Earlier today, Nintendo of Japan confirmed rumours that some accounts had become compromised during a hacking attempt in early April.
Nintendo stated that hackers have used IDs and Passwords “obtained illegally by some means other than our service”. The accounts compromised have been associated with fraudulent purchases, and a Fortnite V-Bucks scam. It is also possible that user information, such as Date of Birth, Age, Country, and Email Address, have also been accessed as a result of the hack.
Nintendo has reached out to affected customers via email, and has disabled logging in to an account via a Nintendo Network ID (NNID). The NNID system was mainly used for Wii U and 3DS consoles with account access, but the Switch had compatibility with that log-in system until today.
If you have been affected by the hack, reset your password, and contact Nintendo Support regarding any dubious purchases on your account.