Summary

A command injection vulnerability allows an unauthenticated user to execute arbitrary code on a server running Cacti, if a specific data source was selected for any monitored device.

Details

The vulnerability resides in the remote_agent.php file. This file can be accessed without authentication. In order to verify that the client is allowed the function remote_client_authorized is called:

if (!remote_client_authorized()) {
	print 'FATAL: You are not authorized to use this service';
	exit;
}

This function retrieves the IP address of the client via get_client_addr and resolves this IP address to the corresponding hostname via gethostbyaddr. After this, it is verified that an entry within the poller table exists, where the hostname corresponds to the resolved hostname. If such an entry was found, the function returns true and the client is authorized:

function remote_client_authorized() {
	// ...
	$client_addr = get_client_addr();
	// ...

	$client_name = gethostbyaddr($client_addr);
           // ...
	$pollers = db_fetch_assoc('SELECT * FROM poller', true, $poller_db_cnn_id);

	if (cacti_sizeof($pollers)) {
		foreach($pollers as $poller) {
			if (remote_agent_strip_domain($poller['hostname']) == $client_name) {
				return true;
			} elseif ($poller['hostname'] == $client_addr) {
				return true;
			}
		}
	}

	cacti_log("Unauthorized remote agent access attempt from $client_name ($client_addr)");

	return false;
}

This authorization can be bypassed due to the implementation of the get_client_addr function. The function is defined in the file lib/functions.php and checks serval $_SERVER variables to determine the IP address of the client:

function get_client_addr($client_addr = false) {
	$http_addr_headers = array(
		'X-Forwarded-For',
		'X-Client-IP',
		'X-Real-IP',
		'X-ProxyUser-Ip',
		'CF-Connecting-IP',
		'True-Client-IP',
		'HTTP_X_FORWARDED',
		'HTTP_X_FORWARDED_FOR',
		'HTTP_X_CLUSTER_CLIENT_IP',
		'HTTP_FORWARDED_FOR',
		'HTTP_FORWARDED',
		'HTTP_CLIENT_IP',
		'REMOTE_ADDR',
	);

	$client_addr = false;
	foreach ($http_addr_headers as $header) {
		if (!empty($_SERVER[$header])) {
			$header_ips = explode(',', $_SERVER[$header]);
			foreach ($header_ips as $header_ip) {
				if (!empty($header_ip)) {
					if (!filter_var($header_ip, FILTER_VALIDATE_IP)) {
						cacti_log('ERROR: Invalid remote client IP Address found in header (' . $header . ').', false, 'AUTH', POLLER_VERBOSITY_DEBUG);
					} else {
						$client_addr = $header_ip;
						cacti_log('DEBUG: Using remote client IP Address found in header (' . $header . '): ' . $client_addr . ' (' . $_SERVER[$header] . ')', false, 'AUTH', POLLER_VERBOSITY_DEBUG);
						break 2;
					}
				}
			}
		}
	}

	return $client_addr;
}

The variables beginning with HTTP_ can be arbitrarily set by an attacker. Since there is a default entry in the poller table with the hostname of the server running Cacti, an attacker can bypass the authentication e.g. by providing the header Forwarded-For: <TARGETIP>. This way the function get_client_addr returns the IP address of the server running Cacti. The following call to gethostbyaddr will resolve this IP address to the hostname of the server, which will pass the poller hostname check because of the default entry.

After the authorization of the remote_agent.php file is bypassed, an attacker can trigger different actions. One of these actions is called polldata:

switch (get_request_var('action')) {
	case 'polldata':
		// ...
		poll_for_data();
		// ...
		break;

The called function poll_for_data retrieves a few request parameters and loads the corresponding poller_item entries from the database. If the action of a poller_item equals POLLER_ACTION_SCRIPT_PHP, the function proc_open is used to execute a PHP script:

function poll_for_data() {
	global $config;

	$local_data_ids = get_nfilter_request_var('local_data_ids');
	$host_id        = get_filter_request_var('host_id');
	$poller_id      = get_nfilter_request_var('poller_id');
	$return         = array();

	$i = 0;

	if (cacti_sizeof($local_data_ids)) {
		foreach($local_data_ids as $local_data_id) {
			input_validate_input_number($local_data_id, 'local_data_id');

			$items = db_fetch_assoc_prepared('SELECT *
				FROM poller_item
				WHERE host_id = ?
				AND local_data_id = ?',
				array($host_id, $local_data_id));
			// ...
			if (cacti_sizeof($items)) {
				foreach($items as $item) {
					switch ($item['action']) {
					// ...
					case POLLER_ACTION_SCRIPT_PHP: /* script (php script server) */
						// ...
						$cactiphp = proc_open(read_config_option('path_php_binary') . ' -q ' . $config['base_path'] . '/script_server.php realtime ' . $poller_id, $cactides, $pipes);
						// ...

The attacker-controlled parameter $poller_id is retrieved via the function get_nfilter_request_var, which allows arbitrary strings. This variable is later inserted into the string passed to proc_open, which leads to a command injection vulnerability. By e.g. providing the poller_id=;id the id command is executed.

In order to reach the vulnerable call, the attacker must provide a host_id and local_data_id, where the action of the corresponding poller_item is set to POLLER_ACTION_SCRIPT_PHP. Both of these ids (host_id and local_data_id) can easily be bruteforced. The only requirement is that a poller_item with an POLLER_ACTION_SCRIPT_PHP action exists. This is very likely on a productive instance because this action is added by some predefined templates like Device - Uptime or Device - Polling Time.

Impact

This command injection vulnerability allows an unauthenticated user to execute arbitrary commands if a poller_item with the action type POLLER_ACTION_SCRIPT_PHP (2) is configured.

Remediation

The following suggestions should be applied to prevent the described vulnerability.

The authorization bypass should be prevented by not allowing an attacker to make get_client_addr (file lib/functions.php) return an arbitrary IP address. This could be done by not honoring the HTTP_... $_SERVER variables. If these should be kept for compatibility reasons it should at least be prevented to fake the IP address of the server running Cacti.

The command injection should be prevented by applying the following changes to the file remote_agent.php:

The variable $poller_id is supposed to be an integer and should thus be retrieved via the function get_filter_request_var instead of get_nfilter_request_var:

function poll_for_data() {
	// ...
           $poller_id      = get_filter_request_var('poller_id');
           // ...

For further hardening against command injections the $poller_id should be escaped with escapeshellarg before being passed to proc_open:

function poll_for_data() {
// ...
$cactiphp = proc_open(read_config_option('path_php_binary') . ' -q ' . $config['base_path'] . '/script_server.php realtime ' . escapeshellarg($poller_id), $cactides, $pipes);
// ...

Patches

For instances of 1.2.x running under PHP < 7.0, a further change a8d59e8 is also required.