The following seems to work for me as contents of
[default] region = eu-west-1 role_arn = arn:aws:iam::12345:role/blubb source_profile = blabla [blabla] aws_access_key_id = XXX aws_secret_access_key = YYY aws_session_token = ZZZ valid_until = DDD
The content of
.aws/config does not seem to matter. So it can be whatever it needs to be for the CLI.