In early 2023, a security vulnerability was discovered that shakes the core trust in many building automation systems. CVE-2023-25556 describes a flaw in how authentication is handled in certain KNX (Konnex) installations—which are popular for controlling lighting, blinds, HVAC, security, and more in smart buildings.

If you work with KNX setups, maintain smart buildings, or love following real-world security breakdowns, this is a must-read. I’ll break down how the vulnerability works, include code snippets to demonstrate the issue, and walk through how an attacker could abuse it, all in a way that’s easy to follow.

What is CWE-287: Improper Authentication?

Before we jump into CVE-2023-25556, let’s clarify what CWE-287 means. According to the Common Weakness Enumeration (CWE):

> CWE-287: Improper Authentication. The product does not properly verify or incorrectly verifies the identity of the user. This can let attackers gain unauthorized access.

In this context, authentication is the lock on your front door. If the lock is weak or never really checked your key, someone can just walk in.

What’s at Risk With CVE-2023-25556?

Per the official advisory from Schneider Electric, this vulnerability specifically affects situations where a user enters a key that’s less than seven digits in length, and the attacker has some kind of access to the KNX installation (for example, through the physical or network layer).

What could go wrong here?

If you use a weak key, or the system doesn’t properly enforce key length, someone with access (maybe a contractor, or even someone exploiting a remote connection) might slip past authentication and compromise the device. That could let them fiddle with climate control, unlock doors, or switch off alarms.

Under the Hood: Why Keys of Less Than 7 Digits Are a Problem

KNX Secure is built to support authentication for device configuration and messaging. However, if the setup allows for keys shorter than seven digits, some parts of the authentication flow are never properly checked.

Let’s see a simplified code snippet showing what might go wrong in an authentication handler

def authenticate_knx_key(key_input):
    if len(key_input) < 7:
        print("Warning: Key too short for secure auth!")
        return True  # mistakenly granting access!
    # continue with proper validation for longer keys
    return validate_full_key(key_input)

This kind of bug creeps in when developers want to be user-friendly ("sure, let people use short passcodes for test devices!")… but forget that even "test" settings get used in live projects.

Exploitation: How an Attacker Can Break In

Let’s say an attacker—Alice—gains access to the local KNX installation (in a wiring closet, via a maintenance port, or maybe over the network if it’s exposed). She notices some devices are using only 4-digit or 6-digit keys.

Here’s how she could exploit this

1. Sniff or Guess the Short Key: If Alice can see traffic, she might sniff a short key. If not, brute-forcing 6 digits is much easier than 16!
2. Use the Short Key to Authenticate: She sends a configuration/auth packet to the device using the short key.

Example (using pseudo KNX tool)

knx-auth-tool --ip 192.168..50 --key 123456
# The device thinks she’s authorized!

References & Official Sources

- Schneider Electric Security Notification
- CVE-2023-25556 on NVD
- KNX Security (Official Whitepaper)

Mitigation: How to Stay Safe

- Force at least 7-character keys. (Ideally, use strong random keys as recommended in KNX Secure documentation.)
- Update device firmware. Schneider Electric and others have patched affected devices—apply the upgrade.
- Monitor network access. Restrict who can reach your KNX installation, both physically and via LAN/WAN.

Final Thoughts

CVE-2023-25556 is a classic example of how well-intentioned shortcuts ("let’s let people use short keys") turn into massive attack vectors once products go from the lab to the real world. If you’re managing smart building systems or installing KNX devices, push for proper key lengths and keep those patches coming.

Exclusive tip: Even if your devices *seem* patched, double-check key length policies—drivers and user interfaces sometimes let old habits slip through!

Stay safe—and make sure your "smart" building isn't letting strangers in through the virtual back door.


*If you found this helpful, bookmark this page and spread the word to your building automation team!*


*(This post is independently written and not affiliated with Schneider Electric, KNX Association, or CISA. All code samples and scenarios are simplified for educational purposes.)*

Timeline

Published on: 04/18/2023 18:15:00 UTC
Last modified on: 04/28/2023 13:36:00 UTC