Internal vs External Penetration Testing: A 2025 Guide

External pentests simulate attacks from outside the network, targeting internet facing assets web servers, VPNs, cloud services with no prior access.
Internal pentests assume an insider or post breach scenario, starting from within the LAN with user level access to find privilege escalation and lateral movement risks.
Both are essential. External tests verify perimeter defenses, while internal tests reveal what an attacker could do once inside. Industry guidelines e.g. PCI DSS requires annual internal and external tests of the Cardholder Data Environment.
Common tools overlap e.g. Nmap, Nessus, Metasploit, but internal testers also use credential dumping tools Mimikatz, Responder, BloodHound to pivot through the network.
In short: don’t skip one or the other. A combined approach gives a fuller picture of risk, satisfying compliance and protecting against both Internet borne attacks and insider threats.

A split-scene 3D infographic animation that transitions from a digital globe view (external attack surface) to a corporate LAN schematic (internal network), symbolizing the shift from perimeter testing to internal exploitation.

Penetration testing comes in two flavors, external and internal. An external penetration test acts like an attacker on the Internet with no inside knowledge, probing your firewalls and public IPs. By contrast, an internal penetration test simulates a threat behind the firewall, say a malicious insider or malware that has already breached the perimeter. Both perspectives are crucial. In today’s landscape hybrid work, cloud apps, AI driven attacks, organizations can no longer assume the network perimeter is impregnable. For example, IBM reports that in 2024 about 83% of organizations experienced at least one insider attack. That underlines why we can’t rely solely on external testing.

In the first 100 words: External tests probe internet facing systems, websites, VPNs, etc., while internal tests probe the LAN from a user’s viewpoint. This matters in 2025 because sophisticated threats and remote endpoints have blurred old perimeters. As one security blog puts it, external pentests simulate internet based attacks on public facing assets like servers or VPNs, whereas internal pentests simulate insider threats or a device that’s already inside to find privilege or lateral movement risks. In this article, we define each type, compare their goals and methods, and explain when and how to use both.

What Is External Penetration Testing?

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An external penetration test focuses on perimeter security. The testers have no credentials and treat your defenses as if they were strangers on the Internet. They use only publicly available information IP addresses, domain names, WHOIS data and try to find exposed entry points. The goal is to see if an outsider could penetrate your network from the outside. External pentesters will scan open ports on firewalls and routers, probe web applications looking for SQL injection, XSS, etc., test VPN or RDP gateways, and check for weak authentication on internet facing services.

Key points for external testing, testers are outside your firewall with zero prior access. They use techniques like open source reconnaissance, port scanning e.g. Nmap, vulnerability scanning Nessus, OpenVAS, and web application testing Burp Suite, OWASP ZAP. Since their traffic passes through your perimeter defenses, firewalls, IDS/IPS, they get a realistic view of what’s visible from the Internet. In short, external tests find holes that real world Internet attackers might exploit.

What Is Internal Penetration Testing?

A dynamic 3D data-environment walkthrough, resembling an animated cyber-labyrinth of servers, nodes, and connections — shifting from calm to critical as the internal attack chain unfolds. Visual storytelling style: immersive data visualization meets cinematic threat simulation.

An internal penetration test starts from inside the network, assuming an attacker has managed to get in or is a malicious insider. Testers are given user level network access for example, a VPN or a compromised workstation account and they try to move laterally and escalate privileges. This simulates scenarios like a disgruntled employee, a compromised PC, or malware that has already breached the firewall. The focus shifts to internal systems: file servers, workstations, internal web apps, Active Directory domains, etc.

Internal pentesters will run LAN port scans, enumerate network shares and services, attempt password attacks using tools like Hashcat/John, and use Windows credential tools like Mimikatz or Responder to harvest credentials. They test for misconfigurations in AD group policies, lack of network segmentation, or vulnerable internal apps. The objectives are to see how far an attacker could go once inside: Can they reach the domain controller? Steal sensitive data? Take over admin accounts? As one source explains, internal tests aim to uncover issues like privilege escalation, lateral movement, and unauthorized access to sensitive data.

Importantly, internal pentests cover the internal perimeter for example, the Cardholder Data Environment in PCI DSS. PCI guidance clarifies that an internal test looks at the internal network of systems handling card data, not just front door systems. In practice, external pen testing checks how effectively your defenses can withstand real world attacks from external threats, while internal pen testing shows the damage an attacker could do if they bypassed those defenses.

Head to Head Comparison

Aspect	External Penetration Test	Internal Penetration Test
Attacker Location	Outside your network, treating it as a black box.	Inside the network behind the firewall insider or assumed breach.
Initial Access	Unauthenticated; testers only know public IPs/domains.	User level credentials or VPN access provided simulating an employee.
Scope	Internet facing assets: web servers, VPN/RDP gateways, mail servers, etc. public attack surface.	Internal LAN: workstations, internal servers, databases, AD controllers, network devices inside the firewall.
Focus Areas	Perimeter security, firewall rules, IDS/IPS, network borders.	Lateral movement, privilege escalation, internal app bugs, segmentation flaws.
Common Tools	Nmap, Nessus/OpenVAS, Burp Suite, Metasploit, Hydra, etc. for external scanning and exploitation.	Nmap internal scans, Active Directory tools BloodHound, CrackMapExec, credential tools Mimikatz, Responder, Wireshark, Metasploit for internal exploits.
Examples of Tests	Exploiting a web app on your public site, guessing VPN passwords, testing open ports on routers.	Dumping domain creds with Mimikatz, exploiting trust relationships, cracking local passwords, testing network shares.
Typical Frequency	Required at least annually or after major changes e.g. per PCI DSS 11.3.	Also annual or after internal changes; used for insider threat assessment and compliance PCI requires both.

Attack Scenarios and Objectives

An animated mirrored sequence (split-screen format) — left side depicting the external attacker journey from outside-in, and the right side showing the internal attacker moving inside the network. Each half follows an attack chain timeline with glowing markers for reconnaissance, scanning, exploitation, privilege escalation, and data exfiltration.

External Testing Scenario: Testers simulate an outsider. They often start with reconnaissance using WHOIS, DNS queries, Google dorking and other OSINT to map your public footprint. Then they scan for open ports and services. For example, if they find port 80 open on a web server, they probe for SQL injection or XSS vulnerabilities. If they compromise one host, they try to use that foothold to move inward. In NIST’s terms: they’re given no real information about the target apart from IP addresses. The objective is to breach the perimeter and extract data simulate data theft or defacement.
Internal Testing Scenario: Testers start with a valid but low level login on the LAN. They scan the internal network for other devices Nmap/Advanced IP Scanner, enumerate shares and services, and attempt to exploit common vulnerabilities, outdated Windows patches, misconfigurations. A big focus is credential harvesting using tools like Mimikatz to dump passwords from memory then using those credentials to escalate privileges or jump to other machines. For example, if a tester finds that a file share is poorly secured, they may grab a domain admin hash and take over the network. The goal is to understand what an attacker could achieve with initial accessi.e., how far they can propagate and what critical data they can reach.

Both test types share some tools, but emphasis differs. Both may use Nmap or Nessus for scanning. External tests rely heavily on web/vulnerability scanners Burp, Nikto, ZAP and brute force tools for login pages. Internal testers use internal focused tools: Mimikatz credential dumping, Responder LLMNR/NBT NS spoofing, BloodHound AD mapping, and password crackers Hashcat, John. For example, Mimikatz is famous for pulling Windows credentials during an internal pentest, while Burp Suite is a staple for attacking web apps externally.

When to Use Each Use Cases

A dynamic decision-flow animation — blending holographic flowchart logic with kinetic data visualization. Each branch of the flow (left for “External”, right for “Internal”) lights up as different triggers or scenarios appear, leading to the conclusion that both are essential for holistic readiness.

External pentests are generally done when you’ve exposed something to the Internet or need perimeter validation. Launching a new website, VPN, or cloud service? That’s an occasion for external testing. Many compliance standards PCI DSS, SOC 2 explicitly require annual external tests of internet facing assets. Also do external pentests after a breach or if you notice attacks targeting your public IPs. In short, any focus on keeping outsiders out calls for an external test.
Internal pentests are warranted when the concern is insider risk or the aftermath of a breach. If you suspect weak internal controls, want to validate network segmentation, or comply with rules emphasizing insider threats, some PCI scopes, ISO 27001 risk assessments, etc., you should do an internal test. For example, if you’ve just restructured your network, deployed a new AD domain, or you handle sensitive data on the LAN, an internal test will show whether someone inside could exploit those changes. Many organizations do internal tests after external ones to see how far an intruder could go under the hood.

In practice, both types complement each other. External tests protect your perimeter; internal tests show the fallout if that perimeter is breached. Industry experts recommend running both. PCI DSS 11.3, for instance, explicitly covers the entire external and internal card data environment. Likewise, ISO 27001’s principles of risk management imply testing all parts of the network. New HIPAA rules proposed in late 2024 are moving toward mandating annual penetration tests of ePHI systems. Bottom line: a strong security program schedules both external and internal pentests at least annually and after big changes.

Advantages and Limitations

A dual-pane kinetic infographic — structured like a futuristic balance scale or two floating holographic panels that shift and pulse in harmony. Each side (External vs Internal) animates independently, showcasing advantages (ascending energy, clean lines) and limitations (descending opacity, red markers). The two panels ultimately balance in the middle, signifying that both are essential and complementary.

External Testing Advantages: Realistic attacker view of the Internet. It objectively assesses firewalls, IDS/IPS, and perimeter configs without any insider knowledge zero trust. By focusing on what’s exposed to attackers, it helps prevent common breach scenarios. External tests also tend to be less disruptive since they don’t touch internal servers. As one resource notes, external tests give a holistic view of your external defenses.
External Testing Limitations: They cannot see inside your network. Any vulnerability behind the firewall remains hidden. If segmentation is poor, an outsider test won’t flag it unless they breach the firewall. Also, firewall rules or NAT can limit what scanners find, testers only see what the outside sees. In short, external tests are blind to internal trust issues, and they depend on correct scoping of IP ranges.
Internal Testing Advantages: Internal tests uncover hidden issues that external scans miss weak internal passwords, misconfigurations, and trust relationships. They realistically simulate insider threats or malware spread. For example, they can reveal if a low level employee can escalate to admin, or if a malicious script on a workstation can spread. This is effective in uncovering vulnerabilities that external testing might miss. Internal tests also help refine incident response you see if your monitoring or controls would catch a real breach.
Internal Testing Limitations: They require more setup. You must provide testers with network access or credentials often via VPN or jump box, which introduces coordination overhead. Internal tests can be riskier: aggressive scans or password cracking can slow networks or trigger alerts. They also won’t find issues on your perimeter if you only do internal tests, you might miss a glaring firewall hole. Plus, internal scopes can be large hundreds of machines, so testing can take longer and cost more. More on cost below.

Neither type is a silver bullet alone. External tests keep the bad guys out; internal tests show what happens if they get in. Together, they provide a full picture of your risk profile.

Compliance and Regulatory Relevance

A rotating 3D compliance timeline and constellation map, where each framework (PCI DSS, HIPAA, ISO/IEC 27001, NIST 800-115, SOC 2) orbits around a luminous “Compliance Core.” Lines of light connect the standards to the test types they mandate — blue beams for external testing, amber for internal testing. Each framework briefly expands into focus as its requirements animate into view.

Most security frameworks expect both perspectives. The PCI DSS Payment Card Industry famously mandates both external and internal tests of your cardholder data environment CDE each year. The PCI Pen Test Guidance spells out that the scope of a penetration test… includes the entire CDE perimeter and any critical systems, covering both public facing external and internal segments. In practice, PCI audits demand that all Internet facing card systems and all in scope LAN systems be tested annually by qualified testers.

For HIPAA U.S. healthcare privacy law, penetration testing wasn’t always explicitly required, but that’s changing. The December 2024 NPRM from HHS would mandate annual pen testing of systems handling ePHI. Even today, best practices and OCR guidance encourage HIPAA regulated entities to include both external and internal pentests as part of risk assessments, since health data breaches can originate from either vector.

ISO/IEC 27001: The standard doesn’t name penetration testing, but it requires regular vulnerability assessments and technical testing of controls. Organizations typically interpret this to include both external and internal tests. Section A.12.6 of ISO 27001 Security of systems and applications implies that testing of internal and external systems should happen routinely.

Other frameworks SOC 2, NIST 800 115, GLBA, etc. all recommend or require testing your defenses. NIST SP 800 115 explicitly notes that a pentest scenario can simulate an inside attack, an outside attack, or both, and that both methods should be considered in planning. In short, compliance and best practice guidance almost universally assume a comprehensive approach: prove your perimeter security with external tests, and validate your internal controls with insider tests.

Cost and Resource Considerations

A hybrid animated infographic and kinetic bar graph with floating holographic panels representing testing types and cost ranges. Each segment unfolds in sequence, comparing external vs internal testing, then merging into a unified value flow diagram that reinforces the message: cost aligns with coverage and impact.

Budgeting also reflects the difference in scope. Generally, external tests are smaller and cheaper than full internal tests. External tests may involve scanning a handful of IP addresses or servers, whereas internal tests often cover dozens or hundreds of workstations and servers. According to industry data, external penetration tests typically range around $5,000- $20,000, while internal penetration tests are often in the $7,000- $35,000 range. Prices vary widely by scope: a simple website test on one server might be low end, while a large internal network test can climb much higher. So in practice, internal pentests often cost more due to the broader asset count and longer testing time.

Resource wise, external tests can usually be done remotely with minimal involvement from the client. Internal tests may require on site access or secure VPN setup and closer coordination with IT to provide test accounts, define safe scanning windows, etc.. Because internal testing is closer to production, it must be carefully planned to avoid disrupting business e.g. scheduling around sensitive hours.

In house teams sometimes handle internal pentesting using their own tools and knowledge of the network, while third party vendors typically perform external tests for an impartial view. However, many organizations hire external specialists for both types to ensure independence. In any case, remember that the testing cost is just one part of the budget you’ll also want to allocate resources for remediation. Internal scopes especially can generate many findings e.g. patching dozens of systems, so fix management and any retesting need planning too.

Best Practices

A flowing process timeline and circular lifecycle infographic, animated like a cyber-operations dashboard with smooth kinetic transitions between phases. Each stage lights up sequentially, showing how structured testing turns findings into actionable security improvements.

To maximize the value of both external and internal pentests, follow these best practices:

Define Clear Scope and Rules: Before testing, document exactly what will be tested IP ranges, apps, endpoints and from what location outside vs inside. Write a Rules of Engagement ROE that covers timing, authorized techniques, and safe words. Good scope planning PCI 11.3 and NIST recommend this ensures testers focus on business critical assets and obey restrictions.
Use Qualified Testers: Pentesting requires skill. Engage experienced, certified professionals OSCP, CISSP, CREST, etc. for each test. External testers should be adept at internet reconnaissance and web exploits, while internal testers need deep knowledge of Windows/Linux networks and AD. Independent third parties reduce bias.
Test Both Regularly: Do both external and internal tests at least annually, and especially after major changes, new infrastructure, mergers, software rollouts. Many treat them as a package: first test the perimeter, then test inside. High risk environments finance, healthcare may even test quarterly or after any significant upgrade.
Follow Robust Methodologies: Adopt industry frameworks PTES, OWASP, OSSTMM or NIST SP 800 115 to ensure thorough coverage. Typical pentest methodology involves phases: planning, reconnaissance, scanning, exploitation, and reporting. Automated tools, scanners, crawlers should be complemented by manual testing and creative attack chains. For example, PCI DSS expects both network and application layer testing, so include both in your plan.
Communicate and Coordinate: While testers need independence, keep key stakeholders informed. Alert the SOC/IT helpdesk to the pentest schedule so that intrusion alerts don’t cause panic. Provide contact points in case something goes wrong e.g. a scan overwhelms a server. For internal tests, arrange safe time windows and ensure backup plans e.g. test accounts locked out.
Use the Right Tools: Combine automated scans with manual hacks. For external tests, tools like Nmap, Nessus, Burp Suite, and Metasploit are standard. For internal tests, add credential tools Mimikatz, Hashcat, internal scanners, and AD audit tools BloodHound, CrackMapExec. Keep tools updated and have multiple techniques ready attackers would.
Report Clearly with Actionable Fixes: A good penetration test report doesn’t just list vulnerabilities it tells a story. It should show the attack paths the testers used from entry point to goal and rank issues by risk. SANS and PCI guidance emphasize detailed reporting with remediation advice. Include screenshots, POCs, and prioritize fixes. Ensure management sees the big picture if unchecked, this issue could lead to X.
Remediate and Retest: Treat the findings as a living process. Immediately patch or mitigate critical issues, then have testers verify fixes either by re scanning or focused retesting. Maintain a remediation tracker so nothing slips. PCI and ISO require that vulnerabilities flagged in a pentest be fixed in a timely manner, so plan for follow up.
Integrate with Overall Risk Management: Feed the results of both tests into your risk register and incident plans. Use them to fine tune network segmentation, employee training, and security controls. For instance, if internal testing finds weak password policies, strengthen those policies. If external testing finds a misconfigured firewall rule, fix it and re-evaluate all external exposures.

By scoping carefully, using skilled testers, and acting on the results, organizations turn pen testing into a cycle of continuous improvement. The goal isn’t a one time checkmark, but an ongoing arms race: find holes before attackers do.

Internal and external penetration tests address different pieces of the same security puzzle. External tests harden your perimeter against outside threats, while internal tests reveal what an attacker could do once inside. In today’s world 2025 and beyond, you can’t afford to rely on one without the other. Combined, they validate your defenses both at the firewall and behind it. By understanding the differences, using the right tools, and following best practices, organizations stay one step ahead of attackers.

Ready to strengthen your defenses? The threats of 2025 demand more than awareness they require readiness. If you’re looking to validate your security posture, find hidden risks, or build a resilient defense strategy, DeepStrike is here to help.

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Our expert penetration testing team delivers clear, actionable guidance to protect your business. Check out our penetration testing services to see how we can uncover vulnerabilities before attackers do. Drop us a line we’re always ready to dive in.

About the Author

Mohammed Khalil is a Cybersecurity Architect at DeepStrike, specializing in advanced penetration testing and offensive security operations. With certifications including CISSP, OSCP, and OSWE, he has led numerous red team engagements for Fortune 500 companies, focusing on cloud security, application vulnerabilities, and adversary emulation. His work involves dissecting complex attack chains and developing resilient defense strategies for clients in the finance, healthcare, and technology sectors.

FAQs

What is the main difference between external and internal penetration testing?

External pentesting simulates attacks from outside your network, targeting public facing assets with no inside access a black box test. Internal pentesting simulates an attacker inside your LAN like a disgruntled employee or post breach malware and focuses on privilege escalation and lateral movement. The key difference is where the attacker starts: outside the firewall vs inside it.

Why is internal penetration testing important?

Internal testing uncovers vulnerabilities that attackers could exploit once past the perimeter. With insider breaches becoming more common IBM reports 83% of orgs had insider attacks in 2024, it’s critical to know how far a threat can go inside your network. Internal tests can reveal weak credentials, poor segmentation, or overly permissive access that an external test alone would miss.

How often should we do external vs internal penetration tests?

Most standards require both types at least annually. For example, PCI DSS 11.3 mandates yearly tests of both your external and internal cardholder data environments. It’s also wise to test after any major change, new systems, network redesign or breach. BrightDefense and NIST recommend doing external tests first and then internal tests as soon as possible afterward to complete the assessment.

Which is more expensive: an external or internal penetration test?

Typically, an internal test costs more due to its broader scope. Industry surveys find external pentests often range around $5K- $20K, while internal tests are roughly $7K- $35K. Exact prices vary by asset count and depth. External tests have fewer targets public servers only, whereas internal tests might cover dozens of machines, hence higher cost and effort.

What tools are commonly used in external vs internal penetration testing?

Many tools overlap, but focus differs. External testers use network scanners Nmap, vulnerability scanners Nessus/OpenVAS, web proxies Burp Suite, and brute force tools Hydra, Medusa against login pages. Internal testers use those plus specialized tools for internal networks: credential dumpers Mimikatz, AD analyzers BloodHound, CrackMapExec, name server poisoners Responder, and password crackers Hashcat on captured hashes. In both cases, exploit frameworks like Metasploit are handy. Essentially, external tests lean on web/app scanners, internal tests lean on Windows/LDAP and privilege escalation tools.

Is a vulnerability scan the same as a penetration test?

No. A vulnerability scan automated tool that finds known flaws is only one part of a pentest. Penetration testing goes further: it actively exploits vulnerabilities to demonstrate the actual impact. For example, a scan might flag a missing patch; a pentester will actually attack that vulnerability e.g. remote code exec to prove an entry into the system. In short, scanning shows what could be wrong; penetration testing shows how bad it can get and includes manual techniques, not just tools.

What certifications should a penetration tester have?

Look for experience and relevant certs. Common credentials include OSCP practical pentesting cert, CEH ethical hacking, CISSP security management, CREST or GIAC GPEN penetration testing. These indicate formal knowledge. The author Mohammed Khalil holds CISSP, OSCP, OSWE, reflecting expertise in network and application hacking. But also ask about hands-on experience: ideally the testers have real engagement experience similar to your environment.

Internal vs External Penetration Testing 2025 Guide