Due to recent unauthorized account access reports, you may want to enable two-step verification for Nintendo accounts.
Switch owners have shown multiple reports stating that they can access their Nintendo account without authorization.
Yesterday, there were reports that the Switch owner ’s account showed unauthorized purchases or access.
Although Nintendo has not confirmed such accounts for users, today it issued a statement to Eurogamer saying that it "has been aware of reports of unauthorized access to certain Nintendo accounts" and is investigating the matter.
For security reasons, Nintendo recommends that all account holders enable two-step verification for their Nintendo accounts. The company also stated that if you find any unauthorized activities, you should take the steps outlined here or contact Nintendo for regular support.
These new reports are only released a few weeks after Nintendo Japan said Third-party login access to data (Thanks to GI.biz).