Port 10520: The Unassigned Port with a Trojan History

What This Port Is

Port 10520 is registered but unassigned. That means it falls in the 1024-49151 range that IANA oversees, a port number that could be assigned to a legitimate service, but currently isn't. It sits empty on the official registry.

Empty ports matter more than you might think.

The Shadow History: AcidShivers

Port 10520 has been flagged as a vector for the AcidShivers trojan (also called Acid Shivers).¹ This malware used the port as a command-and-control channel, a back door into infected systems. The trojan would call home through port 10520, waiting for instructions from attackers.

This wasn't a random choice. Unassigned ports are attractive to malware authors precisely because security teams aren't primed to notice them. A connection on port 22 (SSH) gets attention immediately. A connection on port 10520? That gets lost in the noise.

How to Check What's Listening

If you want to know whether anything is listening on port 10520 on your system:

On Linux/macOS:

netstat -tuln | grep 10520
ss -tuln | grep 10520

On Windows:

netstat -ano | findstr 10520

Cross-platform with Nmap:

nmap -p 10520 localhost

If you see traffic on this port and haven't intentionally opened it, that's a warning sign.

Why Unassigned Ports Matter

This is the crucial part: unassigned ports exist in a blind spot. A service running on port 443 (HTTPS) is expected, visible, monitored. But port 10520? It's in that vast desert between the well-known ports and the ephemeral ports where legitimate programs live temporarily.

Malware loves this space. Administrators don't usually monitor it. Firewalls often allow outbound traffic on random registered ports. The port exists in a state of benign neglect, and that neglect is an opportunity.

The Bigger Picture

Port 10520 isn't important because of what it officially does. It's important because of what it shouldn't be doing. It's a reminder that unassigned doesn't mean safe—it means unguarded. Every empty port is a door that could be used.

If someone on your network opened a connection to port 10520 on a remote system, would you notice? Could you trace it back? Would you even know what to look for?

That's the real story of port 10520.

Check What's Listening

# See all listening ports and the processes using them
netstat -tulpn  # Linux
ss -tulpn       # Modern Linux
netstat -ano    # Windows