//Form Alter, check for form user_form
function base_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {

  if ($form_id == 'user_form') {
    $form['#validate'][] = '_user_form_validation';
  }

}

//Validation example
function _user_form_validation(&$form, \Drupal\Core\Form\FormStateInterface $form_state){

  if ($form_state->hasValue('pass')) {
    $pass = $form_state->getValue('pass');

    if (strlen($pass) < 6 ) {
      $form_state->setErrorByName('pass', t('Password length must be at least 6 characters.'));
    }

  }
}