GHSA-wvh6-f5jh-8gw4

Summary

The chroot check for local files uses a prefix string check to enforce chroot boundaries. The simple string comparison it performs allows paths like /var/www/root_secret/file.html when chroot is /var/www/root.

This allows attacker-controlled document paths/resources to bypass intended local file restrictions.

Details

The validateLocalUri() method is used to check if a local file is within an allowed chroot directory. After normalization with realpath(), this check is performed with a strpos() comparison:

    public function validateLocalUri(string $uri)
    {
        ...
        $realfile = realpath(str_replace("file://", "", $uri));
        ...
        foreach ($dirs as $chrootPath) {
            $chrootPath = realpath($chrootPath);
            if ($chrootPath !== false && strpos($realfile, $chrootPath) === 0) {
                $chrootValid = true;

Due to the normalization, the $chrootPath string does not have a terminating directory separator (/) appended. Because of this, the strpos() check only validates that $chrootPath is a prefix of $realfile. This allows access to folders with similar names that fall outside of the defined chroot restrictions.

For example, a chroot setting of /var/www/ would be normalized to /var/www, removing the trailing /. During strpos(), a $chrootPath of /var/www will also match a $realfile starting with /var/www2, /var/www-admin, or /var/www_backup, despite these being different directories.

PoC

With a directory structure similar to:

/home/dompdf/
  |--> web/
        |--> pdf.php
        |--> cat0.jpg
  |--> web-admin/
        |--> cat1.jpg

And web-accessible Dompdf functionality similar to the following (poc.html):

<?php
require 'vendor/autoload.php';
use Dompdf\Dompdf;
use Dompdf\Options;

$options = new Options();
$options->setChroot(['/home/dompdf/web/']);
$dompdf = new Dompdf($options);

$dompdf->loadHtml($_POST['html']);
$dompdf->render();...