RedirectLeadingSlashesSubscriber.php

<?php

namespace Drupal\Core\EventSubscriber;

use Drupal\Core\Cache\CacheableRedirectResponse;
use Symfony\Component\HttpKernel\Event\GetResponseEvent;
use Symfony\Component\HttpKernel\KernelEvents;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;

/**
 * Redirects paths starting with multiple slashes to a single slash.
 */
class RedirectLeadingSlashesSubscriber implements EventSubscriberInterface {
  
  /**
   * Redirects paths starting with multiple slashes to a single slash.
   *
   * @param \Symfony\Component\HttpKernel\Event\GetResponseEvent $event
   *   The GetResponseEvent to process.
   */
  public function redirect(GetResponseEvent $event) {
    $request = $event->getRequest();
    // Get the requested path minus the base path.
    $path = $request->getPathInfo();
    // It is impossible to create a link or a route to a path starting with
    // multiple leading slashes. However if a form is added to the 404 page that
    // submits back to the same URI this presents an open redirect
    // vulnerability. Also, Drupal 7 renders the same page for
    // http://www.example.org/foo and http://www.example.org////foo.
    if (strpos($path, '//') === 0) {
      $path = '/' . ltrim($path, '/');
      $qs = $request->getQueryString();
      if ($qs) {
        $qs = '?' . $qs;
      }
      $event->setResponse(new CacheableRedirectResponse($request->getUriForPath($path) . $qs));
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events[KernelEvents::REQUEST][] = [
      'redirect',
      1000,
    ];
    return $events;
  }

}