External vs Internal Penetration Testing – How to Choose With a Limited Budget

When planning a penetration test, one of the most common questions organizations face is whether to choose between external vs internal penetration testing. This post will help you understand the key differences between the two approaches and how to choose the one that provides the most value to your organization. While the obvious answer might be, “Why not both?”, real-world constraints, especially budget, often force organizations to prioritize. Knowing what each test offers and how it aligns with your risk profile is essential to making an informed decision.

Comparing the Two Assessments – External vs. Internal

An external penetration test is designed to test the security of your organization’s Internet perimeter. A penetration tester will emulate an external attacker attempting to gather sensitive information, gain unauthorized access to Internet-accessible applications, or breach your internal network. For most organizations, an external attacker is one of the most significant threat vectors they face, and an external penetration test can help evaluate vulnerabilities and the subsequent risk associated with them.

On the other hand, an internal penetration test focuses on the security controls inside your network. In this type of assessment, the tester simulates an attacker who either has insider access or has already breached the perimeter, perhaps through phishing, social engineering, or an unpatched vulnerability. While often overlooked, this scenario can be one of the most damaging, as it assumes the attacker is already past your outer defenses and is now testing the resilience of your internal systems, segmentation, and response capabilities.

Let’s look at the important factors when comparing which of these assessments may be right for you:

Cost

The almighty dollar is usually a significant driver for organizations that are comparing and trying to decide between an external vs internal penetration test. If money weren’t a factor, you should be doing both on at least an annual basis. But that’s not realistic for a lot of organizations. An external penetration test is scoped based on the number of live hosts on your perimeter (an IP address with at least one open port accepting inbound services), so it can range anywhere from $5,000 – $15,000 in most cases. Similarly, an internal penetration test is also scoped based on the number of live hosts/servers/devices on your internal network. This type of assessment can range from roughly $8,000 – $20,000. As you can see, internal penetration testing is usually a more expensive option, given the significant increase in attack surface and the different methodology it requires.

It’s important to note that you shouldn’t stop reading here, though. While cost will most certainly play a role in the decision between an external vs internal penetration test, it should not be the sole basis of a decision. The value and return on investment you see from these two assessments may make the increased cost of an internal penetration test much more appealing, for example.

Expected Value

While both external and internal penetration tests offer valuable insights, the right choice depends on what you’re trying to learn from the assessment. If your organization has never gone through a penetration test before (a vulnerability scan does not count), starting with an external test is usually a smart move. It helps uncover any obvious, high-risk issues that could be exploited from the outside—and gives you some peace of mind that you’re not the easiest target on the internet. On the other hand, if your IT or security resources are stretched thin or your overall program is still maturing, jumping into an internal test might not be the best use of time or money, especially if you’re not ready to act on the findings.

Now, if you are an organization that has had an external penetration test performed for the past 5 years and you’ve got a good handle on what’s on your perimeter, it may be time to branch out and start looking at the internal network. Similarly, if your company doesn’t have any live hosts on the perimeter that accept inbound services and your only window to the Internet is the NATed traffic leaving your firewall, it may not make much sense to have an external penetration test performed. The internal network is the next big step in the maturity of your security program, and an internal penetration test can provide you with much-needed direction for that security roadmap. In an age where social engineering is more prevalent and more sophisticated than ever, you have to consider the threat of an attacker who is already on the network. If you’re having trouble convincing executive leadership of this to get funding for an internal penetration test, maybe a social engineering engagement can help highlight this risk a little more and show just how easy it is to get a foothold on a network.

In any case, there are often a lot of factors that go into choosing between an external vs internal penetration test. We’re really only scraping the tip of the iceberg here by discussing cost and value, which are probably the most significant factors, but definitely not the only factors. Compliance, for example, may be another driving force here. Depending on the compliance standards you need to follow, they may encourage or require you to have both of these assessments performed on an annual basis. If you’re having trouble deciding between the two, please give us a call, and we’ll go over your specific case to help make the right decision for your organization.