NetOn CTF 2021 - Let me in!

Web – 245 pts (9 solves) – Chall author: X4v1l0k

Captchas connected to PHP session cookies are not a secure idea, so let’s break in!



An empty page, with just a taunting ‘Try to catch me!’… Pff, dare provoke me? Alright, alright. Let’s see what you do click nothing… Hah? Upon inspection it sends us to flag.php, but visiting it immediately sends us back to index.php… Let’s curl!

$ curl

reveals the structure of flag.php

                <title>Try to catch the flag!</title>
                <form method="POST">
                                <label for="captcha">Please Enter the Captcha Text</label><br />
                                <img src="captcha.php" alt="CAPTCHA" class="captcha-image">
                                <input type="text" id="captcha" name="captcha_challenge">
                                <input type="submit" value="Send">

So it actually sends us to captcha.php instead of index.php. Okay, let’s curl once more to find… gibberish… oh. Visiting the URL with a browser gives us a captcha image. Mmh, how do we feed this captcha code to the HTML form. Every time we request (refresh) the page, we get a new captcha… How about this: we empty out our cookie jar and visit captcha.php to get a captcha image and a PHP session cookie. Now let’s curl both the captcha code and the session cookie to captcha.php

$ curl -d captcha_challenge=d1ctXg --cookie "PHPSESSID=eh9nn1eko8aj2oe88d9oj542ec"

Succes! Within the returned HTML we find

Nice evade! Take the flag: <b>NETON{7c49af83a2a68304273a8d330cebd93c}</b>

Other combinations of captcha codes and PHP session ID cookies also work!