code: private void setupDefaultUser()
      {
          try
          {
              // insert the default superuser if AUTH_KS.CREDENTIALS_CF is empty.
              if (!hasExistingUsers())
              {
                  process(String.format("INSERT INTO %s.%s (username, salted_hash) VALUES ('%s', '%s') USING TIMESTAMP 0",
                                        Auth.AUTH_KS,
                                        CREDENTIALS_CF,
                                        DEFAULT_USER_NAME,
                                        escape(hashpw(DEFAULT_USER_PASSWORD))),
                          ConsistencyLevel.ONE);
                  logger.info("PasswordAuthenticator created default user '{}'", DEFAULT_USER_NAME);
              }
          }
          catch (RequestExecutionException e)
          {
              logger.warn("PasswordAuthenticator skipped default user setup: some nodes were not ready");
          }
      }
docs: if there are no users yet - add default superuser.