What is an external pentest?
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What is an external pentest?

Daniel Andrew

A penetration test (commonly known as a pentest) is a security assessment which simulates the malicious activities of real-world attackers to identify security holes in your business’s systems or applications. The aim of conducting a pen test is to understand what vulnerabilities are in your business systems, how they could be exploited, and what the business impacts would be if an attacker was successful.

One of the first types of penetration test organisations usually perform is the external pen test. External penetration testing (also known as external network penetration testing) is a security assessment of an organisation’s perimeter systems. Your perimeter comprises all those systems which are directly reachable from the internet. By nature, they are the most exposed systems as they are out in the open and are therefore the most easily and regularly attacked.

The aim of an external pentest is to find ways to compromise your accessible (external) systems and services, gain access to sensitive information, and discover methods an attacker could use to attack your clients or users. In a quality external pentest, the security professional(s) conducting the assessment will replicate the activities of real hackers, including executing exploits to attempt to gain control of systems. They will also test the extent of any weaknesses discovered to see how far a malicious attacker could burrow into your network and what the business impact of a successful attack would be.

External penetration testing usually tests from the perspective of an attacker with no prior access to your systems or networks. This is different to another common type, the internal penetration test. Internal penetration testing instead tests the scenario where an attacker already has a foothold on a compromised machine or is physically in the building. It usually makes sense though to first cover off the fundamentals and consider internal testing only after both regular vulnerability scanning and external penetration testing are being performed.

How to perform external penetration testing

So how does an organisation go about ordering an external penetration test?  

Scheduling an external pentest is usually as simple as engaging with your chosen cyber security consultancy and pointing them at your perimeter systems (a list of domains and IP addresses/ranges). An external pen test is normally run on a “Black Box” basis, which means no privileged information (such as application credentials, infrastructure diagrams, or source code) is provided to the testers. This is similar to where a real hacker targeting your organisation would get started from, after they have discovered a list of your IPs and domains.

There are a few important pointers and due diligence steps that are worth bearing in mind when organising your external penetration test:

External penetration testing or vulnerability scanning?

Those of you that are already familiar with external vulnerability scanning will notice that an external pentest shares some similarities. So, what’s the difference?

Typically, an external penetration test includes a full external vulnerability scan, but that’s just where it gets started. All output from scanning tools will be investigated manually by a pentester to remove false positives, run exploits to verify the extent/impact of the weakness, and “chain together” multiple weaknesses to produce more impactful exploits. Where a vulnerability scanner would simply report that a service has a critical weakness, a penetration test would look to exploit that weakness and gain control of the server. If successful, the pentester will use their newly acquired access to pivot further, and compromise further systems and services.

While vulnerability scanners often identify potential issues, a penetration tester would explore those fully and report on whether the weakness needs attention or not. For example, vulnerability scanners routinely report on ‘Directory Listing’, which is where web servers offer a list of all the files and folders on the server. This is not necessarily a vulnerability on its own, but it does need investigation. If a sensitive file (e.g. a backup configuration file containing credentials) is exposed and listed by directory listing, a simple informational issue (as reported by a vulnerability scanner) could be quickly turned into a high impact risk to your organisation. The penetration tester’s job includes carefully reviewing output from a range of tools, to make sure that no stone is left unturned.

Some further activities which a real attacker would perform which are not performed by vulnerability scanners may also be included, but these vary from consultancy to consultancy. Check the proposal or ask questions before scheduling the pentest if you’d like these to be in scope. For example:  

Finally, it’s important to remember that new critical vulnerabilities are discovered daily, and attackers usually exploit the most serious weaknesses within a week of their discovery. Whilst an external penetration test is important assessment to take deep look into the security of your exposed systems, it best used as an additional service to complement regular vulnerability scanning which should already be in place. You can find a short discussion on how penetration testing and vulnerability scanning can be used to complement each other here.


Intruder offers both external infrastructure penetration testing and vulnerability scanning services, Intruder’s continuous vulnerability scanning service helps you keep on top of the latest vulnerabilities and alerts you to emerging threats which affect your most-exposed systems. To enquire about Intruder’s vulnerability scanning or penetration testing services, please feel free to get in touch, or get started with a free trial today.

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Written by

Daniel Andrew

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