Nintendo announced earlier this morning that the online servers for Splatoon and Mario Kart 8 have been temporarily disabled, most likely due to ENLBufferPwn, an exploit capable of remote execution and hacking of a Nintendo console by doing as little as connecting to the game servers. No word on when they'll be re-enabled. They updated various Switch and 3DS (!!) games to fix this already, but I'm guessing it might be a harder fix on Wii U.