Tuesday, October 15, 2013

Exploiting EasyXDM part 2: & considered harmful

tldr: URL parsing is hard, always encode stuff and Safari has some interesting properties...

This is a second post describing easyXDM vulnerabilities. Reading the first part might come in handy:


"EasyXDM is a Javascript library that enables you as a developer to easily work around the limitation set in place by the Same Origin Policy, in turn making it easy to communicate and expose javascript API’s across domain boundaries". Vulnerabilities were found in 2.4.16 version, and are patched in 2.4.18. They are tracked with a single CVE-2013-5212.

In first post I've described XSS vulnerability in Flash transport used by that library, however the exploit conditions were very limiting. On websites using easyXDM the following code (used e.g. to set up RPC endpoints):
<script type="text/javascript" src="easyXDM.debug.js">
<script type="text/javascript">
    var transport = new easyXDM.Socket({
        local: ".",
        swf: "easyxdm.swf",
can cause XSS when it's loaded by URL like: http://example.com?#xdm_e=https%3A%2F%2Flossssscalhost&xdm_c=default7059&xdm_p=6&xdm_s=j%5C%22-alerssst(2)))%7Dcatch(e)%7Balert(document.domain)%7D%2F%2Feheheh. That will force easyXDM to use vulnerable Flash transport and pass the injected XSS payload. However, the payload will only be used unless Flash file is set up with FlashVars parameter log=true.

Mom, where do flashvars come from?

Let's dig deeper. How is the HTML for the SWF inclusion constructed? Looking at the source code at GitHub (FlashTransport.js):
function addSwf(domain){
  // create the object/embed
  var flashVars = "callback=flash_loaded" + domain.replace(/[\-.]/g, "_") + "&proto=" + 
      global.location.protocol + "&domain=" + getDomainName(global.location.href) + "&port=" +   
      getPort(global.location.href) + "&ns=" + namespace;
  // #ifdef debug
  flashVars += "&log=true";
  // #endif
  swfContainer.innerHTML = ... + "<param name='flashvars' value='" +
  flashVars +
  "'></param>" ....
This 'debug' flag is a preprocessor instruction. The #ifdef / #endif block of code will only be included in easyXDM.debug.js file:
<!-- Process pre proccesing instructions like #if/#endif etc -->
<preprocess infile="work/easyXDM.combined.js" outfile="work/easyXDM.js"/>
<preprocess infile="work/easyXDM.combined.js" outfile="work/easyXDM.debug.js" defines="debug"/>
Exploiting easyXDM.debug.js file described in the first post was straightforward. But if production version of easyXDM library is used instead, there is no log parameter and XSS won't work. What can we do? Like always - look at the code, because code=vulns.

Thou shalt not parse URLs thyself!

In FlashVars construction code getPort and getDomainName functions are used to extract domain and port parameters from current window location (global.location). Let's see what happens with domain name (Core.js):
function getDomainName(url){
    // #ifdef debug
    if (!url) {
        throw new Error("url is undefined or empty");
    // #endif
    return url.match(reURI)[3];
It is being matched against the following regular expression:
var reURI = /^((http.?:)\/\/([^:\/\s]+)(:\d+)*)/; // returns groups for protocol (2), domain (3) and port (4)
In simpler terms - everything after httpX:// and before :digits or a / becomes a domain name. Seems solid, right? WRONG.
Among many tricks bypassing URL parsers (see e.g. kotowicz.net/absolute), HTTP authentication parameters are rarely used. But this time they fit perfectly. You see, hostname (domain name) is not the only thing that comes right after protocol. Not to bore you with RFCs, this is also a valid URL:


If our document was loaded from URL containing user credentials, getDomainName() would return user:password@host (sometimes, there are browser differences here). FlashVars, in that case, would be: 
Still, nothing interesting, but...

Honor thy Encoding and thy Context

(c) Wumo - http://kindofnormal.com/wumo/2013/10/12
In previous example we injected some characters into FlashVars string, but none of them were dangerous in that context. But as you can see:
  var flashVars = "callback=flash_loaded" + domain.replace(/[\-.]/g, "_") + "&proto=" + global.location.protocol + "&domain=" + getDomainName(global.location.href) + "&port=" + getPort(global.location.href) + "&ns=" + namespace;
Values of various parameters are not percent encoded (i.e. encodeURIComponent is not used) If we could only use & and = characters in username part, we could inject additional Flashvars. For example, loading this URL:


(the bold part is actually the username, not a domain name) would cause:
injecting our log=true parameter and triggering the exploit. But can we?

Effin phishers!

Kinda. Credentials in URL were used extensively in phishing attacks, so most current browsers don't really like them. While usually you can use = and & characters in credentials, there are serious obstacles, f.e:
  • Firefox won't return credentials at all in location.href
  • Chrome will percent encode crucial characters, including = and &
However, Safari 6 does not see a problem with loading URL like this: http://h=&ello@localhost/ and returning the same thing in location.href. So - easyXDM 2.4.16 is XSS exploitable in Safari 6 and possibly in some other obscure or ancient browsers. In Safari due to effing phishers using credentials in URL will trigger a phishing warning, so the user must confirm the navigation. Well, Sad Panda^2. But still - it's an easyXDM universal XSS on a popular browser with limited user interaction.


  • Always use context aware encoding!
  • Don't parse URLs manually!

No comments: