Port 1039: Streamlined Blackhole — A Registered Port Nobody Uses

What Port 1039 Is

Port 1039 is a registered user port (TCP/UDP) with the official service name sbl — which stands for "Streamlined Blackhole." ¹ That's all the documentation you get. No RFC. No clear purpose. Just a name.

The Registration Gap

Port 1039 lives in the registered port range (1024–49151), which means someone formally requested it from IANA and received it, but then left no trail about what it's actually for. ² This creates a strange liminal space: the port is "official" without being useful, occupied without being known.

What Actually Uses It

In practice, port 1039 has been observed in two contexts:

Legitimate Use: Windows systems occasionally use port 1039 as a source UDP port for XDMCP (X Display Manager Control Protocol) connections, though this is inconsistent across systems. ³

Malicious Use: Port 1039/TCP became known as the listening port for Dosh, a trojan/backdoor remote access tool for Windows systems. ⁴ When a port has no legitimate purpose, malware settles in.

How to Check What's Listening

On macOS/Linux:

# Check if anything is listening on port 1039
lsof -i :1039

# Check both TCP and UDP
sudo netstat -tuln | grep 1039

On Windows:

netstat -ano | findstr :1039

Using Nmap (from another machine):

nmap -p 1039 <target-ip>

Why This Matters

Port 1039 is a cautionary tale about how the port registry works. Registration doesn't require documentation. A port can exist officially but remain functionally orphaned—and that emptiness becomes exploitable. Every unassigned or poorly documented port is a potential infection point because nothing is there to defend it, and everything there to use it.

If port 1039 appears on your system, check what process owns it. The odds are higher it shouldn't be there.