Popular Slay The Spire mod Downfall was hijacked to spread malware through Steam

But only if you have it as a game in your library NOT as a mod in workshop.

Also only if you launched recently (it has been patched since).

Mods say that it is now safe to play.

Here's a link to the dev logv https://store.steampowered.com/news/app/1865780/view/3865841912968681603