Whether you are aware or not, frames are commonly used in most of the websites we use, for various purposes such as widgets in mashups, containers for advertisements, at the least for loading arbitrary documents into web pages. To serve this purpose <iframe> is used, while <frameset> and <frame> which were initially used for navigation are made obsolete in HTML5.
Frames are used primarily to isolate untrusted content such as remote scripts of widgets/ads etc., from interacting with rest of the DOM. Frames comply with Same Origin Policy if they load remote pages. This means, if an iframe is loaded with a page from same domain, it allows DOM manipulations to and from its parent page. Where as if it is loaded with a page from a different domain, it will restrict DOM manipulations and provides an isolated environment. The below code snippets should make this clear.
<!-- This is allowed -->
<iframe src="sameDomainPage.html"> </iframe>
alert(frames[0].contentDocument.body); //works fine
<!-- This is **NOT** allowed -->
<iframe src="http://google.com"> </iframe>
alert(frames[0].contentDocument.body); //throws error
As a developer, our knowledge of using frames ends here. We are happy with the secure isolation of content and don’t care beyond. But if we look a little deep into how browsers decide what should go into a frame, the scenario becomes scary!
Frame Navigation-who decides what:
Though the same origin policy of browsers isolates frame content of different domains, it has nothing to do with navigation of frames. i.e., if you know the id of a frame, you can navigate it to a different URL. (The browser window can be considered as a top level frame, having a visible address bar which iframes lack).
//Open google.com/ig and execute this script in console
myFrame=document.getElementsByTagName('iframe')[2];
myFrame.src="http://google.com"
Did you see the problems here? So if your trusted components are in iframes, what if someone can redirect the frame to a malicious URL? Which iframe has the permission to navigate which other iframe on a complex mashup page like iGoogle? Even worse, can iframes on a different window/tab navigate the iframes on your page? What about popups? Do you feel the sense of insecurity now?
Well, the fact is, none of these are under the control of web developers. The policies used by browser vendors answer the above questions and developers should know these policies to understand how secure their websites are. Below are the navigation policies used by browsers. These are quite complex to have a deep understanding, so I just mention an outline :
1. Permissive policy: This policy prevailed in all browsers prior to 1999. It simply states that a frame can navigate any other frame. The frames can be in the same window or in different windows. This dangerous policy gave rise to what are called “Cross window attacks”. Several sites like CitiBank kept their password fields in iframes for secure sandboxing. So, an advertisement in another window or a popup containing malicious script can redirect the password frame to an evil frame and grab credentials! IE6, Safari 3 and Flash by default used permissive policy.
2. Window policy: Starting 2001, browsers implemented a new policy called window policy, in order to prevent the cross window attacks. As per this, a frame can navigate frames in its own window and hence external popups/ads cannot affect frames of a different window. However, this gave rise to new line of attacks called “Same window attacks”. Using this policy, advertisements in the same window or evil gadgets (widgets) in mashups (e.g., iGoogle) can redirect legitimate gadgets to malicious URLs. So Gadget hijacking became the fashion of the day due to this policy. Firefox 2 and Opera 9 followed this policy which is dangerous to work with today’s gadget rich Web 2.0 applications.
3. Child Policy: This is a stricter policy, which says that a frame can navigate only its direct child. IE6 team wanted to enable this by default but did not since majority of the existing sites are not compatible with this. Such a strict implementation will prevent even the navigation of legitimate frames which are from same origin as the parent. So this policy could not be implemented.
4. Descendant policy: This is another stricter policy which was designed by IE7 team and it provides the best trade-off between security and compatibility. As per this, a frame can navigate only its descendants. So this prevents the cross window as well as same window attacks. HTML5 working group has standardized this policy
 The above picture explains the policies better. The outer blue box is the browser frame, the inner blocks are iframes and the arrows indicate who can navigate the content of what. Notice that child policy is a subset of descendant., descendant is a subset of window., window is a subset of permissive policies. |
All modern browsers (IE8+, FF3+, Safari 3+, Opera >9) follow the descendant policy to navigate frames and hence are more secure than their older counterparts. This is one of the several hundred reasons why you should get rid of older browsers when you use modern web 2.0 applications.
While we discussed about frame navigation so far, the next problem is how do legitimate frames communicate with legitimate content. e.g., communication between partner gadgets in a mashup. Though there are hacks like using fragment identifier or APIs like HTML5 postMessage, attacks like Reply attack, Recursive mashups attack are made possible with a good understanding of these policies. I shall cover on these and few more interesting scenarios in my upcoming posts e.g., what happens if your website can be framed by attackers and leverage descendant policy for backdoor communication?
Reference: The Stanford security researchers have published an excellent research paper-"Secure Frame Communications in Browsers (pdf)", which is the motivation for this post. Due credits to the authors. Suggest you to go through it for better picture.
Happy secure coding :)