This time in our series on the different types of penetration test, we’re covering “Internal” pen tests, otherwise known as “Internal…
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What is an internal pen test and how is it carried out?

Lars Greiwe

This time in our series on the different types of penetration test, we’re covering internal pen tests, otherwise known as 'internal infrastructure' or 'internal network' penetration tests. Hopefully, from our post on external penetration testing you should now have some idea about penetration testing both in general and on the outside of your business, and this post will explain why you might want to take a look on the inside.

What is an internal network penetration test?

Commonly referred to as an 'internal pen test', the internal infrastructure penetration test focuses on testing attacks which could be carried out by an adversary who has already gained a foothold within your network and is looking to 'elevate' themselves to gain further control and cause more damage. It also deals with security holes that could be taken advantage of by a malicious insider — perhaps a disgruntled employee that wishes to cause damage to areas of the business outside of their usual access level.

This type of pen test typically involves tapping into your network on site, so the tester(s) will need to be given access to your office similar to that of an employee. Alternatively, they could start in your cloud infrastructure, depending on the scope of testing and the scenario to be explored. Testers will then attempt to gain access to sensitive information sources or privileged user accounts which should be off-limits to them, finding ways to subvert any access controls you may have in place.

How is internal pen testing carried out?

The process normally starts with a 'discovery phase' where the tester uses network mapping tools to discover the inner workings and layout of your network. Testers will effectively build up a map of your internal network, and the computers and services available on it, and will use this map to guide their efforts to find holes in your security, and to breach areas they shouldn’t be able to access.

After the discovery phase comes the 'identification phase'. Some examples of the sorts of activity that can take place in this phase are as follows:

The aim of these types of tests is really to find all possible weaknesses in the shortest space of time. An ordinary infrastructure pen test is therefore usually carried out as an audit-style approach, in collaboration with the security team, and can often be very noisy (in terms of security alerts from any monitoring systems you might have). Although this is a good way to discover the majority of the weaknesses you might have, the downside of this approach is that it may not give you the best understanding of how you might fare when targeted by a real attacker.

You can also learn more about how penetration tests are scoped and quoted.

Penetration testing vs red teaming

For larger and more security-mature businesses, it’s possible to go one step further and conduct what’s called a 'red team' exercise. Tests conducted by a red team aim to mirror techniques a real attacker would use as accurately as possible, including trying to avoid detection. As such, a red team is more of a test of your operational defences, and is often carried out without the knowledge of staff members, including those working in security teams. Red teaming will usually involve other types of attack such as phishing, and can offer a more comprehensive, realistic (and expensive!) coverage.

Standard internal pen tests typically take anything from a few days to a couple of weeks, whereas a full red team engagement would likely take longer, running for over a month or even two for larger firms. Pricing varies hugely based on the scale of the job and the experience of the professionals carrying out the tests.

Automated internal penetration tests

Manual penetration testing can be costly, infrequent, and only give you point-in-time insight into your security posture. It’s also impossible for pen testers to manually check for every vulnerability that exists, as there are simply too many.  

Although it’s not possible to fully automate an internal pen test, a tool like Intruder combines continuous network monitoring, automated vulnerability scanning, and proactive threat response, helping you secure your attack surface. See it in action below!


Hopefully this has cleared up some of the questions you may have had regarding internal pen tests. At Intruder, we help you continuously scan your systems, monitor your network, and reduce your attack surface, giving you the protection you need between manual pen tests. Start your 14 day free trial today.

Release Date
Level of Ideal
Before CVE details are published
Limited public information is available about the vulnerability.

Red teamers, security researchers, detection engineers, threat actors have to actively research type of vulnerability, location in vulnerable software and build an associated exploit.

Tenable release checks for 47.43% of the CVEs they cover in this window, and Greenbone release 32.96%.
Day of CVE publish
Vulnerability information is publicly accessible.

Red teamers, security researchers, detection engineers and threat actors now have access to some of the information they were previously having to hunt themselves, speeding up potential exploit creation.

Tenable release checks for 17.12% of the CVEs they cover in this window, and Greenbone release 17.69%.
First week since CVE publish
Vulnerability information has been publicly available for up to 1 week.

The likelihood that exploitation in the wild is going to be happening is steadily increasing.

Tenable release checks for 10.9% of the CVEs they cover in this window, and Greenbone release 20.69%.
Between 1 week and 1 month since CVE publish
Vulnerability information has been publicly available for up to 1 month, and some very clever people have had time to craft an exploit.

We’re starting to lose some of the benefit of rapid, automated vulnerability detection.

Tenable release checks for 9.58% of the CVEs they cover in this window, and Greenbone release 12.43%.
After 1 month since CVE publish
Information has been publicly available for more than 31 days.

Any detection released a month after the details are publicly available is decreasing in value for me.

Tenable release checks for 14.97% of the CVEs they cover over a month after the CVE details have been published, and Greenbone release 16.23%.

With this information in mind, I wanted to check what is the delay for both Tenable and Greenbone to release a detection for their scanners. The following section will focus on vulnerabilities which:

These are the ones where an attacker can point their exploit code at your vulnerable system and gain unauthorised access.

We’ve seen previously that Tenable have remote checks for 643 critical vulnerabilities, and OpenVAS have remote checks for 450 critical vulnerabilities. Tenable release remote checks for critical vulnerabilities within 1 month of the details being made public 58.4% of the time, but Greenbone release their checks within 1 month 76.8% of the time. So, even though OpenVAS has fewer checks for those critical vulnerabilities, you are more likely to get them within 1 month of the details being made public. Let’s break that down further.

In Figure 10 we can see the absolute number of remote checks released on a given day after a CVE for a critical vulnerability has been published. What you can immediately see is that both Tenable and OpenVAS release the majority of their checks on or before the CVE details are made public; Tenable have released checks for 247 CVEs, and OpenVAS have released checks for 144 CVEs. Then since 2010 Tenable have remote released checks for 147 critical CVEs and OpenVAS 79 critical CVEs on the same day as the vulnerability details were published. The number of vulnerabilities then drops off across the first week and drops further after 1 week, as we would hope for in an efficient time-to-release scenario.

Figure 10: Absolute numbers of critical CVEs with a remote check release date from the date a CVE is published

While raw numbers are good, Tenable have a larger number of checks available so it could be unfair to go on raw numbers alone. It’s potentially more important to understand the likelihood that OpenVAS or Tenable will release a check of a vulnerability on any given day after a CVE for a critical vulnerability is released. In Figure 11 we can see that Tenable release 61% their checks on or before the date that a CVE is published, and OpenVAS release a shade under 50% of their checks on or before the day that a CVE is published.

Figure 11: Percentage chance of delay for critical vulnerabilities

So, since 2010 Tenable has more frequently released their checks before or on the same day as the CVE details have been published for critical vulnerabilities. While Tenable is leading at this point, Greenbone’s community feed still gets a considerable percentage of their checks out on or before day 0.

I thought I’d go another step further and try and see if I could identify any trend in each organisations release delay, are they getting better year-on-year or are their releases getting later? In Figure 12 I’ve taken the mean delay for critical vulnerabilities per year and plotted them. The mean as a metric is particularly influenced by outliers in a data set, so I expected some wackiness and limited the mean to only checks released 180 days prior to a CVE being published and 31 days after a CVE being published. These seem to me like reasonable limits, as anything greater than 6 months prior to CVE details being released is potentially a quirk of the check details and anything after a 1-month delay is less important for us.

What can we take away from Figure 12?

Figure 12: Release delay year-on-year (lower is better)

With the larger number of checks, and still being able to release a greater percentage of their remote checks for critical vulnerabilities Tenable could win this category. However, the delay time from 2019 and 2020 going to OpenVAS, and the trend lines being so close, I am going to declare this one a tie. It’s a tie.

The takeaway from this is that both vendors are getting their checks out the majority of the time either before the CVE details are published or on the day the details are published. This is overwhelmingly positive for both scanning solutions. Over time both also appear to be releasing remote checks for critical vulnerabilities more quickly.

Written by

Lars Greiwe

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