Port 3339: Unassigned — A Vacancy in the Registry

What Range This Port Belongs To

Port 3339 sits in the registered ports range (1024–49151), also called the user ports range. IANA maintains this space as a kind of registry: vendors and developers can formally request a port number for their software so other services don't accidentally collide with them.

Port 3339 is listed as unassigned. IANA holds the number but no one has claimed it with an official service.¹

That's not unusual. The registered range contains thousands of unassigned gaps — port numbers that either never attracted a claimant, or whose original service faded away before anyone noticed to clean up the record.

Known Unofficial Uses

ABB System 800xA (Industrial Control)

ABB's System 800xA platform — used to control industrial processes in power plants, factories, and refineries — uses TCP ports 3338 and 3339 for its Aspect Server connections.² The Aspect Server is the core of the 800xA architecture: it stores the "intelligence" of the system, holding graphical elements, object definitions, and configuration libraries.

This is a legitimate, specific use — just never submitted to IANA for official assignment. You'll only encounter it inside industrial control network segments, not on the open Internet.

Historical Malware Association

Some security databases flag port 3339 as having been used by malware at some point in the past.³ These flags are common across the registered range and rarely come with specifics — "a trojan used this port once" describes thousands of port numbers. The flag alone means very little. If you see unexpected activity on port 3339 and you're not running ABB 800xA equipment, that's worth investigating, but the flag itself isn't cause for alarm.

How to Check What's Listening on This Port

If you see port 3339 active on a system and want to know what's using it:

On Linux or macOS:

# Show the process using port 3339
sudo ss -tlnp | grep 3339
# or
sudo lsof -i :3339

On Windows:

# Show process ID using port 3339
netstat -ano | findstr :3339
# Then find the process:
tasklist | findstr <PID>

With nmap (from another machine):

nmap -sV -p 3339 <target-ip>

The -sV flag asks nmap to probe the service and guess what's running — useful when you can't check the host directly.

Why Unassigned Ports Matter

The port registry is the Internet's phonebook for services. When a port is officially assigned, every firewall vendor, intrusion detection system, and network scanner knows what to expect there. When it's unassigned, those systems have no baseline to compare against — anything running on port 3339 is, by definition, outside the norm.

That's not inherently dangerous. Legitimate software frequently uses unassigned ports, especially in closed enterprise or industrial environments where IANA registration is an afterthought. But it does mean that security tools treat unexpected activity here differently than they would on a well-documented port: there's no "correct" behavior to compare against, so anything goes.

Unassigned ports are the blank spaces on the map. Sometimes blank space is just blank. Sometimes something is living there that nobody got around to naming.