Internal vs. External Penetration Testing: The Importance of Both

Internal vs. External Penetration Testing: Why Both Are Important

Many organizations know they need penetration testing. What's less clear is whether external testing, internal testing, or both are necessary to address their actual risk. The answer depends on what question you're trying to answer: can an attacker get in, or what happens once they're already inside?

Penetration testing is an invaluable way to assess and expose the vulnerabilities in an organization's network or systems. There has been an unprecedented increase in cyber threats. Consequently, there is a growing interest in continuous security testing.

Pen tests come in two types: external penetration testing and internal penetration testing. Organizations often gravitate toward one or the other, not both. Some lean on external testing because it maps cleanly to "what can someone see from the outside," which feels like the more obvious risk. Others assume that if their perimeter holds, what happens behind it is a secondary concern. Both assumptions leave gaps. Read on for an in-depth discussion on the value of pen tests in identifying and responding to security issues and how both types work together to simulate real-world attackers and secure your networks.

What Is External Penetration Testing?

External penetration testing addresses perimeter vulnerabilities and evaluates the potential risks of external cyber threats.

Specialist testers attempt to gain entry into the organization's network by leveraging vulnerabilities discovered on the external assets, such as email, websites, and file shares. The goal is to simulate how a bad actor may try to breach the security perimeter externally—through the internet or other public resources—and gain unauthorized access to the organization's systems and data. In practice, this often means uncovering exposed VPN endpoints, forgotten internet-facing assets that fell off the asset inventory, or web applications carrying vulnerabilities nobody flagged because the app itself was assumed to be low-risk.

Pen tests play a key role in protecting the organization's networks, web and mobile applications, and privileged data on-premises or stored in cloud environments. External security testing achieves this goal by mimicking the actions of a real-world threat actor and exposing the threat before an actual attack takes place. Pen testers virtually beat bad actors at their own game.

Common findings from external tests include exposed administrative interfaces, outdated SSL/TLS configurations, unpatched public-facing services, and web application vulnerabilities like SQL injection and cross-site scripting. Individually, any one of these findings might look manageable. The real risk shows up when testers chain them together: an exposed admin panel, a weak credential, and an outdated service can form a realistic path into the network, even when none of the three findings looks severe on its own.

What Is Internal Network Penetration Testing?

Once penetration testers have breached the organization's perimeter security, they focus on laterally moving across internal systems and applications to expose security gaps that threat actors could exploit. Internal penetration testing asks the critical follow-on question: what can an attacker do once they're already inside? Testers commonly find excessive user privileges, flat or poorly segmented networks that let an attacker move freely between systems, and credentials stored or transmitted in ways that make them easy to harvest.

Many companies stop at external pen tests and skip internal testing because they don't think they will likely face insider threats. But internal testing isn't only about insider risk. It also reveals what happens after a compromised credential or a successful phishing attack hands an outside attacker the same starting position as someone already on the network. That distinction matters because the path in doesn't have to come from inside the building for internal testing to be relevant.

Every year, millions of dollars are lost due to insider attacks—sometimes intentional and malicious, but in several cases, internal network penetration testing reveals risks from the unintentional actions of negligent employees, partners, vendors, or suppliers who have authorized access to the organization's networks and applications. According to IBM's 2025 Cost of a Data Breach Report, malicious insider attacks averaged $4.92 million per breach—the highest of any attack vector—and stolen credentials took an average of 246 days to identify and contain.

Internal pen testers may use the same system that was compromised during the external test to perform the internal test. However, many prefer to deploy a designated testing device or laptop within the network, as it offers more stability during the tests. Once the testers gain admin access to the systems, the security is considered breached, and the internal test is complete. Most of the time, testers must try multiple routes of attack to accomplish this goal. Often, the initial attack targets less critical systems, and the information accessed from them is used to target the most vital parts of the system or network.

Key Differences Between Internal and External Penetration Testing

The methodologies are complementary rather than competing. External testing identifies whether your perimeter holds. Internal testing determines what the blast radius looks like when it doesn't. In real-world assessments, the two often connect directly: a low-severity external finding, like a reused password on a forgotten portal, can become the entry point that internal testing then follows straight to a privilege escalation path or sensitive data store.

Why Organizations Need Both Types of Penetration Testing

Are there any reasons to choose between the two types of pen tests? Clearly not.

A comprehensive approach to cybersecurity necessitates a combination of external penetration testing and internal penetration testing. Together, the two function as exposure validation: external testing confirms whether a given path into the network is real, and internal testing confirms how far that path can travel once it's there. The dual approach addresses various security risks and vulnerabilities from different perspectives, producing a more effective defense against bad actors. It covers potential attack vectors from both external and internal sources, strengthening your company's overall security posture.

A synergistic use of internal and external penetration testing methods—combined with vulnerability scanning, continuous monitoring, and incident response planning—helps you answer questions such as:

• Are access control rights stringently enforced?
• How fast and effective are your incident response procedures in case of a security breach?
• Are your systems and applications configured according to the security protocols outlined in your cybersecurity policies?
• Are your company's software and applications up to date with the requisite patches and fixes?

And many more.

Validating Your Company's Security Policies

Have you validated the effectiveness of your company's cybersecurity policies?

Cybersecurity penetration testing can reveal just how well-protected your networks and systems are under your current security policies. Security policies often read well on paper. Testing is where that documentation meets reality, and it's common for gaps to surface in implementation, configuration, or day-to-day enforcement rather than in the policy language itself. Pen test reports provide actionable insights into the types of vulnerabilities discovered and how they can be fixed. They uncover the weak links and gaps in your company's internal cybersecurity policy enforcement to strengthen both internal and perimeter security.

The idea behind penetration testing is to test your company's security policies in simulations that are almost identical to real-life scenarios—and, therefore, assess as accurately as possible the level of protection you have built.

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RedLegg's Approach to Internal and External Pen Testing

Regular and systematic pen testing confirms that your security policies are implemented correctly and provide the intended protection.

Some penetration testers rely solely on automated tests. Automated scanning is fast and consistent, but it's built to find known issues, not to chain smaller findings into a realistic attack path or recognize a privilege escalation opportunity that only becomes obvious through manual exploration. That's why RedLegg uses a proven process that combines automated tools and manual methods for depth and breadth testing.

RedLegg's penetration testing services help you understand the implications of test findings, and our testing experts recommend clear, actionable remediation steps. We approach your network and systems just like an attacker would, helping you separate the vulnerabilities that represent real, exploitable risk from the ones that are largely theoretical, so your team can prioritize remediation where it actually matters.

We can help you with:

• Continuous Penetration Testing
• Network Penetration Testing
• Internal/External Penetration Testing
• Physical Penetration Testing
• Wireless Network Testing
• Vulnerability Scanning
• Application Penetration Testing/Assessments

Talk to an expert and find the gaps in your security—before malicious actors do.

Want more? Learn...

• Pretty Much Everything You Need to Know About Pen Testing
• How Much a Pen Test Costs
• About RedLegg's Pen Test Service