Weaponizing Pt 3: CodePen Redux

Note: CodePen has been notified of these problems and has fixed them. This is an analysis of how their original fix was bypassed and these techniques could possibly be applied to future redirection vulnerabilities.

Since my post about using code playgrounds as attack platforms, CodePen has added some JavaScript that will alert a user if the pen is trying to redirect them to a 3rd party domain. If you use the example redirect code from earlier in this series, you'll be presented with a prompt asking you if you'd like to leave CodePen. This article explains how to work around this code and carry out a redirection.

At the time of this writing, you can use Chrome developer tools or FireBug to see this code in action:

window.__canLeave = false;

window.onbeforeunload = function() {
    if (!__canLeave) {
        return "WARNING! You are leaving the safety of CodePen! Are you sure you want to leave?";
    }
};

setTimeout(function() {
    window.__canLeave = true;
}, 200);

In this code, they are hooking the onbeforeunload window event, which will execute their function when the browser window attempts to "unload" (read: navigate away from) the current page. If you tried our form-based redirect, you may have noticed that they will also attempt to overwrite form actions pointing to other domains.

Bypassing the Prompt
There are a couple of ways we can disable this prompting function. The first, blunt approach, is simply overwriting the window.onbeforeunload hook. We'll replace their function with a blank one of our own, then carry out our redirect.

<script>
window.onbeforeunload = function() {null;}
var target = 'win dow.loca tion="htt p://www.goo gle.com";';
eval(target.replace(/ /gi, ''));
</script>

A slightly more elegant approach is to not modify their function logic, but simply meet the function's requirements for a redirect. Because their __canLeave variable is attached to the window object, we can access it from our closure. So, let's just overwrite it:

<script>
window.__canLeave = true;
var target = 'win dow.loca tion="htt p://www.goo gle.com";';
eval(target.replace(/ /gi, ''));
</script>

Bypassing the Action Rewrite
This one is pretty easy. They're looking for form actions in our HTML, so we can overcome their form action filtering by dynamically assigning our form's action in JavaScript. Chaining our two techniques together gives us the following working redirect code:

<form id="bbb" />
<script>
window.__canLeave = true;
document.getElementById("bbb").action = "http://www.google.com";
document.getElementById("bbb").submit();
</script>

So Long And Thanks For All The Phish
So, besides open redirects, what else can we do with all of this? Well, CodePen has started rolling out professional accounts, and you no longer need Github to make one. Why not use their site to make a pen and go phishing?

Using data URIs combined with redirects, it would be easy to clone their login page in a pen. Add a message saying, "This pen is protected. Please log in to continue." We can now redirect any entered credentials to our own 3rd party site, log them for future use, then bounce the user to a real pen.

Summary
Although the examples in this series were trivial, they could easily be leveraged as a malicious attack platform. As CodePen begins to roll out more of their own features, especially those that people pay for, these techniques will become more exploitable.

Return to Part 1: Overview or Part 2: jsFiddle.

Weaponizing jsFiddle, CodePen, and JSApp

Update : This post is now part of a series. Read part 2 (jsFiddle) and part 3 (codepen).

Sites such as jsfiddle.net and codepen.io allow web developers to quickly prototype and share code. They provide an interactive environment, where you can experiment with HTML, CSS, and JavaScript, then view the results in real time. These code snippets can be saved and shared, and sites providing these services are quickly gaining popularity. While many developers may be wary of strange URL shorteners, they don't think twice to click links from these sites, as they believe they'll just be viewing a sandboxed code snippet.

Today we'll be turning these sites into open redirects. Open redirects are web applications that allow an attacker to redirect you to a site of their choosing, while playing on the user's trust of the vulnerable website. They can be used to launch phishing attacks and redirect users to malicious content.
You can read more about open redirects at the CWE and OWASP.

Although these sites have taken precautions to prevent trivial redirects (such as blocking JS's window.location), they've missed some other techniques which we'll outline below. Such techniques can allow an attacker to deliver malicious iframes or redirects, essentially transforming these sites into URL shorteners.

The Redirection Connection: jsFiddle.net
Both jsFiddle and CodePen attempt to block trivial redirects by stripping out window.location strings. Additionally, jsFiddle even sandboxes your code into a separate iframe. However, it still leaves you other options: either a form-based redirect or inclusion of an arbitrary iframe. After entering your code, you can save your fiddle, go to share, and send your target the URL for the full-screen results.

Using a form-based redirect, we can send a target to an arbitrary URL (in this case, gawker.com). We'll make a form element with an action of our target URL, then automatically submit our form.

Add the following to your HTML text area, save, and share the full-screen URL. 

<form id="fun" action="http://www.gawker.com" />
<script>document.getElementById("fun").submit();</script>

Our second, less sexy option, is an iframe to our target URL. You have a few options here: 1x1px iframe running browser exploits, a full-screen frame, serve up spam, or perhaps some type of UI-redressing techniques. You could also create an official-looking login form and phish for credentials.

For a full screen frame, use the following code in the HTML input area: 

<iframe src="http://www.gawker.com" style="width: 100%; height: 2000px" />

You get a lame jsFiddle nav bar on top, but luckily that won't be a problem using CodePen! You can read about how to remove the jsFiddle nav bar in part 2.

Redirection Indiscretion: CodenPen.io
Both of the above techniques also work on CodePen, and you don't even have the branded navigation bar on top! However, they've also given us a 2nd redirection option. Even though they attempt to strip out "window.location" strings, we can use the tried-and-true combo of eval() + fromCharCode(), and redirect our target to an arbitrary URL.

By converting our desired redirect to it's ASCII character codes, CodePen doesn't recognize our window.location string. You can use sites such as this one to convert your string. Here we've encoded: window.location = "http://www.google.com";

Click on New Pen, enter the code into your HTML section, save, and share the full screen URL.

<script>
eval(String.fromCharCode(119, 105, 110, 100, 111, 119, 46, 108, 111, 99, 97, 116, 105, 111, 110, 32, 61, 32, 34, 104, 116, 116, 112, 58, 47, 47, 119, 119, 119, 46, 103, 111, 111, 103, 108, 101, 46, 99, 111, 109, 34, 59));
</script>

This attack will work as long as the eval() function is available. Even if the CodePen filter attempts to translate fromCharCode() strings, you can utilize other encoding methods such as base64 and URL encoding to accomplish the same attack.

Another simple version of this breaks up the filtered string with whitespaces, then removes the spaces upon eval().

<script>
eval("win dow.loca tion='http://www.google.com';".replace(/\s/g,''));
</script>

Update: Codepen now includes updates to alert users of 3rd party redirects. You can learn how to disable these alerts in part 3.

Bonus round: JSApp.us
As a bonus, we'll also look at jsapp.us, which allows experimentation with NodeJS code. This can also be leveraged to create a redirection service. To share JSApp code, you'll need to create a free account. Click login from the commands on the right, create a new user, and enter your information. You can use a service such as mailinator.com (and its many different domains) to create a throw-away account. Once logged in, you can save your file and deploy it to a JSApp subdomain for sharing.

You can redirect a user to an arbitrary URL using the following code:

var http = require('http');
http.createServer(function (req, res) {
  res.writeHead(302, {'Location': 'http://google.com'});
  res.end();
}).listen();

 I'm sure a NodeJS coder can probably also come up with a couple of other neat tricks to take advantage of.

Summary
When you allow end-users to enter arbitrary code as part of the functionality for your web application, it's hard to do proper input validation. The sites we looked at today used a blacklisting approach (by stripping out certain "bad" strings) which we were able to bypass using basic HTML and JavaScript. Indeed, it could be argued that these features are a legitimate use of their applications. Essentially, it's trivial to turn the sites into nothing more than URL shorteners, which could be used for redirects, serving spam, phishing campaigns, and delivering malicious code.

With small adjustments or stricter filtering policies, weaponizing these platforms could become more difficult. This genre of webapp is a quickly growing, however, and we've only looked at a couple of the more popular sites.

Continue to part 2 of the series.