Port 3527: Unassigned — But Not Unclaimed

What Range This Port Belongs To

Port 3527 sits in the registered ports range (1024–49151). IANA oversees this range and maintains a registry of assignments, but registration isn't mandatory — software can use any port in this range without asking permission. The result is a large middle zone where official assignments and informal conventions coexist without introduction.

Port 3527 is IANA-unassigned. The registry has no entry for it. But that doesn't mean the port is idle.

Unofficial Uses

MSMQ — Microsoft Message Queuing (UDP)

Windows administrators have encountered port 3527 most often through Microsoft Message Queuing (MSMQ), which uses UDP 3527 for an internal ping mechanism. Before MSMQ establishes a session with a remote queue manager, it sends a UDP ping to port 3527 and waits up to one second for a reply. If the remote machine responds, the session opens immediately. If not, MSMQ waits six seconds and connects anyway.¹

Blocking UDP 3527 on a firewall doesn't break MSMQ — but it introduces a six-second delay on every new session, which surfaces as mysterious sluggishness in applications that use message queuing. It's the kind of thing that's hard to diagnose without knowing this port exists.

Veritas Backup Exec (TCP)

Veritas Backup Exec uses TCP 3527 for communication with its backup server component. If another application claims this port first, Backup Exec will fail to bind and must be reconfigured to use a different port.²

Security Note

Port 3527 has appeared in threat databases in connection with a trojan called Zvrop, which used TCP 3527 as a backdoor channel.³ As with any port associated with known malware, unexpected traffic on 3527 from unfamiliar processes warrants investigation.

How to Check What's Listening

If you're troubleshooting unexpected traffic or a port conflict, these commands will tell you what's bound to port 3527:

macOS / Linux:

sudo lsof -i :3527
# or
sudo ss -tulnp | grep 3527

Windows (Command Prompt, run as Administrator):

netstat -ano | findstr :3527

Then cross-reference the PID with Task Manager or:

tasklist /FI "PID eq <pid>"

Why Unassigned Ports Matter

The registered port range contains thousands of unassigned ports, and many of them aren't actually empty. Software vendors, operating systems, and network tools have informally claimed ports throughout the range — sometimes consistently across millions of deployments, sometimes idiosyncratically within a single organization.

Port 3527 illustrates the pattern: IANA has no record of it, but two pieces of widely deployed enterprise software treat it as their own. A firewall rule that blocks it can break backup jobs and introduce silent latency in message queues, with no obvious explanation in the logs.

The lesson isn't that unassigned ports are dangerous. It's that "unassigned" means something more like "uncatalogued" than "unused."