[网鼎杯 2020 青龙组]AreUSerialz

0x00 过程

<?php

include("flag.php");

highlight_file(__FILE__);

class FileHandler {

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

    //初始化执行
    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") {//弱比较，此时op设置i:2或s:2" a"都可绕过
            $res = $this->read();
            $this->output($res);
        } else {
            $this->output("Bad Hacker!");
        }
    }

    
    //出题人干扰函数
    private function write() {
        //判断filename和content是否存在
        if(isset($this->filename) && isset($this->content)) {
            //如果content长度大于100
            if(strlen((string)$this->content) > 100) {
                $this->output("Too long!");
                die();
            }
            //写文件到filename
            $res = file_put_contents($this->filename, $this->content);
            //判断响应
            if($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();
    }

}

//判断传入参数是否在ascii码表指定范围内，在就返回false
function is_valid($s) {
    for($i = 0; $i < strlen($s); $i++)
        if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))
            return false;
    return true;
}

//接收str参数，is_valid判断后反序列化
if(isset($_GET{'str'})) {

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

}