Author: Janek Vind "waraxe"
Date: 12. April 2004
Location: Estonia, Tartu
Web: http://www.waraxe.us/index.php?modname=sa&id=16


Affected software description:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Php-Nuke is popular freeware content management system, written in php by
Francisco Burzi. This CMS (Content Management System) is used on many thousands
websites, because it`s free of charge, easy to install and has broad set of features.

Homepage: http://phpnuke.org



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

Here I am, on the road again, discussing about potential XSS case in phpnuke.
"AGAIN?". Yes, coz phpnuke is surprisingly generous software for finding different
security holes ;)

This XSS case is active, when website uses some specific nuke themes - for example generic
themes "Karate", "Anagram", "Kaput", "Milo", "NukeNews" and many other derivations and
custom themes. By the way, security issues here are phpnuke engine related, not theme related.

Let's be more specific. There is a function in nuke engine, called cookiedecode().
From mainfile.php:

function cookiedecode($user) {
global $cookie, $prefix, $db, $user_prefix;

$user = base64_decode($user);
$cookie = explode(":", $user);
$sql = "SELECT user_password FROM ".$user_prefix."_users WHERE username='$cookie[1]'";
$result = $db->sql_query($sql);
$row = $db->sql_fetchrow($result);
$pass = $row[user_password];
if ($cookie[2] == $pass && $pass != "") {
return $cookie;
} else {
unset($user);
unset($cookie);
}

As we can see, variable $user (from $_COOKIE[], $_GET[] or $_POST[]) gets base64 decoded and then
exploded to array $cookie. Then the code will ask from database the password md5 hash and if retrieved
password matches with browser supplied password, then function returns the array $cookie[] and next
phpnuke theme.php will use this valid username (it's checked in cookidecode) for visual feedback,
for example - "welcome, $username".
So, it seems, that we can't spoof username here, coz we can't fool the checking routine (we dont consider here
sql injection , coz it will be used in my next advisory ;) ). This is, what
code programmer was thinking, but reality is different...

From php manual:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unset

(PHP 3, PHP 4)
unset -- Unset a given variable

Description
void unset ( mixed var [, mixed var [, ...]])

unset() destroys the specified variables. Note that in PHP 3, unset() will always return TRUE (actually,
the integer value 1). In PHP 4, however, unset() is no longer a true function: it is now a statement.
As such no value is returned, and attempting to take the value of unset() results in a parse error.

The behavior of unset() inside of a function can vary depending on what type of variable you are attempting to destroy.
If a globalized variable is unset() inside of a function, only the local variable is destroyed.
The variable in the calling environment will retain the same value as before unset() was called.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
So, this little code in function cookiedecode():
} else {
unset($user);
unset($cookie);
}
will destroy the array $cookie[] only IN LOCAL CONTEXT, but in global scope it will be UNDESTROYED!

Ok, now let's issue request like this

http://localhost/nuke71/index.php?user=MTo8c2NyaXB0PmFsZXJ0KGRvY3VtZW50LmNvb2tpZSk7PC9zY3JpcHQ%2bZm9vYmFy

to the phpnuke enabled website, using vulnerable themes. And we can see, that XSS works!
What's inside of the "user"? If we base64_decode this variable, we see this:

1:<script>alert(document.cookie);</script>foobar

So, in this way, we can exploit XSS and evade all contrameasures in phpnuke, set up against scripting tags etc.




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

Greets to torufoorum members and to all bugtraq readers in Estonia! Tervitused!
Special greets to Stefano from UT Bee Clan!



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

come2waraxe@yahoo.com
Janek Vind "waraxe"

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

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Published on: 2005-01-06 (4178 reads)