Bad actors are abusing large, open-registration, low-moderation Mastodon instances in order to provide direction to the Vidar Stealer trojan horse, which steals passwords, credit card details, bitcoin wallets, etc.

If you run a large, open-registration, low-moderation instance, please consider changing at least one of those qualities.

https://www.bleepingcomputer.com/news/security/vidar-stealer-abuses-mastodon-to-silently-get-c2-configuration/