If you generate PDFs in PHP, you may have run into a popular library called TCPDF. It’s used all over the world to create invoices, reports, and more. But recently, it was discovered that the library had a dangerous security bug letting someone intercept or tamper with online resources—such as images or logos—that your PDFs download from the web.

Let’s break down CVE-2024-56521, how it works, and what you need to do.

What Is CVE-2024-56521?

CVE-2024-56521 is a vulnerability found in TCPDF versions before 6.8. that only shows up if your PHP build has the libcurl extension. When TCPDF tried to download files (like images from HTTPS links) with curl, it disabled critical SSL verification settings:

CURLOPT_SSL_VERIFYPEER

Essentially, these settings tell curl whether to trust the server’s SSL certificate (is this really google.com?) and whether the server’s certificate is properly signed.

By disabling them, any server, even a malicious one, could pretend to be your image source — making classic man-in-the-middle (MITM) attacks way easier.

Why Does This Matter?

If you run a web server that uses TCPDF to generate PDFs with remote images (or other HTTP(s) resources), an attacker on your network could:

Possibly take over your server depending on what the PDF fetches and processes.

If your server is calling out to external URLs over HTTPS and trusts them without checks, it’s a major risk.

The Code Behind the Flaw

Here’s a simplified version of the affected code (from TCPDF's Github before fix:

// Simplified: This disables SSL verification!
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, false); // (should be 2, not false!)

CURLOPT_SSL_VERIFYPEER set to false: curl will not check if the certificate is valid.

- CURLOPT_SSL_VERIFYHOST set to false: curl won’t verify the remote host’s certificate matches the URL.

A malicious server can supply any certificate, and your code will trust it.

How Can This Be Exploited?

Scenario:
Let’s say your server generates a PDF invoice that pulls your logo from https://mycdn.example.com/logo.png. If someone can intercept your network traffic (like on public Wi-Fi or inside your data center), they can:

Pretend to be mycdn.example.com,

2. Serve a fake/malicious image, or even a malicious file (though TCPDF usually only displays images, not executes code).

Code demonstration of unsafe fetch

// In vulnerable TCPDF version <6.8.
$image = 'https://attacker-mitm.example/evil.png';;
$pdf->Image($image, 10, 10, 50, 50, 'PNG');

Even though HTTPS and a "valid" domain are used, TCPDF will happily accept any server pretending to be that image due to disabled SSL checks.

The Fix

From TCPDF 6.8. onwards, the fix restores proper SSL verification. The changeset (see patch here):

// Now safely verify SSL identity
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);

Always set CURLOPT_SSL_VERIFYHOST to 2 (not true, not 1, and never false) for proper host checking. Setting it to false disables the check, which is dangerous.

Update TCPDF Immediately:

Upgrade to version 6.8. or newer from the official releases.

Do you allow user-supplied image URLs?

- Do you fetch remote content over HTTP/HTTPS in your PDF code?

Monitor for Similar Issues:

If you use other PHP libraries to fetch remote resources, always check how they handle SSL peer verification.

Resources and References

- Official CVE Entry CVE-2024-56521
- TCPDF Issue #423 - Security fix
- GitHub Commit Fixing SSL Verification
- libcurl Documentation: SSL Verify Peer
- libcurl Documentation: SSL Verify Host

Bottom Line

CVE-2024-56521 is yet another reminder that SSL verification must always be enabled, especially in code which fetches remote resources for inclusion in your users’ content. If you use TCPDF anywhere before 6.8., update right away—or risk letting attackers tamper with every PDF you generate.

If you found this useful, spread the word—many PDF generators out there are still quietly vulnerable!

Timeline

Published on: 12/27/2024 05:15:07 UTC
Last modified on: 03/24/2025 18:15:21 UTC