Hacked Pokémon are crashing Sword and Shield games over Surprise Trade

Max Raid Sneasel Source: Game Freak

What you need to know

Pokémon HOME just launched, giving players a way to transfer more Pokémon to Sword and Shield.
People have already hacked Max Raid battles.
Players are now reporting that hacked Pokémon being sent over Surprise Trade are crashing their games.

You might want to put a pause on your surprise trades in Pokémon Sword and Shield for a little while. Players are reporting that hacked Pokémon being randomly sent over via the service are crashing their games. This bug appears to be rare, but it's unclear just how many people have been affected.

A Twitter user known as Kaphotics first made the public aware of this bug, according to Eurogamer, which was discovered and verified by Twitter users architdate and Sibuna_Switch.

Beware! There's a bug in Sword/Shield that can crash your game if you receive a maliciously crafted Pokémon via Surprise Trade. It'll crash your game when you try to access online features (via Y-COM), preventing online interactions until a patch is released.
— Kurt (@Kaphotics) February 12, 2020

These hacked Pokémon are created with tools from hacking and modding programs, with people making them deliberately to crash games and then sending them out into the wild over Surprise Trade. Pokémon caught through legitimate methods and traded should not trigger this bug, meaning Pokémon coming from Pokémon HOME are safe for now, so long as they were not illegitimately created to begin with.

Apparently your save data will not be corrupted, but the moment you attempt to receive the "maliciously crafted" Pokémon, your game will crash. Players affected by this bug were unable to launch any online features. It also happens during a normal trade, but your game will only crash once, after which you can release the Pokémon.

You may have also noticed illegitimate Pokémon popping up in Max Raid battles. These are also hacked by modders, though it is unclear whether or not they will crash your game. To stay on the safe side, don't enter a raid with a hacked Pokémon. We have a list of every legitimate Wild Area den and the Pokémon that spawns from them.

Nintendo has yet to comment on the issue.