The vulnerable snippet uses a direct primary key lookup. An attacker can simply iterate the 'id' parameter to scrape every post in the database. The secure implementation employs 'Ownership Scoping'. By requiring both the 'id' and the 'user_id' in the Ecto query (Repo.get_by), the database engine itself enforces the authorization boundary. If a user attempts to access an ID they do not own, the query returns nil. This approach is more performant and less error-prone than fetching the object first and checking permissions in application logic.

def show(conn, %{"id" => id}) do
  user = conn.assigns.current_user
  # SECURE: Scopes the lookup to the authenticated user's context.
  case Blog.get_user_post(user, id) do
    nil -> 
      conn 
      |> put_status(:not_found) 
      |> json(%{error: "Resource not found or access denied"})
    post -> 
      render(conn, "show.json", post: post)
  end
end
In Blog context:
def get_user_post(user, id) do
Repo.get_by(Post, id: id, user_id: user.id)
end