Segmenting a Flat Cardholder Data Environment

Option 1, core ACLs, is the least expensive but weakest. It can block traffic, but it does not provide stateful inspection, application awareness, or useful logging, and the CDE would still share a VLAN with 300 users. The assessor is likely to reject it as segmentation because the CDE is not isolated at Layer 2 and there is no east-west control point.

Option 2, firewall-on-a-stick with the existing pair, is the minimum change that can satisfy the requirement: create a dedicated CDE VLAN, move the 55 CDE devices, and route all CDE-to-corporate traffic through the firewall. Size it from CDE flows, not the 3.2 Gbps total. A 55-device CDE with 40 admin workstations is more likely to peak at 1.0–1.5 Gbps. Even at 2x growth, 2.6 Gbps is 65 percent of the 4 Gbps inspected capacity. If all 3.2 Gbps really traverses the firewall, that is 80 percent of capacity with no headroom, so measure with NetFlow before committing.

Option 3, a dedicated pair, gives the cleanest trust boundary and the best growth path, but it is hard to justify with a limited budget and a 120-day deadline. Recommend option 2, using a dedicated physical interface or zone rather than a subinterface on the core, and keep option 3 as the 12-month plan if CDE traffic exceeds 70 percent of inspected capacity or if the assessor requires separate devices.

Start with default deny in both directions and explicit rules only for required flows:

• CDE servers to Active Directory: DNS TCP/UDP 53, Kerberos 88, LDAP 389, LDAPS 636, SMB 445 only where required, and global catalog 3268 to the domain-controller IPs. Do not allow the entire corporate /16.
• CDE to internal patch repository: HTTPS 443 or WSUS 8530/8531 to the patch server; no direct internet update path.
• CDE to corporate DNS: restrict to the two internal resolvers; block external DNS from the CDE.
• Administration: corporate jump host to CDE management ports 3389 and 22, with MFA on the jump host. No direct user-VLAN-to-CDE rules.
• Vendor support: vendor VPN terminates on the jump host or a dedicated support appliance, not directly into the CDE. Vendor sessions are recorded and time-limited.
• Outbound internet: only through an authenticated proxy for approved update destinations; log all denies.

Also remove or disable the 212 any-any rules after mapping their real traffic. A common failure is to leave a broad any-any rule for a legacy application, which defeats the segmentation. Use the CDE zone to log all denied flows for 30 days before tightening, so legitimate but undocumented traffic is identified. Every rule needs an owner, a ticket reference, and an expiry date.

Put the limited effort where it produces evidence: flow logging on the CDE firewall zone, an IDS or IPS feed on the CDE uplink if the firewall pair supports it, and a weekly review of denied flows. Enable NetFlow or IPFIX on the firewall and core, and alert only on high-value events: a corporate user subnet initiating a connection to a CDE server, a CDE server opening an outbound connection to an unapproved country or IP, or a new any-any rule. A full SIEM build is unnecessary for a 55-device CDE.

Validation tests should be repeatable:

• From 10 randomly selected user devices, attempts to reach a CDE server on 3389 and 445 must fail.
• From the jump host with MFA, the same ports must succeed and be logged.
• A CDE server must resolve internal names, reach the patch server, and fail direct internet DNS.
• A vendor test account must connect only through the jump host.

Metrics: 100 percent of CDE devices in the CDE VLAN, zero direct user-to-CDE flows for 30 days, fewer than 5 high-priority false positives per week, all 212 any-any rules either documented or removed, and a successful quarterly rule review. With three analysts, automate the test scripts and review the exceptions in a weekly 30-minute meeting; do not promise 24/7 monitoring the team cannot sustain.