External Penetration Testing: What to Know Before You Engage

external penetration testing

External penetration testing can mean one of two things: A penetration test conducted against your external attack surface (that is, a test of how a malicious actor might get in) or any penetration test conducted by a 3rd (external) party, rather than by your internal security team.

This article is a detailed guide to the former, that will touch on the latter. It is for companies considering a penetration test, trying to understand how to gain the most value for their investment.

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What Is External Penetration Testing?

External penetration testing, also known as external network penetration testing or ethical hacking, involves mimicking the actions and techniques of an external attacker. 

Ethical hackers, or testers, attempt to breach defenses of your network, app, cloud environment or wifi network from the outside without any internal access or knowledge of the target network. They take the perspective of an attacker without authorized access to internal resources. This helps them uncover vulnerabilities like unpatched software, misconfigurations, default credentials, and insufficient edge security controls.

The testing focuses solely on externally accessible systems like:

  • Websites
  • Web applications
  • VPN concentrators
  • Email servers
  • DNS servers
  • Cloud instances
  • Remote access systems

The Importance of External Penetration Testing

Holes in your system could lead to data breaches, service outages, reputational damage, and regulatory penalties. The goal is to find security weaknesses and vulnerabilities before malicious actors can discover and exploit them.

By proactively surfacing vulnerabilities through simulated attacks, external pentesting allows organizations to find and remediate security gaps before malicious actors do. This hardens the external attack surface and reduces breach risk significantly.

Editor’s Note: Too many companies wait until a compliance framework requires a pentest before arranging one. This is a risky approach. The cost of even a minor data breach; a single leak from a disgruntled employee, can be crushing. The cost of a pentest pails in comparison. Taking a proactive approach to security testing is not just good security practice, it’s good business.

Internal vs. External Penetration Testing

Internal penetration testing is from the perspective of an insider. It evaluates vulnerabilities within your organization’s network perimeter. 

The testers are positioned inside the network perimeter with insider knowledge and access to internal network segments, resources, and authentication systems. This allows deeper analysis of vulnerabilities that require authenticated internal access to exploit.

External penetration testing takes the opposite approach, focusing on the perspective of an outsider. This strategy concentrates exclusively on internet-facing assets and systems that are visible and accessible from outside the network without any internal access. 

The testers have no prior knowledge of or access to internal resources. This provides valuable insights into vulnerabilities that could allow perimeter breaches, such as unpatched services, default credentials, and misconfigurations. 

NOTE: Internal penetration testing is significantly more expensive than external, because of the increased size of the attack surface. A common cost-cutting move is to perform only external penetration testing, or to limit the scope of an internal pentest to critical resources.

Combining Internal and External Penetration Testing Tools

While internal testing evaluates exposure from trusted insiders, external testing quantifies risk from untrusted outsiders. Combining both testing approaches provides comprehensive coverage of an organization’s entire attack surface.

What Are the Benefits of External Penetration Testing?

external penetration testing reporting

External penetration testing provides many valuable benefits, including the following:

1. Identification of Critical Vulnerabilities

External penetration testing identifies high-severity externally exploitable vulnerabilities that could lead to data breaches, denial-of-service (DoS), ransomware attacks, and more.

2. Evaluation of Perimeter Security Controls

Through external penetration testing, the effectiveness and security posture of external facing perimeter security controls like next-gen firewalls, web application firewalls, DDoS protection, VPNs, and remote access systems are assessed.

3. Protection Against Unauthorized Access

External penetration testing assures that external attack surfaces and public assets are properly locked down against outside intruders and unauthorized access.

4. Compliance With Industry Regulations

By demonstrating due diligence, external penetration testing ensures compliance with industry regulations and standards around third-party security testing and risk assessments. Major compliance framework either directly mandates a pentest (such as PCI) or, arguably, are un-maintainable without one (GDPR, HIPAA, ISO 27001 etc).

5. Assessment of Internet-Exposed Risks

External pentesting quantifies the risks resulting from insufficient edge security or exposed services and resources accessible from the internet.

6. Strategic Resource Allocation

External network penetration testing enables better resource allocation and prioritization for strengthening defenses based on critical exposures and vulnerabilities uncovered.

7. Prevention of Brand and Financial Damage

An external pen test prevents brand reputation damage and financial impacts by proactively finding flaws before hackers potentially exploit them in an actual breach scenario.

8. Proactive Defense Hardening

By empowering organizations to fix security gaps and harden defenses, external penetration testing ensures that necessary precautions are taken before threats arise to compromise systems and data.

External testing enables organizations to proactively strengthen their security posture against ever-evolving external threats. The insights gained and vulnerabilities uncovered typically outweigh the costs. 

Discovering critical flaws before hackers is invaluable and will help you secure your network once you find all the potential security issues. 

External Penetration Testing Methodology

Even if you have your own SOC team, you can still look into getting an external penetration test done. A comprehensive external penetration test follows a systematic methodology similar to how real-world attackers operate:


Reconnaissance involves gathering information on target networks, systems, and applications through open-source intelligence (OSINT) research and active footprinting. This consists of harvesting data from WHOIS records, DNS lookups, organization websites, public code repositories, and more to map out the external infrastructure.


Scanning involves identifying live hosts, open ports, and services through non-intrusive techniques like TCP/UDP scanning, SNMP sweeps, DNS zone transfers, and banner grabbing. This maps out the type of systems externally visible and where potential entry points exist.

Vulnerability Detection

Vulnerability detection involves leveraging vulnerability scanners and manual testing to detect security misconfigurations, unpatched services, weak passwords, and exploitable software flaws. It helps find low-hanging fruit an attacker would target.


Exploitation involves attempting to penetrate identified vulnerabilities to demonstrate real-world risk. Testers exploit flaws to achieve outcomes like shell access, data exfiltration, and service disruption. This is done in a controlled manner to quantify the impact.

Lateral Movement

Lateral movement involves pivoting from compromised hosts to attack further systems internally. Since external testing has no internal access, testers simulate lateral movement to provide insights into how breaching perimeter defenses could lead to deeper network compromise.


Reporting involves documenting all vulnerabilities found, how they were successfully exploited, their potential business impact, and actionable remediation advice. Reports provide a risk-rated roadmap for hardening security defenses.

External Penetration Testing Tools

Testers typically use the following tools for external pentesting activities:

  • Nmap: Open source port scanner useful for host and service discovery.
  • Nessus: Scans networks for vulnerabilities and misconfigurations.
  • Burp Suite: Tests web applications for flaws like SQL injection and cross-site scripting.
  • Metasploit: Exploits security holes and executes payloads on targets.
  • John the Ripper: Cracks password hashes obtained during pentests.
  • Wireshark: Captures and analyzes network traffic for reconnaissance.

This is just a short list of penetration testing tools; many more are available, but these are the most well-known that you are likely to come across.

How Long Does an External Pentest Take?

The duration of an external penetration test can range from one week to one month, depending on the size, complexity, and scope of the engagement:

For small environments with just a few external-facing assets, like a corporate website, VPN server, or email server, testing can potentially be completed in less than one week. For medium-sized organizations, testing typically takes two to three weeks.

Large enterprise networks with vast external attack surfaces requiring in-depth testing should expect assessments to take three to four weeks. This is because there are more public-facing systems, web applications, cloud assets, and remote access services to test.

What Affects External Penetration Testing Duration?

Some key factors that influence external pentest duration include:

  • Size of In-Scope Infrastructure: Larger environments require more time to thoroughly test.
  • Complexity: Intricate multi-tiered applications, interconnected systems, and services lengthen testing.
  • Scope: Broad, expansive scopes require more time versus a narrow, limited scope.
  • Access: The availability of testers to systems impacts schedules.
  • Vulnerabilities Discovered: The more flaws uncovered that require exploitation, the longer it takes.
  • Agreed Depth: Testing can be cursory or extremely comprehensive based on preferences.
  • Tester Bandwidth: The number of testers assigned to the engagement impacts speed.

While organizations might be tempted to limit testing timeframes, adequate time should be allotted for thorough, high-quality testing. Rushing assessments can result in missed vulnerabilities.

How Much Does External Penetration Testing Cost?

External penetration testing pricing is generally more affordable than internal testing, ranging from $4,000 to $100,000 for large, complex environments. However, costs ultimately come down to multiple factors.

  • Duration: More testing days means higher costs.
  • Depth: More comprehensive testing with extensive exploitation increases costs.
  • Assets: Large complex environments require more time and resources.
  • Location: Onsite testing is typically more expensive than remote.
  • Company Size: Larger firms generally charge higher rates.

When Does an External Pentest Make Sense?

An external penetration test is advisable if:

  • Your environment relies heavily on internet-facing applications and resources.
  • You want to benchmark your external security posture.
  • Compliance requires an independent external security audit.
  • You are planning to deploy new external-facing assets.
  • You have made significant changes to edge security controls.
  • An application vulnerability scan identified high-risk flaws.

Alternatively, if your network perimeter is tightly restricted with minimal internet exposure, the value of an external pentest is diminished. For highly internal environments, an internal test may be more useful. Combining external and internal testing provides comprehensive insights into an organization’s full security landscape.

Choosing an External Penetration Testing Provider

Selecting the right penetration testing company is crucial for getting maximum value out of an external pentest.

Here are some important factors to keep in mind:

  • Experience: Look for a provider with extensive expertise in external network and application testing. Ask about the types of tests they have performed in the past.
  • Certifications: Leading industry certifications like CEH, OSCP, GPEN, and GWAPT validate up-to-date technical skills. Ask which certs their testers hold.
  • Reputation: Check reviews and ask for references from past clients to gauge the quality of deliverables. A reputable provider should be transparent and provide sample reports.
  • Reporting: Ensure the provider offers comprehensive, actionable reporting with clear remediation guidance, risk ratings, and proof-of-concept examples.
  • Communication: Select a provider that makes communication with stakeholders a priority before, during, and after the test.

When evaluating providers, have them detail their methodology and deliverables. Vet them thoroughly – don’t just take marketing claims at face value. Also, don’t choose based on testing cost alone. The lowest bidder often lacks the expertise to perform rigorous testing. A higher price is justified if the provider produces superior, insightful deliverables.

Rules of Engagement in External Penetration Testing

The rules of engagement in external penetration testing involve what you are allowed to do before the test begins and what you need to do before getting started:

  • Explain the importance of having a detailed scope and clear rules of engagement before starting a penetration test.
  • Highlight key things that should be defined, like tested systems, testing methods, hours of operation, points of contact, and authorization.
  • Note legal considerations when performing security testing.

Preparing for an External Pentest

Here is how you can prepare for an external penetration test:

  • Advise on getting proper executive buy-in and notifying key stakeholders before testing begins.
  • Recommend backing up systems and data in the event something is impacted.
  • Suggest allowing testers access through VPNs or jump boxes to isolate access.

Once you have laid out these rules and set the expectations for your preparation, you can start talking about when the testing will begin.

Interpreting and Acting on Results

The findings from an external penetration test are only useful if acted upon properly. Here are some best practices for interpreting and acting on the results:

  • Review and Validate Findings: Have the technical team review results for accuracy and reproducibility. Validate high-risk flaws through scanning and further exploitation.
  • Prioritize for Remediation: Rank findings based on severity and exploitability. Remediate critical issues first. Categorize results into short and long-term fixes.
  • Develop a Remediation Roadmap: Create a detailed plan for closing security gaps uncovered by testing. Assign remediation tasks to responsible teams and set deadlines.
  • Fix Critical Issues Quickly: For high-severity findings, fix immediately or implement temporary controls to reduce risk until a patch is available.
  • Share Internally: Ensure findings are visible to IT teams for remediation efforts. Develop awareness of risks among leadership as well.
  • Retest Periodically: Schedule recurring external pentests every six or 12 months to assess remediation progress. New vulnerabilities may arise as the environment changes.

Acting quickly on external pentest findings hardens the organization against outside threats. Testing should be viewed as an ongoing process, not a one-time event.

Published by NA Team
This article has been contributed to by multiple members of the Network Assured team....
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