PHP Object Injection

This is a Vulnerability. To view all vulnerabilities, please see the Vulnerability Category page.

Author(s):

• Egidio Romano

Last revision (mm/dd/yy): 01/22/2013

Vulnerabilities Table of Contents

Description

PHP Object Injection is an application level vulnerability which allows an attacker to perform different kinds of malicious attacks. The vulnerability occurs when user-supplied input is not properly sanitized before being passed to the unserialize() PHP function. Since PHP allows object serialization, attackers could pass ad-hoc serialized strings to a vulnerable unserialize() function resulting in an arbitrary PHP object(s) injection into the application scope.

In order to successfully exploit a PHP Object Injection vulnerability two conditions must be satisfied:

• The application must have a class which implements a PHP magic method (such as __wakeup or __destruct) that can be abused to conduct malicious attacks.
• That exploitable class must be declared when the vulnerable unserialize() is being called, otherwise object autoloading must be supported for that class.

Risk Factors

• The impact of this vulnerability could be High but the likelihood is low. So, the severity of this type of vulnerability is Medium.
• This vulnerability can make the application vulnerable to some kinds of attacks such as Path Traversal, SQL Injection or Code Injection.

Examples

Example 1

The example below shows a PHP class with an exploitable __destruct method:

<?php

class Example1
{
   public $cache_file;

   function __construct()
   {
      // some PHP code...
   }

   function __destruct()
   {
      $file = "/var/www/cache/tmp/{$this->cache_file}";
      if (file_exists($file)) @unlink($file);
   }
}

// some PHP code...

$user_data = unserialize($_GET['data']);

// some PHP code...

?>

In this example an attacker might be able to delete an arbitrary file via a Path Traversal attack, for e.g. requesting the following URL:

http://testsite.com/vuln.php?data=O:8:"Example1":1:{s:10:"cache_file";s:15:"../../index.php";}

Example 2

The example below shows a PHP class with an exploitable __wakeup method:

<?php

class Example2
{
   private $hook;

   function __construct()
   {
      // some PHP code...
   }

   function __wakeup()
   {
      if (isset($this->hook)) eval($this->hook);
   }
}

// some PHP code...

$user_data = unserialize($_COOKIE['data']);

// some PHP code...

?>

In this example an attacker might be able to perform a Code Injection attack by sending an HTTP request like this:

GET /vuln.php HTTP/1.0
Host: testsite.com
Cookie: data=O%3A8%3A%22Example2%22%3A1%3A%7Bs%3A14%3A%22%00Example2%00hook%22%3Bs%3A10%3A%22phpinfo%28%29%3B%22%3B%7D
Connection: close

Related Vulnerabilities

• Category:Input Validation Vulnerability

Related Controls

• Input Validation
• Static Code Analysis

Prevention

Do not use unserialize() function with user-supplied input, use JSON functions instead.

References

• PHP: unserialize. http://php.net/manual/en/function.unserialize.php
• PHP: Magic Methods. http://php.net/manual/en/language.oop5.magic.php
• PHP: Autoloading Classes. http://php.net/manual/en/language.oop5.autoload.php
• Shocking news in PHP exploitation. http://www.suspekt.org/downloads/POC2009-ShockingNewsInPHPExploitation.pdf