LinkExternalProtocolsConstraintValidator.php

<?php

namespace Drupal\link\Plugin\Validation\Constraint;

use Drupal\Component\Utility\UrlHelper;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\ConstraintValidator;

/**
 * Validates the LinkExternalProtocols constraint.
 */
class LinkExternalProtocolsConstraintValidator extends ConstraintValidator {
  
  /**
   * {@inheritdoc}
   *
   * phpcs:ignore Drupal.Commenting.FunctionComment.VoidReturn
   * @return void
   */
  public function validate($value, Constraint $constraint) {
    if (isset($value)) {
      try {
        /** @var \Drupal\Core\Url $url */
        $url = $value->getUrl();
      } catch (\InvalidArgumentException $e) {
        return;
      }
      // Disallow external URLs using untrusted protocols.
      if ($url->isExternal() && !in_array(parse_url($url->getUri(), PHP_URL_SCHEME), UrlHelper::getAllowedProtocols())) {
        $this->context
          ->addViolation($constraint->message, [
          '@uri' => $value->uri,
        ]);
      }
    }
  }

}