Nintendo Network, the portal for users on the Nintendo Switch and 3DS, implemented two-factor authentication for all accounts effective now.
The feature is available in the Nintendo Network portal after logging in; the user then should go to the “sign-in and security settings” tab. At the bottom of that column is an option to enable two-step verification.
The two-factor authentication runs through Google Authenticator, which requires a mobile device (iOS or Android). The result is that every new login to Nintendo Network under this ID needs to pass Google Authenticator in addition to the normal login controls.