There are currently vulnerabilities that allow players in Mario Kart Wii to potentially brick your Wii
For More information on this see these for reference
Announcement on Wii Homebrew https://forum.wii-homebrew.com/index.php/Thread/59768-Security-issues-regarding-AltWFC-servers/?s=ddfae57097d391201db4e2350e13019a5c501a1d
[Archived version of the announcement in the event that they delete it] https://web.archive.org/web/20200810205930/https://forum.wii-homebrew.com/index.php/Thread/59768-Security-issues-regarding-AltWFC-servers/?s=ddfae57097d391201db4e2350e13019a5c501a1d
Exploit Release By Star https://mkwii.com/showthread.php?tid=1542&highlight=Star
[Archived Version in case the one above gets deleted] https://web.archive.org/web/20200720004106/https://mkwii.com/showthread.php?tid=1542&highlight=Star
While there isn't a complete solution to this problem as there may be more exploits hidden in the game, There is a solution provided by MkWii Forum member sneeky.
RCE protection by [sneeky] https://mkwii.com/printthread.php?tid=1543
[Archived version in case the top one gets deleted] https://web.archive.org/web/20200716172556/https://mkwii.com/printthread.php?tid=1543
RACE protection from Star https://mkwii.com/printthread.php?tid=1854
[Archived version in case the top one gets deleted] https://web.archive.org/web/20210704175413/https://mkwii.com/printthread.php?tid=1854
While this isn't the best solution the only other one is to not play Mario Kart Wii online
*Note Wiimmfi is very much vulnerable to exploits that haven't been publicly discovered, you are never 100% safe playing Wii games online