bleepingcomputer[.]com/news/security/steam-and-microsoft-warn-of-unity-flaw-exposing-gamers-to-attacks/
neowin[.]net/news/microsoft-mesh-also-impacted-by-unity-issue-patches-rolling-out-for-all-games-too/
discussions[.]unity.com/t/unity-platform-protection-take-immediate-action-to-protect-your-games-and-apps/1688031
While you may have heard of the big name game engines used by expensive AAA games, Unity is the free and low cost option used by quite a few less expensive & giveaway games. Steam has released updated client software to help mitigate the vulnerability. Game devs are supposed to download an update from Unity, recompile the game, and make the new version available to users, but on especially older low cost & free games that's not gonna happen. The good news is that no one's reported anyone actually exploiting the vulnerability, so at least for now you can likely get away with running games using Unity as-is. AntiVirus software *may* also be updated to watch out for and block exploits, and according to Unity, Microsoft is updating Windows to mitigate the vulnerability as well. Generally if you look at the file names in a game's folder you'll see one or more files with Unity in their name if the game uses the Unity game engine.
Unity has also released a patcher, that replaces UnityPlayer.dll, but it's not intended for users, since it may break the game. There's nothing stopping you if you want to try it, though you'd probably want to back up the game's folder beforehand.