Nintendo has taken down the online features of the Wii U versions of Mario Kart 8 and Splatoon today due to an unspecified security vulnerability. VGC reports that the games were taken offline at 11:30PM ET last night and that the company suggests the fix could take days (via IGN). The Nintendo Switch versions of the games are not affected.

At time of writing, Nintendo’s server operational status page currently has this message posted:

Splatoon , Mario Kart 8 — This network service is currently unavailable due to urgent maintenance required to fix a vulnerability related to online play. We do not yet have information on when network services can be restored. We apologise for any inconvenience caused. (Error Code: 106-0811)

As VGC notes, a Nintendo data miner known as OatmealDome tweeted that the security issue could be related to an exploit that allows an attacker to take over a victim’s console by just pairing up with them in the game. According to OatmealDome, whose Twitter profile says they’re also a Dolphin Emulator developer, the vulnerability is “almost certainly” ENLBufferPwn, which Nintendo purportedly defended against by releasing a patch for some games on the Switch and even Mario Kart 7 on the 3DS.

The Wii U’s top-selling game of all time was Mario Kart 8 (just like the Nintendo Switch with the Deluxe version) and sold over 8 million copies. So if there are any active Wii U players out there — I personally have a friend who’s borrowing mine and his kids play it almost daily — then it’s likely Mario Kart 8 online play is being missed somewhere right now.

Nintendo’s Wii U was discontinued in 2017 and had only sold 13.56 million units in its lifetime. The system was widely regarded as a failure and was outsold by its successor, the Nintendo Switch, in just 10 months.

Nintendo is still working on fixing the issues with Mario Kart 8 and Splatoon for Wii U despite the console being defunct — though it’s planning on taking away new Wii U eShop sales this month. Considering Nintendo has released Switch console bundles with Mario Kart 8 Deluxe on an almost annual basis, the company could be expecting fans to have upgraded from the Wii U by now.