Port 10003 — A Registered Port Between Legitimacy and Infection

What This Port Range Means

Port 10003 falls within the registered ports (1024–49151). These are assigned by the Internet Assigned Numbers Authority (IANA) to specific services on a first-come, first-served basis.¹ Unlike well-known ports (0–1023), registered ports aren't universally reserved—your organization can request a registration for almost any application.

Registration creates no enforcement. IANA simply maintains a ledger: "Here's what port 10003 is supposed to be." Whether it actually is that thing on a given network is entirely up to whoever controls that network.

Official Assignments

Port 10003 has legitimate IANA registrations:²

• EMC Documentum Content Server (documentum-s) — Enterprise content management system using TCP/UDP
• Veeam Backup & Replication (veeam) — Backup and disaster recovery platform using TCP

If you see port 10003 open in your network, it's very likely one of these legitimate applications. Documentum is commonly used in enterprises managing large document repositories. Veeam is standard in organizations running backup infrastructure.

The Malware Connection

Port 10003 has been documented as a command-and-control vector for the Lula trojan.³ This isn't the only port hijacked by malware—it's commonplace. The Lula trojan uses this port to communicate with infected systems.

What makes this noteworthy: the port's legitimate assignments are large, enterprise-grade services that operate continuously and are typically monitored. A trojan trying to hide on port 10003 would stand out immediately in a properly maintained network—but it only takes one unmonitored instance to create a beachhead.

How to Check What's Actually Listening

The difference between what a port should be and what it is matters.

On macOS/Linux:

lsof -i :10003
sudo netstat -tln | grep 10003
sudo ss -tln | grep 10003

On Windows:

netstat -ano | findstr :10003

What to look for:

• The process name and PID (is it actually Veeam or Documentum?)
• The local address (is it listening on all interfaces or restricted?)
• Whether it's established connections or just listening
• Whether it's running from the expected application path

If something is listening on port 10003 and it's not Documentum or Veeam, that's your signal to investigate further.

Why Unassigned Ports Matter

Ports 1024–49151 exist because someone eventually needs them. For every port with an official assignment, there's a story: a company solved a problem, built software, and requested a reserved port number to avoid conflicts.

But IANA's registry is a ledger, not a firewall. Port 10003's official status as "Documentum" doesn't prevent malware from using it. And that's exactly why you need to verify what's actually listening rather than trusting the assignment.

Unassigned ports teach a critical lesson about networks: authority and reality are not the same thing. IANA can say what a port is for. Your firewall, your monitoring, and your curiosity determine what it actually carries.