Grav: The system, site, and theme Twig variables bypass the content sandbox entirely and are never covered by config_denied_paths

Summary

Grav\Common\Twig\Twig::init() unconditionally puts the raw system, site, and theme config arrays into $this->twig_vars. Twig::processPage() builds the variables for the sandboxed, editor-authored page-content render by copying that same base array ($sandbox_vars = $twig_vars;) and replacing only the config key with a filtered SandboxConfig facade. The system, site, and theme keys are carried into the sandboxed render completely untouched.

Because these are plain PHP arrays, not objects, Twig's sandbox SecurityPolicy (the allowed_classes/allowed_methods/allowed_properties lists in system/config/security.yaml) has no jurisdiction over them at all. The sandbox only gates method calls and property access on objects. Dot notation or subscript access on an array is always allowed by Twig regardless of any sandbox policy. So {{ system.cache.redis.password }} in page content renders the value directly, with the sandbox doing nothing to stop it, and with security.twig_sandbox.config_denied_paths never even being consulted, since that list only filters the separate config facade object, not the system array.

This means: even on a default install where twig_content.config_access is false (its documented default) so the config Twig variable is empty inside sandboxed renders, an attacker with page-content edit access (or a stored-XSS-style Twig injection into page content, if twig_content.process_enabled is on) can still read system.*, site.*, and theme.* in full, including any admin-configured secret nested under those trees.

Affected product and version

Product: Grav CMS, getgrav/grav Confirmed present in: 2.0.15, commit c2b46866857a93a0aa7048e7ed707ed3ed45dbc3

Affected code

system/src/Grav/Common/Twig/Twig.php, in init(), the base variable set (around line 300):

$this->twig_vars += [
        'config'            => $config,
        'system'            => $config->get('system'),
        'theme'             => $config->get('theme'),
        'site'              => $config->get('site'),
        'uri'               => $this->grav['uri'],
        ...
    ];

system/src/Grav/Common/Twig/Twig.php, in processPage(), where the sandboxed render variables are built (around line 419-429):

if ($item->shouldProcess('twig') || $item->isModule()) {
    $name = '@Page:' . $item->path();
    $this->setTemplate($name, $content);
    // Replace `config` with a denied-path-filtered facade for the
    // sandboxed render so editors can't exfiltrate plugin secrets
    // via `config.toArray()` (GHSA-j274-39qw-32c9). The modular
    // theme render below is unsandboxed and keeps the raw Config.
    $sandbox_vars = $twig_vars;
    $sandbox_vars['config'] = $this->buildSandboxConfig();
    try {
        $output = $content = $local_twig->render($name, $sandbox_vars);
    ...

Only $sandbox_vars['config'] is replaced. $sandbox_vars['system'], $sandbox_vars['site'], and $sandbox_vars['theme'] still point at the exact same raw arrays that were assigned in init().

system/config/system.yaml shows a concrete real secret field that lives under system:

cache:
  redis:
    socket: false
    password:                                    # Optional password
    database:

Root cause

Two separate things have to both be true for this to be reachable, and they both are:

1. The sandbox's SecurityPolicy only checks object method calls and object property access (checkMethodAllowed, checkPropertyAllowed in Twig's Sandbox\SecurityPolicy). It has no concept of restricting array key access, because Twig's own design does not treat plain array reads as something a sandbox policy needs to arbitrate. config_denied_paths is implemented entirely inside SandboxConfig, a wrapper object with its own get()/offsetGet() that consults the denied list, that facade is what makes config safe. system/site/theme never get wrapped in anything like it, they are passed straight through as arrays.

2. processPage()'s sandboxed variable set is built by copying the entire pre-existing $twig_vars array and only patching the one key (config) that the GHSA-j274-39qw-32c9 fix was scoped to. system, site, and theme were already sitting in that array before the sandboxed path was ever reached, and nothing removes or filters them for that specific render.

Proof of concept, verified, real output

I verified this at two levels: first that the raw Grav source really does copy system into the sandboxed variables unfiltered (shown above via direct file reading of system/src/Grav/Common/Twig/Twig.php, not a paraphrase), and second, since I do not have a fully bootstrapped live Grav site available in this sandbox (composer install needs packagist.org, unreachable here), I verified the actual mechanism, that Twig's sandbox cannot restrict array access no matter how strict the policy is, by running it against the exact, real Twig source Grav has pinned.

Step 1, get the exact Twig commit Grav's composer.lock points at:

$ python3 -c "
import json
d = json.load(open('composer.lock'))
for pkg in d['packages']:
    if pkg['name'] == 'twig/twig':
        print(pkg['source'])
"
{'type': 'git', 'url': 'https://github.com/getgrav/Twig.git', 'reference': '24d7a0e821cf573496d99e05d6bd9d1a42f822c7'}

Step 2, clone that exact commit:

$ git clone https://github.com/getgrav/Twig.git twig-src
$ cd twig-src && git checkout 24d7a0e821cf573496d99e05d6bd9d1a42f822c7
HEAD is now at 24d7a0e8 Merge branch 'twigphp:3.x' into 3.x

Step 3, PoC script. This builds a SecurityPolicy with an empty allowed_classes, allowed_methods, and allowed_properties list, deliberately stricter than Grav's real policy, to show that even a maximally locked down object policy still cannot stop array key access, then renders {{ system.cache.redis.password }} against a system variable shaped exactly like what $config->get('system') returns in real Grav:

<?php
// twig_sandbox_poc.php
spl_autoload_register(function ($class) {
    if (strpos($class, 'Twig\\') === 0) {
        $rel = str_replace('Twig\\', '', $class);
        $path = '/home/claude/twig-src/src/' . str_replace('\\', '/', $rel) . '.php';
        if (file_exists($path)) {
            require_once $path;
        }
    }
});
require '/home/claude/twig-src/src/Resources/core.php';
require '/home/claude/twig-src/src/Resources/escaper.php';

use Twig\Environment;
use Twig\Loader\ArrayLoader;
use Twig\Extension\SandboxExtension;
use Twig\Sandbox\SecurityPolicy;

// Modeled on Grav's real system/config/security.yaml twig_sandbox block:
// a couple of harmless tags/filters allowed (escape is allow-listed in the
// real config since autoescape is forced on), and zero allowed classes,
// methods, or properties, stricter than Grav's real policy even is.
$policy = new SecurityPolicy(
    ['if', 'for'],
    ['upper', 'lower', 'escape'],
    [],
    [],
    []
);

$twig = new Environment(new ArrayLoader([
    'page_content' => '{{ system.cache.redis.password }}',
]));
$twig->addExtension(new SandboxExtension($policy, true));

// Exactly what $config->get('system') returns as a plain PHP array in real
// Grav, and exactly what Twig::init() assigns to $twig_vars['system'].
$system_config_array = [
    'cache' => [
        'driver' => 'redis',
        'redis' => [
            'socket' => false,
            'password' => 'REDACTED-REAL-SECRET-VALUE-abc123',
            'database' => 2,
        ],
    ],
];

try {
    $output = $twig->render('page_content', ['system' => $system_config_array]);
    echo "Template : {{ system.cache.redis.password }}\n";
    echo "Rendered output : " . $output . "\n";
    echo "Sandbox blocked it : " . ($output === '' ? 'YES' : 'NO, the secret was rendered in plain text') . "\n";
} catch (\Twig\Sandbox\SecurityError $e) {
    echo "Sandbox threw a SecurityError (blocked): " . $e->getMessage() . "\n";
}

Step 4, run it:

$ php twig_sandbox_poc.php

Actual output:

Template : {{ system.cache.redis.password }}
Rendered output : REDACTED-REAL-SECRET-VALUE-abc123
Sandbox blocked it : NO, the secret was rendered in plain text

For reference, running the same script before I added escape to the allowed filters (autoescape is forced on, so every {{ }} in real Grav goes through the escape filter first) correctly failed closed:

Sandbox threw a SecurityError (blocked): Filter "escape" is not allowed in "page_content" at line 1.

which confirms the harness is actually exercising the sandbox's enforcement path, not silently skipping it, and that the only reason system.cache.redis.password got through is the array access itself, not a policy misconfiguration in my test.

This demonstrates the mechanism precisely: no matter how the allowed_classes/allowed_methods/allowed_properties lists in system/config/security.yaml are configured, and independent of config_denied_paths entirely, a raw array handed to the sandboxed template is fully readable. Combined with the direct source reading in the "Affected code" section above, showing that system, site, and theme are exactly such raw arrays and are carried unfiltered into processPage()'s sandboxed render, this is a complete, verified chain from source to impact. I was not able to additionally capture a live HTTP round trip against a running Grav install with real page content, for the same reason as my other reports, no bootstrapped instance available in this sandbox, but every step of the actual code path has been verified against the real source, not reconstructed or assumed.

Impact

Any content author who can enable Twig processing on a page (process.twig: true in page frontmatter, gated by security.twig_content.process_enabled, or unconditionally for modular page content per the comment in processPage()) can read the entire system, site, and theme configuration trees, including any secret that happens to live there, such as system.cache.redis.password in core, and whatever plugins may nest under site.* for their own settings, since plugin config lives elsewhere (plugins.*) but site owners commonly stash site-specific integration keys under site.* custom fields. This works regardless of twig_content.config_access, which was presumably assumed to be the single gate for config exposure in sandboxed content, it is not, system/site/theme were never part of that gate.

Suggested fix

The config_denied_paths fix pattern (a filtering facade) does not apply here since these are plain arrays, not an object with its own get(). The direct fix is to stop injecting the raw arrays into the sandboxed render, options in rough order of how much they preserve existing template behavior:

1. In processPage(), after copying $sandbox_vars = $twig_vars;, also strip or replace system, site, and theme for that specific sandboxed call, the same way config already gets replaced. A SandboxConfig-style facade wrapping $config->get('system') with its own denied-path list would let you keep the currently-useful subset (e.g. system.pages.* for things page authors are expected to read) while still hiding secrets.
2. Alternatively, since config already gives filtered access to the same data (config.get('system.cache.driver') etc. through SandboxConfig), consider whether system/site/theme need to be separate top level variables in the sandboxed render at all, versus just being reachable via the already-filtered config facade.

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

CWE-200, Exposure of Sensitive Information to an Unauthorized Actor (secondary: CWE-668, Exposure of Resource to Wrong Sphere, describing the sandbox-bypass mechanism itself)

=========================================================== 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: Privileges Required is set to Low because reaching this requires page-content edit access, which is exactly the privilege level the entire content sandbox exists to constrain, someone with edit rights but who should not have operator-level secrets. If your threat model treats page-content editors as fully trusted, please rescore. I set Confidentiality to High rather than Low because the exposed tree can contain live credentials (a cache backend password, and whatever else operators or plugins choose to nest under system/site), not just configuration shape.

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

Moderate