All containers on a host are basically able to communicate with each other, with the host, and with any other host. If this communication is not restricted, malware or an attacker can exploit this to attack other containers or hosts (for example).

Furthermore, there is the danger that containers will be accessible from the outside, even if this is not desired. This could allow an external attack against services that should only be accessible internally. This threat is increased by the fact that less attention is often paid to internal services. If a vulnerability is tolerated in a service that is only used internally but is also accessible from the outside, this can put the entire corresponding operation at considerable risk.