// composer require "twig/twig"
require 'vendor/autoload.php';

class Template {
  private $twig;

  public function __construct() {
    $indexTemplate = '<img ' .
      'src="https://loremflickr.com/320/240">' .
      '<a href="{{link|escape}}">Next slide &raquo;</a>';

    // Default twig setup, simulate loading
    // index.html file from disk
    $loader = new Twig\Loader\ArrayLoader([
      'index.html' => $indexTemplate
    ]);
    $this->twig = new Twig\Environment($loader);
  }

  public function getNexSlideUrl() {
    $nextSlide = $_GET['nextSlide'];
    return filter_var($nextSlide, FILTER_VALIDATE_URL);
  }

  public function render() {
    echo $this->twig->render(
      'index.html',
      ['link' => $this->getNexSlideUrl()]
    );
  }
}

(new Template())->render();

<?php
    $url = filter_var($_GET['url'], FILTER_VALIDATE_URL);
	var_dump($url);
	$url = htmlspecialchars($url);
	var_dump($url);
	echo "<a href='$url'> Next Slide </a>";
?>

function RemoveXSS($val) {
       $val = preg_replace('/([\x00-\x08,\x0b-\x0c,\x0e-\x19])/', '', $val);
       $search = 'abcdefghijklmnopqrstuvwxyz';
       $search .= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
       $search .= '1234567890!@#$%^&*()';
       $search .= '~`";:?+/={}[]-_|\'\\';
       for ($i = 0; $i < strlen($search); $i++) {
          $val = preg_replace('/(&#[xX]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
          $val = preg_replace('/(&#0{0,8}'.ord($search[$i]).';?)/', $search[$i], $val); // with a ;
       }

       $ra1 = array('javascript', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'blink', 'link', 'style', 'script', 'embed', 'object', 'iframe', 'frame', 'frameset', 'ilayer', 'layer', 'bgsound', 'title', 'base');
       $ra2 = array('onabort', 'onactivate', 'onafterprint', 'onafterupdate', 'onbeforeactivate', 'onbeforecopy', 'onbeforecut', 'onbeforedeactivate', 'onbeforeeditfocus', 'onbeforepaste', 'onbeforeprint', 'onbeforeunload', 'onbeforeupdate', 'onblur', 'onbounce', 'oncellchange', 'onchange', 'onclick', 'oncontextmenu', 'oncontrolselect', 'oncopy', 'oncut', 'ondataavailable', 'ondatasetchanged', 'ondatasetcomplete', 'ondblclick', 'ondeactivate', 'ondrag', 'ondragend', 'ondragenter', 'ondragleave', 'ondragover', 'ondragstart', 'ondrop', 'onerror', 'onerrorupdate', 'onfilterchange', 'onfinish', 'onfocus', 'onfocusin', 'onfocusout', 'onhelp', 'onkeydown', 'onkeypress', 'onkeyup', 'onlayoutcomplete', 'onload', 'onlosecapture', 'onmousedown', 'onmouseenter', 'onmouseleave', 'onmousemove', 'onmouseout', 'onmouseover', 'onmouseup', 'onmousewheel', 'onmove', 'onmoveend', 'onmovestart', 'onpaste', 'onpropertychange', 'onreadystatechange', 'onreset', 'onresize', 'onresizeend', 'onresizestart', 'onrowenter', 'onrowexit', 'onrowsdelete', 'onrowsinserted', 'onscroll', 'onselect', 'onselectionchange', 'onselectstart', 'onstart', 'onstop', 'onsubmit', 'onunload');
       $ra = array_merge($ra1, $ra2);

       $found = true; 
       while ($found == true) {
          $val_before = $val;
          for ($i = 0; $i < sizeof($ra); $i++) {
             $pattern = '/';
             for ($j = 0; $j < strlen($ra[$i]); $j++) {
                if ($j > 0) {
                   $pattern .= '(';
                   $pattern .= '(&#[xX]0{0,8}([9ab]);)';
                   $pattern .= '|';
                   $pattern .= '|(&#0{0,8}([9|10|13]);)';
                   $pattern .= ')*';
                }
                $pattern .= $ra[$i][$j];
             }
             $pattern .= '/i';
             $replacement = substr($ra[$i], 0, 2).'<x>'.substr($ra[$i], 2);
             $val = preg_replace($pattern, $replacement, $val); 
             if ($val_before == $val) {
                $found = false;
             }
          }
       }
       return $val;
    }

// index.php
<?php 
$url = $_GET['url'];
if(isset($url) && filter_var($url, FILTER_VALIDATE_URL)){
    $site_info = parse_url($url);
    if(preg_match('/sec-redclub.com$/',$site_info['host'])){
        exec('curl "'.$site_info['host'].'"', $result);
        echo "<center><h1>You have curl {$site_info['host']} successfully!</h1></center>
              <center><textarea rows='20' cols='90'>";
        echo implode(' ', $result);
    }
    else{
        die("<center><h1>Error: Host not allowed</h1></center>");
    }

}
else{
    echo "<center><h1>Just curl sec-redclub.com!</h1></center><br>
          <center><h3>For example:?url=http://sec-redclub.com</h3></center>";
}
?>

// f1agi3hEre.php
<?php
$flag = "HRCTF{f1lt3r_var_1s_s0_c00l}"
?>