Docker Can Silently Bypass UFW Firewall Rules, Exposing Container Ports Publicly

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A known but widely overlooked security flaw causes Docker to bypass UFW firewall rules on Linux servers, leaving container ports exposed to the public internet even when UFW is configured to block them. The issue stems from Docker injecting its own iptables rules at startup, which take precedence over UFW's deny rules before they are evaluated. As a result, services like admin dashboards mapped to ports such as 8080 remain publicly accessible despite firewall restrictions. The fix involves appending a custom iptables configuration block to UFW's after.rules file and reloading the firewall, which forces Docker traffic through a chain that respects UFW rules. Once applied, standard UFW commands can be used to control container port access, including restricting specific ports to trusted IP addresses only.

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