We recently posted that Mario Kart 8 had been modded using a browser hack on Nintendo's Wii U which allowed modder's to make some small tweaks to the game. The exploit which allowed modder's to use on-the-fly RAM edits to mod Mario Kart 8 has been known for a couple of weeks but became common knowledge after a Youtuber posted a video and also did a livestream showing off the Mario Kart 8 mods.
Nintendo have now released a new update (5.0) for the Wii U console which patches this known exploit and therefore the userspace exploit can not be used.
The userspace exploit using the web browser allows basic code execution within the permissions that the browser has. To avoid updating to 5.0 and losing the ability to run homebrew on your Wii U you should not update past 4.1.0 until a way to exploit 5.0 has been found.
For those of you that haven't yet update to 4.1.0 you can go and buy a copy of Mario Kart 8 and let the disc install the update for you.
Also be sure to keep your console offline.