How to protect certain static routes?

Depends on what kind of “protection” you want.

Does your app have any existing protected routes?

You could have HTTP Basic Auth, phx.gen.auth email+password accounts, integration with external services for auth, …

You can add a custom plug in your pipeline that would wrap Plug.Static and implement custom protection logic there.

# assuming a Phoenix project
defmodule MyAppWeb.Endpoint do
  # ...
  # private static files
  plug :private_static, Plug.Static.init(at: "/private", from: :my_app)
  # public static files
  plug Plug.Static, at: "/", from: :my_app, only: MyAppWeb.static_paths()
  # ...

  defp private_static(%Plug.Conn{path_info: ["private" | _]} = conn, plug_static_opts) do
    if private_static_allowed?(conn) do
      # NOTE: if it doesn't match any files, then it would pass-through to router
      Plug.Static.call(conn, plug_static_opts)
    else
      conn |> send_resp(403, "forbidden or whatev") |> halt()
    end
  end

  # pass-through non /private/** routes
  defp private_static(conn, _opts), do: conn

  defp private_static_allowed?(conn) do
    # your custom logic
    true
  end
end

And you can also do it in the router. It might be even easier that way.

I would be using phx Auth

Thanks for your example!!

In that case combine the plugs generated by phx.gen.auth (mix phx.gen.auth — Phoenix v1.8.8) with the example from @ruslandoga to gate access to Plug.Static.

His example is great, as it leaves space for your own custom logic.