Listen to this Post
Introduction:
Most organizations still pour their security budgets into perimeter firewalls, assuming that blocking threats at the internet edge is enough. But as Charles Crampton, CEO of Akzium, highlights, the real danger now lurks inside the network—compromised IoT cameras, vendor laptops, or employee workstations can move laterally to cripple backups, domain controllers, and storage systems. This shift demands a “deny-by-default” internal firewall strategy that complements edge defenses, creating micro-perimeters around critical assets rather than relying on static Layer 3 switch ACLs.
Learning Objectives:
- Implement stateful internal firewall rules on Linux and Windows to block lateral movement.
- Configure policy-based segmentation between IoT, management, and production VLANs.
- Use logging and visibility tools to detect internal reconnaissance and blast radius reduction techniques.
You Should Know:
- Deploying a Basic Internal Firewall on Linux with nftables (Step‑by‑Step)
Switch ACLs only filter by IP/port and are easily misconfigured. A stateful internal firewall on a Linux jump host or router can enforce session tracking and deny-by-default between internal segments. Below is a hardened nftables example for a server that should only accept SSH from a specific management VLAN and allow outbound updates—nothing else.
Step‑by‑step guide:
1. Install nftables (if not present):
`sudo apt install nftables -y` (Debian/Ubuntu) or `sudo yum install nftables -y` (RHEL/CentOS)
2. Flush existing rules and create a new table for internal filtering:
sudo nft flush ruleset sudo nft add table inet internal_fw
3. Create base chains with default drop policy:
sudo nft add chain inet internal_fw input { type filter hook input priority 0 \; policy drop \; }
sudo nft add chain inet internal_fw forward { type filter hook forward priority 0 \; policy drop \; }
sudo nft add chain inet internal_fw output { type filter hook output priority 0 \; policy accept \; }
4. Allow established/related traffic (stateful inspection):
`sudo nft add rule inet internal_fw input ct state established,related accept`
5. Permit SSH only from management subnet (e.g., 10.10.10.0/24):
`sudo nft add rule inet internal_fw input ip saddr 10.10.10.0/24 tcp dport 22 accept`
6. Log dropped packets for visibility:
`sudo nft add rule inet internal_fw input log prefix “Internal-FW-DROP: ” counter drop`
7. Make persistent: `sudo nft list ruleset > /etc/nftables.conf` and enable via systemd.
What this does: This turns your Linux server into a mini internal firewall that blocks all lateral movement except explicitly allowed SSH from your management VLAN. Attackers compromising an IoT device on a different subnet cannot reach this server’s SSH, dramatically shrinking the blast radius.
- Hardening Windows Internal Segmentation with PowerShell and Windows Defender Firewall
Windows endpoints and servers often reside in the same VLAN, allowing lateral tools like PsExec or WMI to move unchecked. Use built‑in Windows Defender Firewall with advanced security to create internal firewall rules that block inbound lateral movement while keeping outbound internet access.
Step‑by‑step guide:
- Open PowerShell as Administrator and list existing rules: `Get-1etFirewallRule`
- Block all inbound SMB (port 445) from non‑domain controller subnets (critical to stop ransomware lateral spread):
New-1etFirewallRule -DisplayName "Block Lateral SMB" -Direction Inbound -Protocol TCP -LocalPort 445 -Action Block -RemoteAddress "192.168.2.0/24","10.0.0.0/8" -Description "Prevent lateral movement from internal segments"
- Allow RDP only from a specific jump host IP:
New-1etFirewallRule -DisplayName "RDP from Admin Host" -Direction Inbound -Protocol TCP -LocalPort 3389 -Action Allow -RemoteAddress "172.16.5.10"
- Enable logging for dropped packets (to monitor internal scanning):
`Set-1etFirewallProfile -1ame Domain,Private,Public -LogFileName C:\Windows\System32\LogFiles\Firewall\pfirewall.log -LogAllowed False -LogBlocked True` - Apply via Group Policy for domain‑joined machines: Use `Group Policy Management Console` → Windows Defender Firewall → Inbound Rules to deploy segmentation across hundreds of devices.
Why this matters: Attackers frequently use PsExec to move laterally using SMB. By blocking inbound SMB from internal subnets (except authorized sources), you break common attack chains without affecting edge firewall subscriptions.
- Building a Deny‑by‑Default Internal Firewall Using pfSense (Open Source Alternative)
Commercial edge firewalls require costly annual subscriptions. You can deploy pfSense as an internal firewall on commodity hardware or a VM, with no reoccurring web/DNS filtering fees, to segment IoT cameras, backup networks, and management interfaces.
Step‑by‑step guide:
- Install pfSense (minimum 2 NICs). Assign WAN (uplink to edge firewall) and LAN (internal segments).
- Create VLANs under Interfaces → Assignments → VLANs: e.g., VLAN 10 (IoT), VLAN 20 (Cameras), VLAN 30 (Backup).
- Assign each VLAN to a new interface and enable DHCP if needed.
- Set default firewall rule on each VLAN interface to block all inter‑VLAN traffic:
– Firewall → Rules →
→ Add rule: Action=Block, Protocol=Any, Source=Any, Destination=Any. 5. Create allow rules for specific, justified flows: e.g., allow backup server on VLAN 30 to initiate backup to storage on VLAN 40. 6. Enable logging under Status → System Logs → Firewall to see blocked lateral attempts. 7. Test segmentation by pinging from an IoT device to a camera subnet – it should fail unless explicitly allowed. What this accomplishes: You’ve built an internal firewall that enforces micro‑segmentation with full stateful inspection. IoT devices cannot reach backup repositories, even if they are on the same physical switch. This reduces blast radius to near zero for internal compromises. <ol> <li>Using AI‑Driven Analytics to Detect Lateral Movement in Internal Firewall Logs</li> </ol> Internal firewalls generate massive logs. Modern AI/ML models can identify anomalous connection patterns—like a camera suddenly trying to connect to a domain controller on port 445—much faster than manual rule auditing. <h2 style="color: yellow;">Step‑by‑step guide (using open‑source ELK + custom ML):</h2> <ol> <li>Forward internal firewall logs (nftables, pfSense, or Windows) to a central syslog server. </li> <li>Install Elasticsearch, Logstash, Kibana (ELK) on a hardened management host. </li> <li>Use Logstash to parse firewall logs into structured fields (src_ip, dst_ip, dst_port, action). </li> <li>Deploy a Python‑based anomaly detector (isolation forest or LSTM) that models normal lateral flow: [bash] from sklearn.ensemble import IsolationForest Train on historical allowed flows (src_subnet, dst_subnet, port) model = IsolationForest(contamination=0.01) model.fit(normal_flows)
Why this elevates security: AI transforms static ACLs into adaptive threat prevention, catching zero‑day lateral moves that signature‑based edge firewalls would miss. This directly addresses Crampton’s point about “better logging and visibility.”
- Practical Commands to Audit Your Internal Network for Lateral Movement Blind Spots
Before deploying internal firewalls, you must discover existing risky paths. Use these commands from a compromised workstation perspective (pen‑tester style) to see where switch ACLs fail.
Linux (attacker simulation):
- Scan for live hosts on internal subnet: `nmap -sn 192.168.1.0/24`
- Check open SMB shares: `smbclient -L //192.168.1.100 -1`
- Enumerate domain controllers via DNS: `nslookup -type=SRV _ldap._tcp.dc._msdcs.
` - Test firewall rules with crafted packets: `hping3 -S -p 3389 192.168.1.200` (check if RDP is accidentally open)
Windows (PowerShell as low‑privilege user):
- Find reachable management interfaces: `Test-1etConnection -ComputerName 192.168.1.50 -Port 443`
- List network adapters and routes: `Get-1etRoute | ft DestinationPrefix, NextHop`
- Use built‑in `net view` to see exposed file shares: `net view \\192.168.1.10`
- Execute WMI query to test lateral movement path: `Get-WmiObject -Class Win32_Process -ComputerName 192.168.1.30`
Mitigation commands (internal firewall deployment): After auditing, apply the nftables or PowerShell rules from sections 1 and 2 to cut off each discovered lateral path.
What Undercode Say:
- Key Takeaway 1: Edge firewalls are necessary but insufficient; internal segmentation with stateful inspection is the only way to stop lateral movement after a perimeter breach.
- Key Takeaway 2: Internal firewalls do not require expensive annual subscriptions for web/DNS filtering, making them cost‑effective for IoT, camera, and backup networks.
Analysis (10 lines): Charles Crampton correctly identifies a blind spot that many enterprises overlook – the assumption that a hardened perimeter protects against insider threats or compromised endpoints. His emphasis on “dozens of internal firewalls” that never touch the internet edge reflects a mature zero‑trust architecture where each critical asset gets its own micro‑perimeter. The technical reality is that Layer 3 switch ACLs lack stateful tracking, making them trivial to bypass with spoofed packets or connection reuse. Internal firewalls also provide granular logging that SIEMs can feed into AI models for behavioral detection. Cost is a major driver: businesses often avoid segmentation because edge firewall subscriptions scale poorly. By decoupling internal firewalls from those subscriptions, organizations can segment aggressively without recurring fees. Practical implementations using nftables, Windows Defender, or pfSense are within reach of any IT team. The missing piece is cultural – teams must shift from “protect the border” to “assume breach and segment internally.” Crampton’s post serves as an excellent blueprint for that transition.
Expected Output: (This section is part of the template; it repeats the key points above. The actual output here is the article itself, so no additional text is needed.)
Prediction:
- +1 More enterprises will adopt internal firewall appliances or virtual editions by 2027, driven by ransomware insurance requirements for micro‑segmentation.
- +1 AI‑powered internal firewall log analysis will become a standard SIEM feature, reducing mean time to detect lateral movement from weeks to minutes.
- -1 Legacy switch ACLs will continue to cause breaches because IT teams overestimate their effectiveness against stateful, encrypted lateral traffic.
- -1 Without internal firewalls, the proliferation of IoT and vendor devices will expand the attack surface until internal networks become as dangerous as the public internet.
- +1 Open‑source internal firewall solutions (pfSense, nftables, Opnsense) will gain enterprise adoption as subscription fatigue pushes budget‑conscious security teams away from proprietary edge renewals.
▶️ Related Video (72% Match):
🎯Let’s Practice For Free:
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
💎 Smart Architecture | 🛡️ Secure by Design | ⭐ Trusted by Thousands
IT/Security Reporter URL:
Reported By: Charlescrampton Most – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
