[MajorSecurity Advisory #57]PHP <=5.3 - preg_match() full path disclosure

[David Vieira-Kurz <david@xxxxxxxxxxxxxxxxxx>]

[MajorSecurity Advisory #57]PHP <=5.3 - preg_match() full path disclosure

Details
=======
Product: PHP <=5.3
Security-Risk: moderated
Remote-Exploit: yes
Vendor-URL: http://www.php.net/
Vendor-Status: informed
Advisory-Status: published

Credits
============
Discovered by: David Vieira-Kurz
http://www.majorsecurity.info

Affected Products:
----------------------------
PHP 5.3 and prior
PHP 5.2.11 and prior

Original Advisory:
============
http://www.majorsecurity.info/index_2.php?major_rls=major_rls57

Introduction
============
"PHP is a widely-used general-purpose scripting language that is 
especially suited for Web development and can be embedded into HTML."
- from php.net

More Details
============
1. Full Path Disclosure
-----------------------------------
There is a full path disclosure vulnerability concerning the 
preg_match() php function which allow attackers to
gather the real path of the server side script.

The preg_match() PHP function takes strings as parameters and will raise 
warnings when values that are passed are arrays rather then strings.
To get the path of the current script, you simply need to pass the 
arguments as arrays rather then expected strings
and then simply read the warning message generated by PHP to see the 
error including the full path of the current running script.

Proof of concept:
http://localhost/cms/modules/system/admin.php?fct=users&op[]=

Warning: preg_match() expects parameter 2 to be string, array given in 
/htdocs/cms/include/common.php on line 105

Solution
================
I would NOT recommend to just react by "security through obscurity" and 
turn off the error messages, error reporting etc.
This is not a solution because there are a lot of users that are having 
a shared hosting server where they aren't able to manipulate
the "php.ini" configuration file - even ini_set() is forbidden on some 
shared hoster servers.
So they still would have the full path disclosure there.

Workaround
================
I would recommend to meticulously go through the code forcing PHP to 
cast the data to the desired type, in this case the (string) casts
to eliminate the Notice or Warning messages.

Example:
<?PHP
if(isset($_GET['page'])) {
    if (is_array($page = $_GET['page'])) { 

         $casted = (string)$page;
     } else {
        $page = htmlspecialchars($_GET['page'],ENT_QUOTES,'UTF-8');
        validate_alpha($page);
       }
}
function validate_alpha($page) {
           return preg_match("/^[A-Za-z0-9_-]+$/ ", $page);
} ?>

Vendor communication
================
The PHP Developer team has been informed that there is this vulnerability.

MajorSecurity
================
MajorSecurity is a German penetrationtesting and security research 
company which focuses on web application security. We offer professional 
penetrationtestings, security audits,
source code reviews and reliable proof of concepts.
You will find more Information about MajorSecurity at
http://www.majorsecurity.info/