After a break we continue with the CTF Natas series, now is the turn for natas8
[bash]
Natas Level 7 → Level 8
Username: natas8
URL: http://natas8.natas.labs.overthewire.org
[/bash]
Using the flag obtained in the previous challenge, we go to the URL showed in the description and we will see the following screen.
It’s just a simple web page with a basic input form, if we type nonsense we get an error message displaying Wrong secret, we proceed to click the the View sourcecode
[php]
<html>
<head>
<!– This stuff in the header has nothing to do with the level –>
<link rel="stylesheet" type="text/css" href="http://natas.labs.overthewire.org/css/level.css">
<link rel="stylesheet" href="http://natas.labs.overthewire.org/css/jquery-ui.css" />
<link rel="stylesheet" href="http://natas.labs.overthewire.org/css/wechall.css" />
<script src="http://natas.labs.overthewire.org/js/jquery-1.9.1.js"></script>
<script src="http://natas.labs.overthewire.org/js/jquery-ui.js"></script>
<script src=http://natas.labs.overthewire.org/js/wechall-data.js></script><script src="http://natas.labs.overthewire.org/js/wechall.js"></script>
<script>var wechallinfo = { "level": "natas8", "pass": "<censored>" };</script></head>
<body>
<h1>natas8</h1>
<div id="content">
<?
$encodedSecret = "3d3d516343746d4d6d6c315669563362";
function encodeSecret($secret) {
return bin2hex(strrev(base64_encode($secret)));
}
if(array_key_exists("submit", $_POST)) {
if(encodeSecret($_POST[‘secret’]) == $encodedSecret) {
print "Access granted. The password for natas9 is <censored>";
} else {
print "Wrong secret";
}
}
?>
<form method=post>
Input secret: <input name=secret><br>
<input type=submit name=submit>
</form>
<div id="viewsource"><a href="index-source.html">View sourcecode</a></div>
</div>
</body>
</html>
[/php]
This is supposed to be the backend code of the HTML page we just saw, the important part of this challenge is in the PHP code functions, taking a quick look the data flow looks like this:
- Check if submit key exists on $_POST
- Pass $_POST[‘secret’] to encodeSecret function
- encodeSecret function will apply some transformation to the secret and return it
- The transformed secret must be equal to 3d3d516343746d4d6d6c315669563362, otherwise we are getting the Wrong secret error we saw already
As I say before, the important part is happening inside the encodeSecret function, the code is basically doing this:
secret -> base64_encode -> strrev -> bin2hex -> 3d3d516343746d4d6d6c315669563362
So we need to perform exactly the same operations but in reverse order to obtain the original secret, ie: the old bin2hex should be hex2bin, I don’t know if we should call this reverse engineering, anyway ¯\_(ツ)_/¯
3d3d516343746d4d6d6c315669563362 -> hex2bin -> strrev -> base64_encode -> secret
We can use PHP from the command line and do this:
[bash]
$ php -r "echo base64_decode(strrev(hex2bin(‘3d3d516343746d4d6d6c315669563362’)));"
oubWYf2kBq
$
[/bash]
We get the secret: oubWYf2kBq, we try it on the input form.
The flag for the next level, natas9, is: W0mMhUcRRnG8dcghE4qvk3JA9lGt8nDl
In this challenge we take advantage of a security vulnerability called Source code disclosure and then we did basic reverse engineering on the PHP code.
Happy hacking 🙂