Why Internal Infrastructure Pen Testing Is as Important as External

Penetration testing (or ‘pen testing’) is a critical cyber security practice that helps businesses identify and fix vulnerabilities before attackers can exploit them. However, most businesses prioritise external threats, such as phishing, malware, and network breaches, while overlooking threats and risks that exist within the network. Having an external focus creates a dangerous blind spot, as attackers who breach the perimeter or malicious insiders (already in the network) can exploit weaknesses, escalate privileges, then move laterally to access sensitive data.

Which is where internal infrastructure penetration testing comes in. Where external infrastructure pen testing focuses on preventing outside breaches, internal infrastructure pen testing simulates real-world attack scenarios inside the network itself.

This is accomplished by identifying misconfigurations, weak access controls, and privilege escalation paths, allowing businesses to proactively strengthen their security before an attacker can take advantage.

The overlooked internal threat landscape

Typically, organisations invest heavily in security measures such as firewalls, intrusion detection systems and other external measures to keep attackers out– but what happens once an attacker gets in, or an insider turns malicious?

The fact is not all threats are external, and internal security weaknesses can be exploited by an attacker who’s already breached the perimeter, or by a malicious insider. And once inside, it’s often game over unless proper defences are in place within the network.

Common internal attack vectors

• Insider threats: it’s important to note that not every attack is a faceless hacker. Sometimes the threat is within an organisation in the form of a disgruntled employee, a contractor with more access than necessary, or even well-intentioned staff who make genuine mistakes.
• Compromised credentials: weak, re-used, or stolen passwords are still one of the biggest security risks encountered. If an attacker is able to obtain valid credentials for high privilege users, or to escalate from a low privilege user, external defences can be bypassed and access gained to the internal system.
• Lateral movement: once inside, an attacker can start moving laterally across the network, escalating privileges whilst searching for sensitive data and critical systems. Even with a single point of entry, a network without proper segmentation and monitoring can lead to a full-scale breach.

These threats often go undetected due to businesses assuming their internal network is safe. Highlighting the importance of internal infrastructure penetration testing, it helps expose these risks before attackers do. Which in turn ensures the networks are secure from the outside to the inside.

How internal penetration testing strengthens security

Instead of looking for ways to break into a network, like with external infrastructure pen testing, internal infrastructure pen testing assumes that an attacker (or malicious insider) already has access, with a focus on what they can do next. Can they escalate privileges? Can they move laterally? Can they access sensitive data?

Answering these questions helps uncover internal security gaps that traditional external infrastructure pen testing might miss.

Key benefits and why internal penetration testing matters

• Uncovers weak network segmentation & access control gaps: many organisations have a ‘flat’ network design due to cost and ease of maintenance, but this means that once an attacker gains access, they can freely move between systems. Internal infrastructure pen testing helps to identify segmentation weaknesses and ensure critical data and systems are properly isolated and better secured. This covers both intentional segmentation and accidental lack of segmentation.
• Tests privilege escalation protections: just because someone has access to a network doesn’t mean they need access to everything within the network. Internal infrastructure pen testing can assess whether an attacker (or employee with limited access) is able to escalate privileges to gain administrator-level control over critical systems and data.
• Exposes poor internal security hygiene: outdated software, misconfigurations and default passwords may not seem like serious risks, but they provide attackers with easy entry points into the system. Internal infrastructure pen testing can help highlight these issues before they become security liabilities.

Internal infrastructure vs external infrastructure pen testing: Why you need both

Relying solely on external infrastructure penetration testing can be likened to locking your front door but leaving all your internal doors open.

Similarly, many businesses treat cybersecurity like building a fortress – stacking up firewalls, intrusion detection systems, and endpoint protection to keep attackers out. But what if the threat is already inside and decides to cause damage?

This is the fundamental difference between external and internal infrastructure penetration testing. One protects outside threats, while the other helps detect what could happen if the perimeter is breached, highlighting why businesses need both approaches to stay secure.

Internal vs. external: How they compare

Pen Test vs. Red Team: Understanding the key differences

Feature	External infrastructure pen testing	Internal infrastructure pen testing
Focus	Attacks from outside the network	Attacks originating from the inside of the network
Primary threats	Open ports, misconfigured firewalls	Insider threats, lateral movement, weak access controls
Goal	Identify and patch perimeter vulnerabilities	Assess security weaknesses after initial access is gained
Best for	Ensuring the external attack surface is locked down	Simulating real-world attack scenarios from an insider’s perspective
Key benefit	Reduces opportunities for attackers to access network	Stops an internal breach from escalating

Why a combined approach is essential

Not all attacks start externally: sometimes the risk is already inside, whether in the form of an insider threat, from a compromised account, or a misconfigured system.

External infrastructure security cannot stop lateral movement: the best firewalls won’t help if an attacker gets past them and your network has weak segmentation.

Internal and external infrastructure pen tests uncover different risks: running both tests ensures comprehensive security for your network as perimeter weaknesses and internal security gaps are covered.

Best practices for internal infrastructure penetration testing

As with other penetration tests, internal infrastructure pen testing is not a one-and-done exercise but rather should be an ongoing part of a business’s security strategy. Threats can and do evolve, systems change, and new vulnerabilities continuously emerge. Therefore, it is vital for businesses to approach internal pen testing strategically to get the most out of it.

How to make internal infrastructure pen testing effective

• Test regularly, especially after major IT updates: networks are not static and every software update, infrastructure change, or system migration can introduce new security gaps. Regular internal infrastructure pen testing helps ensure security controls remain effective.
• Use assumed breach testing to simulate real attacks: rather than just looking for vulnerabilities, assumed breach testing starts with a worst-case scenario where the attacker is already inside the network. This approach helps security teams to understand how far an internal threat actor can get, what data they can access and how quickly privileges can be escalated.
• Combine internal infrastructure testing with red teaming: penetration testing is about finding weaknesses, while red teaming goes a step further by simulating an attacker's tactics, techniques, and, and procedures (TTPs). Combined, these approaches provide a more comprehensive assessment covering not just vulnerabilities but also gaps in detection and response. For best results, ensure the internal infrastructure is well secured before performing red team activities.
• Testing alone isn’t enough – remediation is key: the real value of pen testing comes not from just fixing the vulnerabilities found, but continuously improving the network security based on the insights from the tests.