Drupal is a powerhouse in the content management system (CMS) world, used by everyone from small businesses to giant media outlets. However, it’s not immune to vulnerabilities. In June 2024, a serious security issue—CVE-2024-55638—was disclosed affecting Drupal Core, which introduces a new risk for sites running outdated versions. Let’s break down what this means for your website, how an attack works, and what you need to do right now.
Quick Facts: What Is CVE-2024-55638?
CVE-2024-55638 is a Deserialization of Untrusted Data vulnerability in Drupal Core, leading to Object Injection. When an application unserializes data from an untrusted source, it can be tricked into loading crafted objects that trigger unintended behaviors. In the worst case, an attacker might achieve Remote Code Execution (RCE) — meaning they could run code on your server.
Drupal 7: All versions from 7. to before 7.102
- Drupal 8/9/10: From 8.. up to (but not including) 10.2.11
Drupal 10.3: From 10.3. up to (but not including) 10.3.9
If you’re running any of those, you’re exposed.
> Reference:
> Drupal Security Advisory SA-CORE-2024-004 (Original Announcement)
> NIST NVD Entry
Why This Is Dangerous
By itself, this vulnerability does *not* create a direct exploit—there’s no single place in the Drupal codebase doing unserialize($_GET['input']) or something obviously malicious. Instead, Drupal’s code contains a chain of functions called a gadget chain that can be abused if any insecure deserialization occurs (that’s: unserialize() or similar being called on data that can be controlled by an attacker).
If you or a contributed module introduces a deserialization vulnerability (e.g., processing data from user uploads or requests), the presence of this gadget chain makes it much easier for attackers to turn a basic bug into a full server takeover.
What is Deserialization?
*Serialization* turns an object into a string (so it can be stored or sent somewhere); *deserialization* turns the string back into an object. In PHP, it looks like this:
$serialized = serialize($myObject); // string
$object = unserialize($serialized); // object again
The problem? If an attacker sends specially-crafted serialized data, and the code unserializes it, bad stuff can happen.
What’s an Object Injection?
In PHP, certain objects have special “magic methods” (__wakeup, __destruct, __toString etc.) that get called automatically when they’re unserialized or destroyed. If the codebase includes dangerous logic in those methods, attackers can trigger them during the deserialization process. This is called Object Injection.
Example Exploit Scenario
Exploitability:
This vulnerability requires an attacker to find a spot where deserialization of untrusted data happens. This could be in:
An API endpoint handling serialized user input (never do this!)
Attack Flow:
Find input the website accepts (maybe in a POST body or file upload) that is unserialized.
2. Send a malicious serialized payload using Drupal gadget chain classes such that, when unserialized, the magic methods get triggered.
Drupal's gadget chain can be abused with a payload like
// Example: Crafting a payload using Drupal's classes for demonstration.
// (In reality, this chain is obscure and complex—involving real Drupal classes.)
class Evil {
public $callback;
function __construct($cmd) {
$this->callback = $cmd;
}
function __wakeup() {
eval($this->callback);
}
}
// Attacker serializes the object with malicious code:
$payload = serialize(new Evil('system("whoami");'));
echo $payload; // Output to be sent to server
If the above string is received by vulnerable code that unserializes it, it will run the command whoami on the server!
*Note*: Drupal's real gadget chain is more complex and uses core classes, not the simple Evil class above. But the idea is the same.
How Does Drupal’s Gadget Chain Work?
Attacker-crafted payload references a sequence of built-in classes and known magic methods. Once PHP's unserialize hits those, the chain cascades until, ultimately, arbitrary code or command execution occurs.
Example (very simplified)
O:25:"Drupal\Core\File\FileSystem":1:{s:4:"path";s:13:"/tmp/exploit";}
This is a placeholder—use of the actual gadget chain is restricted to responsible security research. The real chain will be far more intricate, often requiring knowledge of Drupal’s internals.
How To Protect Yourself
1. Upgrade Immediately
Patch your site to the latest available Drupal 7/10.2/10.3 version:
- Drupal 7.102
- Drupal 10.2.11
- Drupal 10.3.9
2. Audit Your Code
Audit contributed and custom modules for any unserialize usage.
3. Monitor Your Logs
Watch for suspicious serialized data coming into your application.
4. Limit PHP Object Usage
Avoid using PHP objects (if possible) in serialized contexts exposed to users.
Drupal Security Advisory:
CVE Info:
OWASP Cheat Sheet: Deserialization Attacks:
https://cheatsheetseries.owasp.org/cheatsheets/Deserialization_Cheat_Sheet.html
Drupal Release Notes:
https://www.drupal.org/project/drupal/releases
Final Thoughts
CVE-2024-55638 does not mean your site is instantly hacked, but it does mean that any spot in your codebase that unserializes untrusted data can be used as a beachhead for a complete takeover. Fix it by patching—and remember, always sanitize the data you handle!
Timeline
Published on: 12/10/2024 00:15:22 UTC
Last modified on: 12/16/2024 18:15:11 UTC