future and see if the distribution of data changes:
    how many people are using this service as intended?

    Scope for Evil

    The API supports updating JSON data by sending an HTTP PUT request. It would only take a few minutes to overwrite all data stored by Myjson. Myjson doesn’t advertise a secure service, and they obviously aren’t worried that data is disclosed. They ought to be worried that somebody might trash everything they have stored.

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