Port 523 sits in the well-known port range (0-1023), assigned by IANA to IBM's DB2 Database Administration Server (DAS). It's a port with a complicated legacy: officially assigned, widely deployed, and now deprecated because the security model it represented is no longer acceptable.1
What Ran on Port 523
Port 523 (both TCP and UDP) was the registered port for IBM DB2's Database Administration Server, an administrative interface for managing DB2 database instances. The DAS handled administrative tasks like starting and stopping database instances, managing configurations, and coordinating distributed database operations.2
The critical detail: no authentication was required for requests to this port. Any client that could reach port 523 could send administrative commands to the DB2 server.3
This wasn't a bug. This was the design.
The Well-Known Port That Shouldn't Be
Port 523 occupies the well-known port range—ports 0-1023 that are assigned by IANA for standard Internet services. These are the ports that define the Internet's foundational protocols. Port 80 for HTTP. Port 443 for HTTPS. Port 25 for SMTP.
And port 523 for unauthenticated database administration.
The inclusion of port 523 in this range reflects a different era of computing—when database servers lived behind corporate firewalls, when "access to the network" implied "authorized user," when security through obscurity felt sufficient.
That era is over. Port 523 is deprecated.4
Why It Was Deprecated
The problem with port 523 wasn't just the lack of authentication—it was the fundamental assumption that network access control was sufficient security. As networks became more complex, as database servers became Internet-facing, as threat models evolved, the DAS became indefensible.
IBM deprecated the Database Administration Server and its unauthenticated port 523 in favor of more modern administrative tools that require proper authentication and encryption. The default database port moved to 50000, with authentication built in from the start.5
What Port 523 Represents
Port 523 is a historical marker. It represents a moment when database administration was considered an "internal" operation that didn't need the same security rigor as user-facing services. It shows what happens when convenience beats security in the design phase—the technical debt compounds until deprecation becomes the only path forward.
The well-known port range includes brilliant protocol designs that have lasted decades. It also includes port 523—a cautionary tale about building security as an afterthought.
Checking for Port 523
To see if anything is listening on port 523 on your system:
Linux/macOS:
Windows:
If you find something listening on port 523, investigate immediately. The legitimate service is deprecated. An active listener is either a legacy DB2 installation that needs upgrading, or something pretending to be one.
Security Considerations
Port 523 has been used by malware and trojans precisely because it was expected to be open on database servers and accepted unauthenticated connections.6 The combination of official legitimacy and weak security made it an attractive target.
If you're running IBM DB2:
- Don't use the DAS
- Don't open port 523
- Use modern DB2 administrative tools that require authentication
- Keep database servers behind firewalls with strict access controls
The Broader Lesson
Unassigned ports are unremarkable. Deprecated ports tell stories.
Port 523 tells the story of what happens when security is considered a deployment concern rather than a design requirement. It tells the story of protocols that worked perfectly in their original context becoming liabilities when that context changed. It tells the story of the slow, painful process of replacing infrastructure that millions of systems depend on.
Every deprecated port in the well-known range is a lesson learned the hard way.
Frequently Asked Questions
Related Ports
- Port 50000: Default IBM DB2 database connection port
- Port 50001-50100: Common range for additional DB2 instances
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