I just did some more research into this:

When a client tries to log in, the server sends a response back with a JWT token inside the body. The client then stores this token inside an isomorphic cookie. This cookie is then used to identify the user by adding it to all requests after that.

I haven't really checked post requests, but if it is the same So let's say you visit website X, that has nothing to do with Lemmy, they could do a Fetch request in the background to lemmy.world to post spam on your behalf. Even a CSRF token could no longer protect you, as the website could just do a fetch request first to get a CSRF token.

I hope I'm wrong in some way, but to me this feels like a huge security risk.