On September 22nd, 2017, Nintendo created an app authentication program for online Nintendo accounts.
This Two-Factor Authorization is a security measure that makes it difficult for other people to access your account.
To enable this new security feature, head to Nintendo’s login web portal here, then go to the Sign-in and Security page.
Note that if you have not already, you will need to verify an e-mail address before you an enable the Two-Factor Authorization. An email will send you a verification code to continue.
When you have it enabled. There will now be a requirement that every new log in for your Nintendo account requires an additional authorization besides the login and password. You will be asked to download a Google Authenticator App on your smart device, and scan the QR Code (or input a key). Finally, the app will give you a 6-digit code that you then enter to confirm the authorization.
From then on, you will use the app in conjunction with signing in – which will prevent others from accessing your Nintendo Account without your knowledge. Once you have provided your email and password, you will be asked to provide a code now found in your Google Authenticator app – and this code will change constantly.
Note: After first setting up your authentication, you will be given a list of backup codes in case you are unable to access your app for any reason – copy these codes to a safe place!