request.host can be `127.0.0.1:8123` https://github.com/aio-libs/aiohttp/blob/0b8be7f6c8db0d59afe02109df9be874d0d637e6/aiohttp/web_request.py#L447 yarl 1.13.0+ caught this because host is now validated via https://github.com/aio-libs/yarl/pull/954 Pinning yarl to < 1.13.0 will also prevent the issue since the validation doesn't happen until that version.