Network Pentesting: Discovering Vulnerabilities in Corporate Networks

Not all penetration tests are created equal. Organizations must tailor the scope and methodology of the engagement based on their specific security maturity and objectives. Network pentesting is generally categorized into two primary environments: External and Internal. Furthermore, the level of information provided to the testing team dictates the specific approach (Black Box, White Box, or Grey Box).

External vs. Internal Testing

External Network Pentesting focuses on the organization's perimeter defenses. The testers position themselves on the public internet and attempt to breach the network from the outside in. They target external-facing assets such as web servers, firewalls, VPN gateways, and email servers. The goal is to determine if an anonymous attacker can bypass the perimeter and gain a foothold within the internal network.

Internal Network Pentesting operates under the assumption that the perimeter has already been breached. The tester is granted access to the internal corporate network—often mimicking the access level of a standard employee or a compromised workstation. The objective here is to evaluate the strength of internal controls. Can an attacker escalate privileges from a standard user to a Domain Administrator? Can they pivot across different network segments and access highly sensitive databases? Internal testing exposes the terrifying reality of lateral movement.

Black Box, White Box, and Grey Box Approaches

The amount of information the organization provides to the testing team significantly impacts the engagement.

• Black Box Testing: The tester is provided with absolutely zero prior knowledge of the target network, other than the company name. This most closely simulates a real-world, unprivileged attacker. The tester must spend significant time conducting reconnaissance to map the network infrastructure blindly.
• White Box (Crystal Box) Testing: The tester is provided with complete transparency, including network diagrams, source code, IP addresses, and administrative credentials. This approach is less about simulating an external attack and more about conducting a deep, exhaustive audit to find every possible configuration flaw.
• Grey Box Testing: A hybrid approach. The tester is provided with limited information, such as standard user credentials to an internal application or a partial list of IP ranges. This is highly effective for simulating an insider threat or an attacker who has already compromised a low-level account.

A professional penetration test is not a chaotic free-for-all; it is a highly structured, systematic process. While specific methodologies vary slightly, they generally adhere to a standardized lifecycle, such as the Penetration Testing Execution Standard (PTES).

1. Pre-Engagement and Scoping

Before a single packet is sent, the testing team and the organization must define the Rules of Engagement (RoE). This critical phase dictates the scope of the test: exactly which IP addresses, domains, and systems are authorized targets, and which are strictly off-limits (e.g., critical production databases that cannot risk a crash). The RoE also establishes testing windows, communication protocols, and emergency "stop-test" procedures.

2. Reconnaissance and Intelligence Gathering

Often referred to as Open-Source Intelligence (OSINT), this phase involves gathering as much passive information about the target as possible without directly interacting with their systems.

Testers will scour public databases, DNS records, and WHOIS registrations to map the organization's external footprint. They will analyze social media platforms like LinkedIn to identify employee names, job titles, and potentially used technologies. This intelligence gathering is crucial for identifying target IP ranges and crafting highly personalized social engineering campaigns (if included in the scope).

3. Network Scanning and Enumeration

Once the passive reconnaissance is complete, the testers transition to active engagement. They utilize powerful scanning tools (such as Nmap) to probe the target IP addresses. The goal is to discover live hosts, identify open ports, and fingerprint the specific services and operating systems running on those ports (e.g., identifying that port 80 is running an outdated version of Apache web server).

Following the scan, the testers move to enumeration. This involves actively querying the discovered services to extract more detailed information. They might query an SNMP service to pull a list of active network routes, or enumerate an SMB share to identify user accounts and accessible file directories.

4. Vulnerability Assessment

With a comprehensive map of the network and running services, the testers begin searching for weaknesses. They will run automated vulnerability scanners (like Nessus or OpenVAS) to identify known flaws, such as missing security patches, outdated software versions, or default passwords.

However, a penetration test is not just a vulnerability scan. Automated scanners frequently generate false positives and often miss complex, chained vulnerabilities. The human tester must manually verify the scanner's findings and utilize their expertise to identify logical flaws that an automated tool could never detect.

5. Exploitation and Gaining Access

This is the defining phase of the penetration test. Armed with a verified list of vulnerabilities, the testers attempt to exploit them to gain unauthorized access to the systems.

This might involve launching a Buffer Overflow attack against a vulnerable legacy application, executing an SQL Injection to bypass an authentication portal, or utilizing a tool like Metasploit to deliver a precise exploit payload. If the exploit is successful, the tester gains a "shell"—a command-line interface allowing them to interact directly with the compromised machine.

6. Post-Exploitation and Lateral Movement

Gaining an initial foothold is rarely the end goal; it is merely the beginning. Once a tester compromises a machine, they enter the post-exploitation phase.

The immediate objective is Privilege Escalation. If the tester compromised a low-level web server account, they will search for local vulnerabilities or misconfigurations to escalate their privileges to "root" or "SYSTEM," granting them total control over that specific machine.

From there, the tester initiates Lateral Movement. They will dump the password hashes from the compromised machine's memory using tools like Mimikatz, and attempt to use those credentials to access other machines on the internal network. The ultimate goal of an internal pentest is often to compromise the Active Directory Domain Controller, effectively granting the tester administrative control over the entire corporate network.

7. Reporting and Remediation

The final and most critical phase is the delivery of the Penetration Testing Report. A successful exploit is useless if the organization does not understand how to fix it.

The report must be comprehensive, translating complex technical findings into actionable business intelligence. It should include an executive summary detailing the overall risk posture, followed by a deeply technical breakdown of every vulnerability discovered, the exact steps required to reproduce the exploit, and, crucially, specific, prioritized remediation recommendations to patch the flaws.

Network penetration testing requires a diverse toolkit capable of automating mundane tasks and executing highly complex exploits. While tools do not replace the critical thinking of the tester, they are essential for efficiency.

• Nmap (Network Mapper): The undisputed king of network scanning. Nmap is used for host discovery, port scanning, and service fingerprinting. It is the foundational tool used to map the initial attack surface.
• Metasploit Framework: A massive, open-source exploitation framework. Metasploit provides testers with a vast database of verified exploits and payloads, streamlining the process of attacking known vulnerabilities and managing the resulting command shells.
• Burp Suite / OWASP ZAP: While primarily focused on web applications, these interception proxies are crucial during network pentests when targeting internal web portals, administrative interfaces, or custom APIs.
• BloodHound: An essential tool for internal Active Directory pentesting. BloodHound uses graph theory to visually map complex, hidden relationships and permission structures within AD, allowing testers to quickly identify highly complex, multi-step attack paths leading to Domain Admin privileges.
• Mimikatz: A post-exploitation tool used to extract plaintexts passwords, hashes, PIN codes, and kerberos tickets from memory. It is heavily utilized during lateral movement phases.