Hijacking Opera’s Native Page using malicious RSS payloads

Well, this one is a continuation of my previous post on Cross Site Scripting issues relating to RSS feed readers. In that post, I mentioned Scenario (3), but didn’t discuss any details or PoC since Opera Team was actively fixing it. This issue is now fixed in the latest security update v10.01 from Opera Team.

In this exploit, an attacker uses a maliciously crafted RSS payload to achieve full control over the Victim’s Opera Browser. The attack works by convincing a user to visit a RSS feed link. When the user opens the url in Opera, there are two things that take place. The first one being Javascript in various RSS feed entries gets executed in the context of the calling site. This part was discussed in the previous post and can be used to execute XSS in the context of that site. The second thing that occurs is the untrusted rss feed content lands up in the Opera’s Feed Subscription Page (also the reason for this post). Since this is a native page, it runs in a higher privileged zone than the internet zone (something similar to chrome:// in Firefox and Chrome).

So, if you find a way to execute your malicious javascript in the feed subscription page, you can essentially execute native opera functions and ultimately use it to control the Victim’s Opera browser. It looks like Opera’s Team did think about the implications of putting untrusted user content in this page and hence only permitted a certain whitelist of html tags. In addition, for some html tags such as “A” and “IMG”, it required certain preconditions to be met. See the code snippets captured using Opera inbuilt debugger DragonFly (you can also use Firebug lite).

Whitelisted HTML Tags Definition – Opera Feed Subscription Page (Source – DragonFly)

HTML Tag Sanitizer/Filter Function – Opera Feed Subscription Page (Source – DragonFly)

If you had tried the simple xss attacks like <img src=”x:x” onerror=”some javascript”/> or something like <a onmouseover=”some javascript”>link</a>, these won’t work here (hint: check out preconditions defined above). It is important to understand what you are attacking and if read this code, you will figure out what constitutes a valid malicious payload that will evade this filter or sanitizer on the Opera Subscriptions Page.

So, here is an example PoC exploit code which executes the opera.feeds.subscribeNative function to automatically register a feed in Opera browser without user consent.

https://securethoughts.com/security/rssatomxss/opera10exploit2.atom
(Tested on Opera 10.00 Stable Build 1750)