mailcow is a dockerized email package, with multiple containers linked in one bridged network. A security vulnerability has been identified in mailcow affecting versions < 2024-01c. This vulnerability potentially allows attackers on the same subnet to connect to exposed ports of a Docker container, even when the port is bound to 127.0.0.1. The vulnerability has been addressed by implementing additional iptables/nftables rules. These rules drop packets for Docker containers on ports 3306, 6379, 8983, and 12345, where the input interface is not `br-mailcow` and the output interface is `br-mailcow`.