Poor Network Segmentation: an open door for attackers

Introduction

In a company, the IT network allows users, servers, applications, and devices to communicate with each other. However, when this communication is not properly controlled, it can become a real security risk.

Poor network segmentation allows an attacker, after compromising a simple user workstation, to move more easily within the information system. The attacker can then access sensitive servers, exploit internal vulnerabilities, compromise privileged accounts, or reach critical environments.

For a CISO, CIO, or IT professional, network segmentation is therefore not just a technical matter. It is a key component in protecting the information system.

What Is Network Segmentation?

Network segmentation consists of dividing a company’s network into several separate zones based on usage and level of criticality.

For example:

• user network;
• server network;
• administration network;
• guest network;
• backup network;
• DMZ;
• critical systems zone.

The objective is simple: not all machines should be able to communicate freely with each other.

A user workstation does not always need direct access to a database. A guest network should not have access to the internal network. A server exposed to the Internet should not be able to communicate freely with critical systems.

Network segmentation therefore helps limit communications to only the flows that are truly necessary.

Why Is Poor Segmentation Dangerous?

When a network is too open, an attacker can progress more easily after an initial compromise.

Imagine that an employee clicks on a phishing email. Their workstation becomes infected. If the network is poorly segmented, the attacker can scan the internal network, identify accessible servers, test credentials, and attempt to access sensitive resources.

This is known as lateral movement.

The more open the network is, the easier this movement becomes. Conversely, good segmentation limits possible paths and significantly slows down the attacker.

The Most Common Mistakes

Segmentation issues often come from simple but dangerous bad practices.

The first mistake is having a network that is too flat, where user workstations, servers, printers, IP cameras, and business equipment can communicate almost freely.

The second mistake is creating VLANs without real filtering between them. A VLAN does not provide sufficient protection if all communications are allowed.

The third mistake involves overly permissive firewall rules, such as rules that allow all ports, all protocols, or broad IP address ranges.

Finally, sensitive environments are sometimes poorly isolated: production, administration, backup, testing, or third-party access. This lack of separation greatly increases the risks in the event of an incident.

A Simple Example

Let’s take the case of a server containing sensitive data.

In a poorly segmented architecture, user workstations can communicate directly with this server. If one of these workstations is compromised, the attacker can attempt to access the server, scan its ports, or exploit a vulnerability.

In a better-segmented architecture, this server is placed in a dedicated zone. User workstations cannot access it directly. Only specific authorized services can communicate with it, on precise ports.

This separation does not necessarily block the initial attack, but it limits its propagation and reduces its impact.

Impacts on the Company

Poor network segmentation can have major consequences:

• unauthorized access to sensitive data;
• rapid spread of ransomware;
• compromise of critical servers;
• difficulty containing an incident;
• non-compliance with certain security requirements;
• financial loss and damage to the company’s reputation.

It is therefore a technical, operational, and strategic risk.

How to Improve Network Segmentation

The first step is to map the network: identify VLANs, servers, flows, applications, critical devices, and sensitive zones.

Then, assets must be classified according to their level of criticality. A domain controller, a customer database, a backup platform, or an administration tool should not be treated like ordinary resources.

Next, clear security zones must be defined and the principle of least privilege must be applied: each resource should only access what it truly needs.

Firewall rules must be precise, documented, and regularly reviewed. It is also important to test segmentation through audits, configuration reviews, and internal penetration tests.

Best Practices to Remember

To strengthen network segmentation, it is recommended to:

• separate user networks from critical servers;
• filter communications between VLANs;
• isolate backups;
• protect administration zones;
• limit third-party access;
• place exposed services in a DMZ;
• regularly review firewall rules;
• monitor abnormal flows between zones;
• regularly test possible attack paths.

Conclusion

Poor network segmentation can turn a limited compromise into a major incident. When an attacker manages to enter the network, the absence of clear separation gives them more freedom to progress.

Conversely, well-designed segmentation limits lateral movement, protects critical assets, and makes incident containment easier.

For CISOs, CIOs, and IT professionals, the real question is not only: “Do we have VLANs?”

The real question is: “Is our network segmentation truly effective against an attacker?”

Call to Action

Your company may already have VLANs and firewall rules. But this does not necessarily mean that segmentation is effective.

A network segmentation audit or an internal penetration test helps identify overly permissive flows, possible attack paths, and critical zones that are insufficiently protected.

Regularly assessing your segmentation means reducing the risk of lateral movement and strengthening the resilience of your information system.