[tasn]

The same way that it can reject certain headers, like it could have done in this case. It's no different, still in-band.

[kccqzy]

The middleware doesn't have to reject it. It could decide to just wrap it and pass it along. The backend code can then be able to distinguish which was sent by the client and which was added by the middleware. And that's the point. The middleware can do as little or as much filtering it desires, without causing any confusion to the backend.