Historically, Paradox used simple SteamStub DRM—trivial to crack. But in late 2023 and 2024, they quietly upgraded their protection for EU4.
While single-player usually works flawlessly with DLC unlockers, hosting multiplayer games can be unreliable and may require additional workarounds. Patch 1.37 - Europa Universalis 4 Wiki csrinru eu4 updated