Network Penetration Testing — Breaking Into the Infrastructure Layer

This is Part 4 of my VAPT internship series at CYBERENTITYZ. If you've been following along, Part 1 covered reconnaissance and information gathering, and Part 2/3 walked through scanning and enumeration. This time we're going one layer deeper — into the network infrastructure itself.

What Is Network Penetration Testing?

Network penetration testing is the practice of probing an organization's network infrastructure — routers, firewalls, switches, VPN gateways, and the traffic flowing between them — to find exploitable weaknesses before an attacker does. Unlike web app testing, which focuses on application logic, network pentesting targets the plumbing: how devices talk to each other, how segments are isolated, and how perimeter defenses are configured.

It typically splits into two flavors:

• External network testing — simulating an internet-based attacker hitting your perimeter (firewalls, VPN endpoints, exposed services).
• Internal network testing — simulating an attacker who's already inside (a compromised laptop, a malicious insider, a rogue device on a LAN), focusing on lateral movement and segmentation failures.

Why It Matters

Network misconfigurations are consistently among the most common critical findings in any VAPT engagement — often more so than application-layer bugs. A few reasons this keeps happening:

• Default credentials left on switches and routers.
• Flat networks with no VLAN segmentation, so one compromised endpoint means the whole network is exposed.
• Firewall rules that are too permissive, written once and never revisited.
• VPN configurations that don't enforce MFA or use outdated protocols.
• Legacy protocols (SMBv1, Telnet, unencrypted SNMP) still running because "nothing's broken."

A single weak point in any of these can give an attacker a foothold that application-layer defenses never see coming. This is exactly why frameworks like MITRE ATT&CK and ASD Essential 8 dedicate entire tactic categories to lateral movement, command-and-control, and network-based persistence — the infrastructure layer is where small misconfigurations turn into full domain compromise.

The Core Toolkit

Nmap — Mapping the Terrain

Nmap is where almost every network test starts. Beyond a basic port scan, it's used for service version detection, OS fingerprinting, and scripted vulnerability checks via the NSE (Nmap Scripting Engine).

nmap -sS -sV -O -p- 192.168.1.0/24
nmap --script vuln 192.168.1.10

nmap -sS -sV -O -p- 192.168.1.0/24
nmap --script vuln 192.168.1.10

The first command does a full TCP SYN scan across all 65535 ports with service/version and OS detection. The second runs Nmap's vulnerability-detection scripts against a specific host — useful for quickly flagging known CVEs tied to identified service versions.

Netcat — The Swiss Army Knife

Netcat earns its "TCP/IP Swiss Army knife" reputation in network testing for three things: banner grabbing, manual service interaction, and setting up listeners for reverse/bind shells during later exploitation phases.

nc -nv 192.168.1.10 80
nc -lvnp 4444

nc -nv 192.168.1.10 80
nc -lvnp 4444

The first connects manually to a service to grab its banner and test raw responses. The second sets up a listener — the classic first half of a reverse shell catch.

VLAN Hopping

VLANs are supposed to segment traffic logically even when devices share physical switch infrastructure. VLAN hopping defeats that isolation through two main techniques:

• Switch spoofing — an attacker's machine negotiates a trunk link with the switch by mimicking a switch itself (abusing Dynamic Trunking Protocol), gaining access to all VLANs carried on that trunk.
• Double tagging — the attacker crafts a frame with two 802.1Q VLAN tags; the first switch strips the outer tag and forwards the frame, exposing it to a VLAN it shouldn't have reached.

This is one of those findings that looks "theoretical" until you demonstrate it live on a client's switch and watch the segmentation argument collapse.

Man-in-the-Middle (MITM) Attacks

Once inside a flat or poorly segmented network, MITM techniques let an attacker intercept and sometimes alter traffic between two hosts. Common techniques include ARP spoofing/poisoning (tricking hosts into sending traffic through the attacker's machine), DNS spoofing, and rogue DHCP servers handing out malicious gateway information.

Tools like Ettercap, Bettercap, and arpspoof are the usual suspects here. The value of demonstrating this in a report isn't just "we intercepted traffic" — it's showing exactly what was exposed: plaintext credentials, session tokens, internal hostnames.

Firewall Bypass

Firewalls are policy enforcement points, and policy is only as good as its weakest rule. Common bypass approaches include:

• Port/protocol manipulation — using fragmented packets or non-standard source ports to slip past poorly written rules.
• Tunneling — wrapping disallowed traffic inside an allowed protocol (DNS tunneling, HTTPS tunneling) to exfiltrate data or maintain C2 channels.
• Misconfigured rule ordering — firewalls process rules top-down; a permissive rule placed above a restrictive one silently defeats the restriction.

Nmap's -f (fragment packets) and --source-port flags are common starting points for testing how rigorously a firewall actually enforces its rules versus how it's documented to behave.

Lateral Movement

Once an initial foothold is established, lateral movement is how an attacker expands access across the network — moving from a single compromised workstation toward domain controllers, file servers, and other high-value targets. Techniques here overlap heavily with credential-based attacks I've covered in earlier posts (Impacket's secretsdump, pass-the-hash, pass-the-ticket) combined with network-layer enumeration to find the next hop.

This is where MITRE ATT&CK's Lateral Movement tactic (TA0008) becomes the practical map — techniques like Remote Services (T1021) and Exploitation of Remote Services (T1210) are exactly what shows up in real engagement reports.

[IMAGE 6 — Diagram] Suggested: A simple "attack path" graphic showing a chain: Initial Foothold → Internal Recon → Credential Access → Lateral Movement → Domain Admin. This ties the whole article together visually and works well as a closing diagram before the conclusion.

Closing Thoughts

Network penetration testing is less glamorous than chasing a slick web app exploit, but it's where the largest blast-radius findings tend to live. A flat network, an over-permissive firewall rule, or an unpatched switch can undo every other security control an organization has invested in. That's also what makes it one of the most valuable skills to build hands-on — the gap between reading about VLAN hopping and watching it work on your own lab switch is where the real learning happens.

Next up in this series: deeper into Active Directory lateral movement and credential attacks, building on the Impacket and BloodHound work from earlier posts.

This post is part of an ongoing VAPT internship series documenting hands-on offensive and defensive security learning at CYBERENTITYZ. Previous posts covered reconnaissance/information gathering and scanning/enumeration.