The OpenTTD Team reports:
Using a slow read type attack it is possible to prevent anyone
from joining a server with virtually no resources. Once
downloading the map no other downloads of the map can start, so
downloading really slowly will prevent others from joining.
This can be further aggravated by the pause-on-join setting in
which case the game is paused and the players cannot continue
the game during such an attack. This attack requires that the
user is not banned and passes the authorization to the server,
although for many servers there is no server password and thus
authorization is easy.