Google AdWords API PHP client library <= 6.2.0 - Arbitrary PHP Code Execution

[Massaro]

# Date: 06.11.2015
# Title: Google AdWords API PHP client library <= 6.2.0 Arbitrary PHP Code Execution
# Exploit Author: Dawid Golunski
# Vendor Homepage: https://developers.google.com/adwords/api/docs/clientlibraries
# Software Link: https://github.com/googleads/googleads-php-lib
# Version: <=6.2.0


=============================================
- Release date: 06.11.2015
- Discovered by: Dawid Golunski
- Severity: Medium/High
=============================================


I. VULNERABILITY
-------------------------

Google AdWords API PHP client library <= 6.2.0    Arbitrary PHP Code Execution
(googleads-php-lib)

II. BACKGROUND
-------------------------

- AdWords API

https://developers.google.com/adwords/api/docs/

"The AdWords API is a collection of web services that you can use to build 
applications that manage AdWords accounts and their associated campaign data.
While the AdWords API is based on SOAP 1.1, high-level client libraries are 
provided to help you develop applications more quickly."

AdWords API client libraries are available for different platforms
such as PHP, .NET, Java etc. 
These can be found at:

https://developers.google.com/adwords/api/docs/clientlibraries

III. INTRODUCTION
-------------------------

The Google AdWords API client library for PHP contains a WSDL Interpreter
class which is described in a comment within the source code as:

"
* The main class for handling WSDL interpretation.
 *
 * The WSDLInterpreter is utilized for the parsing of a WSDL document for rapid
 * and flexible use within the context of PHP 5 scripts.
"

The class contains a function savePHP() which allows to convert the WSDL 
document received from a remote end into a PHP file. 
The funcion is vulnerable to Path Traversal and Code Execution vulnerabilities.

IV. DESCRIPTION
-------------------------

googleads-php-lib contains the following function which is meant to load WSDL 
document (XML data) from a remote Google AdWords server:

---[ build_lib/WSDLInterpreter/WSDLInterpreter.php ]---

  protected function loadWsdl($wsdlUri, $proxy = null) {
    // Set proxy.
    if ($proxy) {
      $opts = array(
          'http' => array(
              'proxy' => $proxy,
              'request_fulluri' => true
          )
      );
      $context = stream_context_get_default($opts);
      libxml_set_streams_context($context);
    }

    $this->dom = new DOMDocument();
    $this->dom->load($wsdlUri,
        LIBXML_DTDLOAD|LIBXML_DTDATTR|LIBXML_NOENT|LIBXML_XINCLUDE);

-------------------------------------------------------

For security reasons Google AdWords API should only be accessed via HTTPS. 
However, the above code does not set appropriate SSL settings on the 
https:// stream context. 
It fails to assign Certificate Authority (CA), turn the verify_peer 
option to ON, specify allowed ciphers etc.
It uses the stream_context_get_default() function to get the default context,
which on all PHP versions below PHP 5.6.x (see references), does not validate 
the CA by default. 
Because of this, application may retrieve data from untrusted sources 
pretending to be adwords.google.com.

Further on, the WSDLInterpreter class contains the following savePHP function:

---[ build_lib/WSDLInterpreter/WSDLInterpreter.php ]---

  /**
   * Saves the PHP source code that has been loaded to a target directory.
   *
   * Services will be saved by their validated name, and classes will be
   * included with each service file so that they can be utilized independently.
   *
   * @param array $options Options for the SoapClient
     */
    public function __construct($wsdl, $options, $user) {
      $options["classmap"] = self::$classmap;
      parent::__construct($wsdl, $options, $user, self::SERVICE_NAME,
          self::WSDL_NAMESPACE);
    }
  }
}

----------------------------------------

If such class gets included it will execute the malicious code due to the 
injected __destruct() method, which creates /tmp/adwrods_api_hacked file.

At this point the attacker can control the name of the class (through service name), 
the path to the resulting PHP file, and is also able to inject any PHP code.

Going further, He could also close the class definition statement and write an arbitrary 
PHP code in the main file. 
This would allow the attacker to create a stand alone script which he 
could request remotely via the Web server if he managed save it within the web
root.  
In this way the attacker could create a stand alone PHP command shell and get
access to the system.


VI. BUSINESS IMPACT
-------------------------

The severity of this issue is lowered to medium/high as despite the possibility
to execute arbitrary code, the attacker must impersonate adwords.google.com
server to be able to inject malicious XML. 
If there is a possibility for such an attack, the severity of the issue
can grow to high/critical.

VII. SYSTEMS AFFECTED
-------------------------

Google AdWords API PHP client library in versions up to 6.2.0 contain the 
vulnerable WSDLInterpreter code.

VIII. SOLUTION
-------------------------

Upgrade Google AdWords API PHP client library to the latest version.

IX. REFERENCES
-------------------------

This advisory:

[url]http://legalhackers.com/advisories/Google-AdWords-PHP-Client-library-PHP-Code-Execution.txt[/url]

Related, Google AdWords API client libraries - XML eXternal Entity Injection (XXE) vuln:
[url]http://legalhackers.com/advisories/Google-AdWords-API-libraries-XXE-Injection-Vulnerability.txt[/url]

[url]https://github.com/googleads/googleads-php-lib[/url]

[url]https://github.com/googleads/googleads-php-lib/blob/master/ChangeLog.md[/url]

[url]https://developers.google.com/adwords/api/docs/[/url]

[url]https://developers.google.com/adwords/api/docs/clientlibraries[/url]

PHP 5.6.x openssl certificates in PHP streams:
[url]http://php.net/manual/en/migration56.openssl.php[/url]

X. CREDITS
-------------------------

The vulnerability has been discovered by Dawid Golunski
dawid (at) legalhackers (dot) com

[url]http://legalhackers.com[/url]

XI. REVISION HISTORY
-------------------------

May 18th, 2015:  Advisory created and sent to Google Security Team

Nov 5th,  2015:  Google, after half a year, confirm the vulnerability has been patched

Nov 6th,  2015:  Advisory released publicly

XII. LEGAL NOTICES
-------------------------

The information contained within this advisory is supplied "as-is" with
no warranties or guarantees of fitness of use or otherwise. I accept no
responsibility for any damage caused by the use or misuse of this information.

.