Port Scans: Identifying Open Doors in Your Network Perimeter

Port scanning is a foundational activity in any penetration test, and for good reason. It’s often the first sign that an attacker is assessing your network—and sometimes, it's your own pentest team trying to beat them to it. In a recent engagement, an internal red team exercise revealed several misconfigured and vulnerable services exposed to the internet through open ports.

🔍 What Did the Scan Reveal?

Using tools like Nmap and Masscan, testers identified multiple publicly accessible services that should have been behind firewalls or isolated via network segmentation:

• Open SSH ports (22) on production web servers

• Exposed MySQL databases (3306) without access restrictions

• Old versions of HTTP services (80/8080) still running with default configurations

• SNMP (161) available on internal gateways

These findings signaled poor perimeter hygiene and weak access controls.

🧪 Testing Methodology

The testing team followed a systematic approach:

1. Network Discovery

   • Subnet scanning

   • ARP mapping of internal network

2. Port Identification

   • Full TCP and UDP scans on live hosts

3. Service Fingerprinting

   • Identifying software versions and potential CVEs

4. Vulnerability Mapping

   • Cross-referencing discovered services with known exploits

All findings were documented and verified manually to avoid false positives.

📉 Why Open Ports Matter

Leaving ports open—even unintentionally—can:

• Invite automated scanning bots that catalog services for exploit campaigns

• Expose outdated services vulnerable to known CVEs

• Lead to unauthorized access if weak or default credentials are used

• Act as an entry point for pivoting deeper into your infrastructure

🛡️ Defense Recommendations

Based on the scan results, several mitigations were suggested:

• Restrict public access: Use firewall rules and access control lists (ACLs)

• Enforce segmentation: Place sensitive systems in isolated VLANs

• Use port knocking or VPNs for access to administrative services

• Disable unused services and ports immediately

• Conduct regular scans to catch configuration drift

🧠 Final Thoughts

Port scanning might seem like an old-school tactic, but it remains one of the most powerful tools in both attacker and defender arsenals. Regular internal scans are a simple yet effective way to ensure your network isn't leaving doors open to unwanted visitors.

As always in cybersecurity: what you don’t know will hurt you.