CVE-2024-56527 - How a Simple Missing Function in TCPDF Led to XSS Vulnerability

Summary:
A vulnerability was discovered in TCPDF before version 6.8.. The problem? The Error() function doesn't use htmlspecialchars() on error messages, opening the door for Cross-Site Scripting (XSS) attacks. In this post, we'll break down what this means, see code samples, and learn how attackers might exploit it, all in easy-to-understand language.

1. What Is TCPDF?

TCPDF is a popular open-source PHP class for generating PDF files. Web developers use it to turn web content into nicely formatted PDFs.

More about TCPDF: https://tcpdf.org

2. The Vulnerability Explained

CVE-2024-56527 highlights a security issue in TCPDF's Error() function. When something goes wrong, Error() displays a message. But before TCPDF version 6.8., it directly echoed whatever message you gave it – without escaping HTML characters.

Why is this Dangerous?

It means that if an attacker can control the error message (for example, by triggering an error with crafted input), they can inject malicious HTML or JavaScript code, leading to XSS attacks.

3. The Vulnerable Code

Here’s a simplified (stripped-down) look at how the problematic function might appear in old versions of TCPDF (before 6.8.):

function Error($msg) {
    // Just outputs the error message
    echo '<b>TCPDF ERROR:</b> '.$msg;
    exit;
}

Notice it just echoes the message directly, no filtering, no escaping.

A secure version would look like this

function Error($msg) {
    // Escape HTML special characters to prevent XSS
    echo '<b>TCPDF ERROR:</b> '.htmlspecialchars($msg, ENT_QUOTES|ENT_SUBSTITUTE, 'UTF-8');
    exit;
}

4. How an Exploit Could Happen

Imagine an application lets a user upload a filename, and that filename is used in generating a PDF. If something goes wrong (like an invalid filename), TCPDF's Error() function might show the filename in the error message:

// $filename comes from user input
tcpdf->Error("Error opening file: $filename");

If the user uploads a file with a special name

"><script>alert('XSS')</script>

Then, the error page would output

<b>TCPDF ERROR:</b> Error opening file: "><script>alert('XSS')</script>

The browser would run the JavaScript! The attacker can steal cookies, hijack sessions, etc.

DON'T DO THIS

<?php
$filename = $_GET['file'];
if (!file_exists($filename)) {
    Error("File not found: $filename");
}
?>

Requesting ?file="><script>alert('XSS')</script> will trigger the XSS.

6. How to Fix It

Upgrade!
The best way is to update to TCPDF 6.8. or later. The maintainers fixed the bug by adding htmlspecialchars() to the error output.

If you can't upgrade right away, patch your copy

echo '<b>TCPDF ERROR:</b> '.htmlspecialchars($msg, ENT_QUOTES|ENT_SUBSTITUTE, 'UTF-8');

7. Official References

- TCPDF GitHub Release Notes
- CVE-2024-56527 (Mitre)

8. Summary Table

| Version | Safe? | Details |
|-----------|--------|--------------|
| < 6.8. | ❌ No | Vulnerable |
| >= 6.8. | ✅ Yes | Fixed |

9. Final Thoughts

Always sanitize output, especially anything controlled by user input! Even mature libraries can make simple mistakes with big consequences. Check your TCPDF version and patch today.

Stay safe, keep your servers patched, and happy coding!

*This post is exclusive to you and uses original explanations. If you share, please credit the source links above for official information.*

Timeline

Published on: 12/27/2024 06:15:23 UTC
Last modified on: 03/13/2025 14:15:33 UTC