UserLoginForm::validateAuthentication

public UserLoginForm::validateAuthentication(array &$form, FormStateInterface $form_state)

public function validateAuthentication(array &$form, FormStateInterface $form_state) {
  $password = trim($form_state->getValue('pass'));
  $flood_config = $this->config('user.flood');
  if (!$form_state->isValueEmpty('name') && strlen($password) > 0) {
    // Do not allow any login from the current user's IP if the limit has been
    // reached. Default is 50 failed attempts allowed in one hour. This is
    // independent of the per-user limit to catch attempts from one IP to log
    // in to many different user accounts.  We have a reasonably high limit
    // since there may be only one apparent IP for all users at an institution.
    if (!$this->flood->isAllowed('user.failed_login_ip', $flood_config->get('ip_limit'), $flood_config->get('ip_window'))) {
      $form_state->set('flood_control_triggered', 'ip');
      return;
    }
    $accounts = $this->userStorage->loadByProperties(array('name' => $form_state->getValue('name'), 'status' => 1));
    $account = reset($accounts);
    if ($account) {
      if ($flood_config->get('uid_only')) {
        // Register flood events based on the uid only, so they apply for any
        // IP address. This is the most secure option.
        $identifier = $account->id();
      }
      else {
        // The default identifier is a combination of uid and IP address. This
        // is less secure but more resistant to denial-of-service attacks that
        // could lock out all users with public user names.
        $identifier = $account->id() . '-' . $this->getRequest()->getClientIP();
      }
      $form_state->set('flood_control_user_identifier', $identifier);

      // Don't allow login if the limit for this user has been reached.
      // Default is to allow 5 failed attempts every 6 hours.
      if (!$this->flood->isAllowed('user.failed_login_user', $flood_config->get('user_limit'), $flood_config->get('user_window'), $identifier)) {
        $form_state->set('flood_control_triggered', 'user');
        return;
      }
    }
    // We are not limited by flood control, so try to authenticate.
    // Store $uid in form state as a flag for self::validateFinal().
    $uid = $this->userAuth->authenticate($form_state->getValue('name'), $password);
    $form_state->set('uid', $uid);
  }
}