What I Learned At My First CTF

Posted on  by


The other day, my company ran a ‘Capture The Flag’ exercise, where developers attempted to complete a series of hacking challenges against a running server (set up by our VP of R&D).

The majority of the exercise revolved around application security, where the attacker would gain more and more information to gain further and further access.

To start, we were asked to identify what sofware the site was running. A quick look at the page source of the site showed the following:

<meta name="generator" content="Drupal 7 (http://drupal.org)" />

making it easy to determine that the site was running Drupal. With that in hand, I was able to use online scanning software to identify the exact version was 7.31 (apparently contained in a changelog that is sitting accessible).

A quick search for ‘Drupal 7.31 cve’ brought me to Exploit DB which handily contained a python script which allowed me to create an admin user in Drupal and log in to the server.

The next challenge was to try to identify users on the machine and passwords to things like databases. I spent a bunch of time poking around and trying to figure this out until I realized that Drupal, by default, allows content to be rendered as PHP. From the docs:

Body: where you put the text for the page. The "Input format" controls what code can go in the Body field. There are three default options: filtered HTML, PHP code, and full HTML.

With the ability to execute PHP, this made a few tasks pretty easy. I was able to leverage code execution to enumerate file paths on the server, and then open and render specific files. Of note was the settings.php file of Drupal to gather information on the database, as well as list the /home directory to learn more about some users.

The next step was to essentially obtain access to the machine itself, which I did not figure out in the time alloted, but I believe would build on the basis that I can potentially write files to the file system using PHP. Perhaps trying to add my own public key to a user’s .ssh folder? Or perhaps trying to create a reverse shell? The next CTF will tell.