Port 539 exists in two worlds. In one world—the official IANA registry—it belongs to a protocol called apertus-ldp (Apertus Technologies Load Determination).1 In the other world—the one where security researchers and system administrators actually live—it's associated with Microsoft Remote Procedure Call (MSRPC) services and treated as a potential security vulnerability.2
What Port 539 Is Supposed to Be
According to IANA, port 539 is assigned to Apertus Technologies Load Determination Protocol for both TCP and UDP.1 The protocol was intended for load determination—presumably measuring or distributing system load across networked resources.
The problem: there's almost no public documentation about this protocol. No RFC. No widely available specification. No community of users discussing implementation details. It's a registered port for a protocol that appears to have never achieved widespread adoption.
What Port 539 Actually Is
In practice, port 539 appears in security contexts as part of Microsoft's RPC infrastructure.23 Windows systems use a range of ports for remote procedure calls—dynamic ports above 1024 that allow programs to execute code on remote machines.
Port 539 falls into this range and has been identified in penetration testing guides as an MSRPC port that can be exploited for:3
- User enumeration — Discovering valid usernames on a system
- Privilege escalation — Gaining higher access levels
- Remote code execution — Running code on the target machine
- Lateral movement — Moving between systems in a network
Historical vulnerabilities like MS08-067 demonstrated how RPC services could be exploited for remote code execution.2
The Security Problem
Here's the honest truth: whether port 539 is running apertus-ldp or being used by Windows RPC services, it probably shouldn't be exposed to the Internet.4
RPC services are designed for internal networks. They carry sensitive data, often unencrypted. They expose system internals that attackers can enumerate. Security best practices say: block this at the firewall.4
The fact that the port has an official assignment to one protocol but is used in practice for something entirely different creates confusion—and confusion in security is dangerous.
Checking What's Listening
To see if anything is actually using port 539 on your system:
On Linux/macOS:
On Windows:
If you see a process listening here, investigate what it is. If it's not something you intentionally configured, that's a red flag.
Why This Matters
Port 539 represents a broader truth about the port system: official assignments don't always match reality. Protocols get registered but never widely deployed. Other services—especially on Windows—use ports dynamically, sometimes colliding with official assignments.
The gap between IANA's registry and what's actually running in production networks creates blind spots. Port scanners see 539 and report "apertus-ldp," but the service listening might be something completely different.
The Forgotten Protocol Pattern
Apertus Technologies Load Determination joins a long list of protocols that got port assignments but never achieved widespread use. The port exists. The name exists in databases. But the protocol itself? It's effectively a ghost—a registration without a body of deployed systems using it.
Meanwhile, other services move in. Microsoft RPC uses the port dynamically. Security researchers include it in their scanning profiles. The official assignment becomes trivia, and the practical reality becomes the security concern.
What You Should Do
- Block port 539 at your firewall — Both inbound and outbound, unless you have a specific, documented need for it
- Monitor for unexpected listeners — If something starts listening on 539, investigate immediately
- Don't assume the service matches the name — Just because a scanner reports "apertus-ldp" doesn't mean that's what's actually running
- Treat it like an RPC port — Apply the same security considerations you'd apply to ports 135, 593, and other Windows RPC ports
Frequently Asked Questions About Port 539
Related Ports
- Port 135 — Microsoft RPC Endpoint Mapper, the primary RPC service port
- Port 593 — HTTP RPC Endpoint Mapper, alternative RPC binding
- Port 1024-5000 — Default dynamic RPC port range on Windows systems
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