If you’re running any kind of server infrastructure, you’ve probably heard the advice to ”close unused ports” so many times it sounds like a cliché. But here’s the thing – this advice exists because open ports remain one of the most exploited attack vectors in real-world breaches. I’ve seen firsthand how a single forgotten open port can become the entry point for a devastating security incident.
The truth is, port security isn’t just theoretical security theater. It’s the difference between a secure server and one that’s actively being probed by attackers right now. Let me share some real incidents and what we can learn from them.
The WannaCry Wake-Up Call
In May 2017, the WannaCry ransomware attack infected over 200,000 computers across 150 countries in just a few days. The attack vector? Port 445, used by Windows SMB protocol. Organizations that had left this port exposed to the internet found themselves locked out of their own systems, with attackers demanding Bitcoin ransoms.
What made this particularly devastating was how preventable it was. Microsoft had released a patch months earlier, but many organizations hadn’t applied it. More critically, many had SMB ports exposed directly to the internet when they should have been behind firewalls or VPNs.
The lesson here is brutal but simple: if a port doesn’t need to be publicly accessible, don’t leave it open. SMB is designed for internal networks, not the public internet. Yet countless servers had it exposed simply because nobody had audited their port configuration.
When Development Ports Go to Production
I once consulted for a mid-sized e-commerce company that experienced a data breach. During development, their team had opened port 27017 (MongoDB’s default port) for easier testing. When they pushed to production, nobody remembered to close it. Worse, their MongoDB instance had no authentication enabled – another common development shortcut.
Attackers found the open port within days. They didn’t need sophisticated hacking tools or zero-day exploits. They just scanned for open MongoDB ports, connected, and started exfiltrating customer data. The entire breach could have been prevented with two simple steps: closing the port to public access and enabling authentication.
This taught me that development conveniences are production vulnerabilities. Every port you open for testing should be documented and reviewed before going live. Better yet, use different configurations for development and production environments from the start.
The Redis Incident That Shouldn’t Have Happened
In 2015, thousands of Redis instances were compromised because they were running on port 6379 with no password protection and bound to public IP addresses. Attackers didn’t just steal data – they injected SSH keys, giving themselves permanent backdoor access to the servers.
Redis, like many database systems, was never designed to be exposed to the internet. It’s meant to be an internal service, accessed only by your application servers. Yet system administrators installed it with default settings and never changed the binding address from 0.0.0.0 (all interfaces) to 127.0.0.1 (localhost only).
The lesson: default configurations are rarely secure configurations. When you install any service, your first question should be: ”Does this need to be accessible from outside?” If not, bind it to localhost and add firewall rules to restrict access.
Docker’s Surprise Port Exposure
Here’s a more modern example that catches people off guard. Docker, by default, can bypass your firewall rules when you publish ports. I’ve seen several incidents where administrators thought their firewalls were protecting services, only to discover Docker had exposed ports directly to the internet.
One company ran Elasticsearch in a Docker container for internal log analysis. They published the port to make it accessible to their application servers, assuming their firewall would protect it. Docker, however, added its own iptables rules that effectively bypassed their security. Within hours, their Elasticsearch cluster was discovered and exploited.
The fix requires explicitly telling Docker not to bypass the firewall or carefully managing Docker’s network configuration. But most people don’t know this until it’s too late. Container technologies add complexity to port management – you can’t just set firewall rules and forget about it.
Lessons That Apply to Every Server
After years of dealing with port security issues, here’s what actually matters:
Inventory your open ports regularly. Don’t assume you know what’s running. Use both internal and external scans to see what’s actually accessible. What you think is closed might not be.
Default deny is your friend. Start with all ports closed and only open what you specifically need. Document why each port is open and who requested it.
Layer your security. Even if a port needs to be open, add authentication, use VPNs for administrative access, and implement rate limiting. One security measure will fail – multiple layers might save you.
Monitor continuously. Port configuration drift is real. Services get added, configurations change, and ports that were once closed get opened during troubleshooting and never reclosed.
Automated scanning beats manual audits. Human memory is unreliable, especially in dynamic environments. Automated tools will catch the SSH port you forgot about or the database that’s suddenly exposed after a configuration change.
The Bottom Line
Every one of these incidents was preventable. None required sophisticated security tools or massive budgets. They just needed someone paying attention to basic port hygiene.
The attackers aren’t always nation-states with zero-day exploits. Often, they’re opportunists running automated scans looking for the low-hanging fruit – exposed databases, unsecured management interfaces, forgotten development services. Don’t be the low-hanging fruit.
Your servers are being scanned right now. The question isn’t whether attackers will find your open ports – it’s what they’ll find when they do.