In one such application, when certain usernames were submitted to the server, the server returned a page that was larger than the typical error page. I needed to find a way to submit lots of usernames via POST requests in hopes of enumerating valid usernames based on the returned page. One application that would be able to do this is THC-Hydra. Hydra is an incredibly powerful brute forcing tool that supports many different types of services, not only HTTP. You can view the supported protocols here.
I could have used hydra in this scenario, but I wanted to figure out a way to write my own brute forcing script so that I could give it some customization. After some research, I learned that CURL, a program prepackaged in most flavors of linux, would be a great tool for what I needed to do.
So what exactly did I need to do? I needed to take a list of usernames and loop through each one, sticking the username in the data to be submitted via POST requests to the server. I fired up burp suite and grabbed a copy of the extra data that the server needed, including the username and password fields.
Once I had that, I used curl with the -d switch. I then piped the results to grep and searched for a string that was only returned on the pages where the username was a valid one.
Curl -s -d "allthePOSTdatagoes&here&username=USERNAMEHERE&password=PASSWORDHERE" | grep "validpage"
The -s switch in Curl just says to be "silent" and dont print the results of each request to stdout. Throw that in bash script and you have your very own, simple, fairly fast account brute forcer . At this point, the attacker is halfway done with a successful brute force attack. All he would need to do now is take one of the valid usernames and do the very same thing for the password field until the script successfully guesses the password.
How to fix: To fix this vulnerability, the invalid login pages that are returned must be the EXACT same. As I have stated before, even just a few bytes difference in the pages (for example, a simple spelling error) could tip off an attacker, and allow him to successfully enumerate valid usernames. If an attacker does not know valid usernames, his job is twice as hard and would require MUCH more time to brute force an account, because he has to blindly guess combinations of usernames and password to get access.