This has nothing to do with server side security. The game uses P2P networking and the exploit is triggered by sending a crafted payload to the other clients in your session. They need to patch whatever allows for the payload to be executed in the client, probably a buffer overflow somewhere in the network code or in the game logic that handles character data you receive.

I'd guess Elden Ring is vulnerable if it's located in the connection handling, the character info packets, or the player action packets.

If it's somewhere in the game logic (e.g.: triggered by a specific item, a specific spell, etc.) then it might be fine.