MySQL  8.0.15
Source Code Documentation
Protocol::AuthMoreData:

We need to make sure that when sending plugin supplied data to the client they are not considered a special out-of-band command, like e.g.

ERR_Packet, Protocol::AuthSwitchRequest: or OK_Packet. To avoid this the server will send all plugin data packets "wrapped" in a command \1. Note that the client will continue sending its replies unrwapped: Protocol::AuthSwitchResponse:

Payload
TypeNameDescription
int<1> 0x01 status tag
string<EOF> authentication method data Extra authentication data beyond the initial challenge
See also
wrap_plguin_data_into_proper_command, server_mpvio_write_packet, client_mpvio_read_packet