Great question—let’s break this down step by step since TCP port 0 is a bit of a special case that often confuses scanners and admins alike.

TCP port 0 is not a valid, listenable port in the traditional sense. Here’s the breakdown of how it works:

• When an application requests to bind to port 0 (via system calls like bind()), the operating system automatically assigns it an available ephemeral port (typically in the range 1024–65535, though this varies by OS).
• There is no service that ever actually listens on port 0 itself. It’s purely a placeholder that tells the OS to pick a dynamic port for the application.
• Incoming traffic is never directed to port 0; it goes to the ephemeral port assigned by the OS once the application is bound.

These are definitely false positives. Here’s why:

• Since no service can listen on TCP port 0, there’s no actual software running on that port to have vulnerabilities.
• Credentialed scans like Nessus might flag port 0 due to a quirk in how the scanner interprets port data, or if the scan configuration accidentally includes port 0 in its target range. Thousands of vulnerabilities reported on a non-existent listening port are a clear red flag for misinterpretation by the tool.
• It’s also possible the scanner is misreporting ephemeral ports as port 0, but even then, each ephemeral port would correspond to a specific running service—not a generic "port 0" with thousands of flaws.

Follow these steps to resolve the issue:

• Verify no process is actually using port 0: Run commands on the target Linux/Unix server to check for listeners on port 0:

  netstat -tulpn | grep ":0"
  # Or use ss for newer systems:
  ss -tulpn | grep ":0"
  

  You should get no results here, as no legitimate process listens on port 0.
• Adjust your Nessus scan policy: Edit your credentialed scan configuration to exclude port 0 from the target port list. Most scanners include port 0 by default in "all ports" scans, so explicitly removing it will prevent future false positives.
• Mark the findings as false positives: In Nessus, go to the scan results, select all port 0 vulnerability entries, and mark them as False Positive with a note explaining why (e.g., "TCP port 0 is reserved and no service listens here").
• Audit ephemeral port usage: If you’re concerned about dynamic port security, review your OS’s ephemeral port range (check /proc/sys/net/ipv4/ip_local_port_range on Linux) and ensure that only trusted applications are using these ports. Monitor for unusual ephemeral port activity with tools like tcpdump or your SIEM solution.

内容的提问来源于stack exchange，提问作者Shakir