Validating Network Segmentation

A large enterprise had been regularly conducting network segmentation testing through an existing vendor as part of its security program. However, as part of a compliance-driven requirement, there was a need to specifically validate that no unauthorized traffic was flowing from the Cardholder Data Environment (CDE) to non-CDE networks.

While prior assessments indicated that segmentation controls were in place, there was limited evidence confirming their effectiveness at a granular level. The objective of this engagement was to independently validate segmentation and identify any gaps that may have been missed in earlier exercises.

The approach focused on comprehensive validation of east-west traffic across VLANs, with particular attention to communication paths originating from the CDE. Instead of relying on standard testing methods, targeted scripts were deployed to actively assess open ports, service accessibility, and communication flows between CDE and non-CDE environments.

This deeper level of testing revealed previously unidentified gaps, including unintended communication paths and open ports that allowed traffic flow where restrictions were expected. These issues stemmed primarily from configuration gaps and incomplete enforcement of segmentation controls, rather than absence of security mechanisms.

All findings were documented with clear mapping to impacted network segments and risk areas. The client's teams implemented corrective actions, including tightening firewall rules, restricting unnecessary ports, and reinforcing segmentation boundaries between CDE and non-CDE environments.

A revalidation exercise was conducted post-remediation to confirm that all identified gaps had been effectively addressed. The follow-up testing verified that traffic from CDE to non-CDE environments was fully restricted, meeting the intended segmentation objectives.

This engagement highlighted that even mature environments with ongoing assessments can have blind spots in segmentation validation. By adopting a more targeted and validation-driven approach, the organization was able to ensure true isolation of its CDE, strengthening both its security posture and compliance readiness.