Monday, September 10, 2012

Cross Domain Data Access via JavaScript: window.open


The Analysis

In 2008, we prepared a quick short demo  about "Cross-Domain Autcomplete Data Access" or "How Bad Guys Steal your Login Info Smartly". 

Let's learn about another not-so-old cross-domain vulnerability in Firefox 4 - Firefox 11 discovered by Jordi Chancel, Eddy Bordi, and Chris McGowen.  The bug relied on the Firefox's  processing of the JavaScript "window.open()" API.   The proof-of-concept exploit comprised of two components:
  • A client-side page that does a redirection trick with JavaScript API - history.back(), history.forward() and 
  • A server-side page that does a redirection trick with  JavaScript API - history.forward()  and a server-side timing redirection to an ARBITRARY web site 
Brandon the explained the root cause in a simple way in the  Bugzilla post:
When using window.open and some APIs to navigate the opened document, it is possible to navigate the opened document to a different site, while the location bar doesn't stay in sync with the new location.
The visible part was attacker's controlled web site with contents from his targeted web site. The background end result was being able to inject his controlled scripts into his targeted web sites. 

As we can learn from Chris's demo,  the execution of the JavaScript at the localhost was actually triggered at an off-site site, www.google.com, according to his mentioned script domain.domain property.   

The vulnerability went far beyond the URL spoofing.  In a normal URL spoofing vulnerability, the script execution ties only to the localhost itself. 

The issue was filed under Mozilla Advisory 2012-27 and CVE-2012-0474.

Lesson Learnt

Apparently, this window.open trick was used as URL spoofing test vector since the early days of Firefox 1.x/2.x and Internet Explorer 6.x.  It is surprised to see new versions of Firefox re-introduced the old issue. 

Finding zero-day or browser-based flaws give invaluable advantage to attackers as they do not need to exploit web applications; hence decreasing the chance of their attack being noticed and increasing the likelihoods of their attack against victims being successful.

It has been recommended to use a single-browser based approach on accessing critical web sites.  This advice was not feasible for those web 2.0 applications which have plethora of complex third-party integration. For example, you have to use your Google account to post comments or purchase applications from Google store.  

However, critical applications such as Banking should not rely third-party systems to do their operations so users can use the "single-browser, single domain" approach to safely use the service. 



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