Insufficient Network Segmentation

Insufficient network segmentation occurs when an organisation's network architecture fails to properly isolate systems, zones, and security domains from one another. In a flat or poorly segmented network, all systems can communicate freely with each other, meaning that a compromise of any single system provides an attacker with direct network-level access to every other system in the environment. This includes workstations, servers, databases, management interfaces, and critical infrastructure components.

Proper network segmentation involves dividing the network into distinct zones based on function, sensitivity, and trust level, then enforcing strict access controls at the boundaries between zones. Common segmentation strategies include separating user workstations from server infrastructure, isolating management networks (out-of-band management), creating DMZ zones for internet-facing services, and establishing dedicated segments for sensitive environments such as PCI cardholder data environments, industrial control systems, and development/test networks.

The absence of effective segmentation is consistently identified as a critical finding in penetration tests and breach investigations. The majority of high-profile data breaches involved lateral movement that would have been prevented or significantly impeded by proper network segmentation. Regulatory frameworks including PCI DSS (Requirement 1), HIPAA, and NIST CSF all mandate network segmentation as a fundamental security control.

After gaining initial access to a single system within a flat network, attackers perform internal reconnaissance to discover all reachable hosts and services. Tools like Nmap, masscan, and Ping Castle allow rapid scanning of entire RFC 1918 address ranges from the compromised host. In a poorly segmented environment, the attacker discovers that workstations can reach database servers directly on port 3306 or 1433, management interfaces (iDRAC, iLO) are accessible from the user VLAN, and there is no access control between production and development environments.

This unrestricted access dramatically accelerates lateral movement. The attacker can directly target high-value systems from any foothold: pivot from a compromised workstation to a database server, access management interfaces to gain out-of-band control of servers, reach network device administration ports, or communicate with systems in what should be isolated security zones. Techniques such as pass-the-hash, remote service exploitation, and credential reuse are all enabled by the lack of network-level barriers between the attacker and their targets.

The impact is compounded when combined with other common vulnerabilities. For example, an attacker who captures NTLM hashes via LLMNR poisoning can relay those hashes to any system on the network if segmentation does not prevent the relay traffic. Similarly, a compromised web server in a DMZ that can reach internal database servers, domain controllers, or file shares due to poor segmentation transforms a perimeter breach into a full internal compromise. Attackers may also abuse allowed traffic flows, such as DNS or HTTPS, to tunnel command-and-control traffic through segmentation boundaries.

• Unrestricted lateral movement allowing attackers to pivot from any compromised system to every other system in the environment
• Direct access to critical assets (databases, domain controllers, management interfaces) from user workstations
• Failure to contain breaches, enabling a single compromised endpoint to threaten the entire organisation
• Regulatory non-compliance with PCI DSS, HIPAA, and other frameworks that require network segmentation
• Expanded blast radius for ransomware and worm-like malware that propagates across flat networks
• Exposure of management and out-of-band interfaces to untrusted network zones
• Inability to implement effective monitoring and detection due to uncontrolled east-west traffic flows
• Difficulty achieving least-privilege network access and zero-trust architecture principles

1. Conduct a comprehensive network architecture review to document all current network zones, VLAN assignments, and inter-zone traffic flows using network topology diagrams and firewall rule analysis
2. Design a segmentation strategy based on asset classification: separate user workstations, production servers, database tier, management networks, DMZ, development/test, and any regulated environments (PCI CDE, HIPAA) into distinct network segments
3. Implement microsegmentation using next-generation firewalls, software-defined networking (SDN), or host-based firewalls (Windows Firewall with Advanced Security, iptables/nftables) to enforce granular access controls between segments
4. Restrict inter-VLAN routing to only explicitly approved traffic flows, following a default-deny approach where all traffic between zones is blocked unless there is a documented business justification
5. Isolate all management interfaces (IPMI, iDRAC, iLO, switch/router management, hypervisor management) on a dedicated out-of-band management network accessible only from authorised jump boxes
6. Deploy network access control (NAC) solutions to enforce device authentication and authorisation before granting network access, ensuring only approved devices reach their assigned segments
7. Implement regular segmentation testing (at least annually and after any significant network changes) to verify that controls are functioning as intended, as required by PCI DSS
8. Monitor east-west traffic for anomalous patterns using network detection and response (NDR) tools or flow analysis to detect attempts to traverse segment boundaries

Network segmentation testing should begin from each identified network zone to determine what other zones and systems are reachable. From a compromised workstation or a testing laptop placed in the user VLAN, perform comprehensive port scanning against server subnets, management networks, database networks, and any regulated environments: nmap -sT -Pn -p 21,22,23,80,135,139,443,445,1433,3306,3389,5432,5900,8080,8443 -oA segtest server_subnet. Use masscan for rapid large-scale scanning: masscan 10.0.0.0/8 -p445,3389,22 --rate 10000 -oG segtest.gnmap.

Document all reachable services from each zone and compare the results against the organisation's documented network segmentation policy and firewall rules. Key indicators of insufficient segmentation include: user workstations able to reach database ports (1433, 3306, 5432), any system able to reach management interfaces (IPMI port 623, iDRAC/iLO on 443), cross-environment access (development to production), and DMZ hosts able to initiate connections to internal network segments. Use traceroute to identify whether traffic between zones passes through a filtering device: traceroute -T -p 445 target_ip.

For advanced testing, attempt to pivot through compromised systems to reach otherwise segmented networks. Use SSH tunnelling (ssh -D 9050 user@pivot_host), Chisel (chisel server -p 8080 --reverse and chisel client pivot_ip:8080 R:socks), or Ligolo-ng to establish tunnels through compromised hosts. Test whether the segmentation can be bypassed through allowed protocols such as DNS tunnelling (using dnscat2 or iodine) or ICMP tunnelling. Document each segment boundary that was tested, whether it was effective, and the specific traffic flows that should be restricted but were found to be permitted.