It was only a matter of time before the Wii U got cracked wide open, just like the Wii did. A brand new browser exploit has been found that allows you to run unsigned code. This basically lets you make the Wii U run whatever program you wish. One of the first things hackers did was patch Mario Kart 8 on the fly. This allows you to race on custom tracks, use custom menus, patch in custom racers, and more.
Unfortunately, this also opens the floodgates for a lot of hacks that are less friendly. For example, you can now use a software side cheat device to alter your game’s values. This could allow you to do things like, steer perfectly, auto aim items at the enemy, perfectly drift, and more. Heck, it could even allow you to simply make yourself invincible or super fast. The Nintendo servers have no anti-cheat software on them right now, so at best it allows cheaters full reign over the online world, and at worst will cause frequent desynchs which will render many races unplayable.
The one saving grace is that your Wii U needs to be outdated for this crack to run. However, hackers did find a way to make the Wii update itself while retaining cracks. So who knows what will happen in the future. At least this means hackers will be able to rebalance the new Smash when it comes out.