Pentest vs. Vulnerability Scan: What’s the difference?

Quick Definition: Vulnerability scans are automated processes for finding known weaknesses across systems, while penetration tests are manual attacks that find and exploit weaknesses to gain access to broader networks.

If you're only a few years into your IT career, or even if you are an industry veteran with limited exposure to the security side, you've probably heard the terms' vulnerability scan' and 'penetration test' thrown around. Without the appropriate context, you've probably heard them used pretty interchangeably, as if they are the same set of practices and procedures.

This is far from reality, however: vulnerability scans and penetration tests are similar in principle but very different in execution, time, expense, and purpose. Today we'll break down the differences in these two security practices, how they work, what's involved, and when and why you would choose one over the other (or, spoiler alert, why you possibly should do both).

What is a Vulnerability Scan?

Vulnerability scans are processes that probe applications, networks, and hosts to identify known weaknesses and report on these findings. They don't try to actually hack our servers; a good analogy is walking around a house looking for doors and windows left open or with easily bypassed locks.

How Vulnerability Scans Work

There are several tools that run vulnerability scans. They are generally configured by providing a list of hostnames or IPs and then kicking off a scan. Scans can be set to run on a schedule and also email reports to staff upon completion.

These scanners rely on publicly available lists of known vulnerabilities, known as CVEs (Common Vulnerabilities and Exposures). A CVE will identify a known vulnerability in an application or OS, describing the vulnerability, known affected versions, and summarizing its severity with a rating. Here is an example of the infamous Log4j vulnerability. Scanners look for these vulnerabilities and report the relevant CVEs to you.

Vulnerability Scan Tools

There are a number of tools available for performing vulnerability scans, including:

• Nessus: One of the most commonly used commercial tools, Tenable's Nessus is available in self-hosted and cloud-hosted versions.

• OpenVAS: A popular open-source alternative with a powerful scanning engine and broad community support.

• Qualys: One of many SaaS cloud-based scanners on the market.

When to Use Vulnerability Scans

Vulnerability scans are best suited for:

• Regular Security Assessments: Running vuln scans weekly or even daily is a great best practice for keeping a constant eye on your environments as new CVEs are published daily.

• Compliance Requirements: Most security auditing frameworks mandate regular vulnerability scans.

• Early Detection: If a bad guy has your organization in their sights, you can be sure they are using these same tools against you. Finding and remediating vulnerabilities before they can be exploited is a constant game we must play.

While these scans are important and incredibly useful, they have their shortcomings:

• False Positives: Automated tools aren't perfect; they can misidentify applications, versions, or other factors and report vulnerabilities that don't exist. It's up to you to understand your network and systems so that you can quickly identify false positives.

• Surface-level Scans: Scanners aren't intelligent enough to dig deep and attempt to exploit things like the business logic or workflows within an app. That conveniently introduces our next topic: 

What is a Penetration Test?

Penetration tests (or pentests) are attacks performed by ethical hackers (also known as white hats) who you have hired. They use a mix of automated tools and manual processes to discover and exploit weaknesses in a system. 

Instead of just finding and reporting on vulnerabilities, pentesters (with permission) attempt to actively hack and gain access, move laterally within the network, and discover what further access can be achieved and what data can be compromised. Pentests are used to discover these unknown issues so you can fix them.

How Pentests Work

Pentests are commonly performed as engagements with third-party providers whom you hire to do the pentest on a specific system. Using an objective third-party can help provide independent assurance and attestation of your security.

A pentest engagement begins with a scope that specifies the specific systems, hosts, domain names, etc. that are fair game for testers. For example, a scope could include domain names for the test environment for your SaaS app. Scope is closely tied to your return on investment (too narrow a scope might miss key components) and budget (too large a scope runs up the cost by testing unnecessary components).

Pentesters are typically individuals with a broad and deep range of experience in IT systems, networking, operating systems, and web app development. They must be deeply technical, persistent, creative, and think like a hacker. They typically begin with automated tools to gather data and learn about the system to find potential entry points, like open ports or vulnerabilities. 

In the case of web apps, they will spend lots of time using, navigating, and understanding the app, looking for more potential exploits. If an entry point is found, they will continue to enumerate within the network, finding ways to move laterally, discovering other systems and data, and identifying the means to exploit them, and so on. The end result of a pentest is an extensive report detailing findings, if and how they were able to exploit any part of the system, and recommendations for remediation.

Pentest Tools

The pentest toolset is broad, with a combination of open source tools and custom scripts. Here are some tool highlights:

• Kali Linux: This Linux distribution is preloaded with numerous common penetration testing tools organized into categories such as information gathering, vulnerability scanners, exploitation, password attackers, etc.

• Burp Suite: This tool specifically targets web apps and API, with features like proxying requests, crawling sites, and building automations to find exploits.

• Metasploit: This is a powerful tool for developing and executing exploits. 

When to Use Pentests

Pentests are designed to simulate real-world risk, such as how an actual hacker might attack you. They can be expensive, but they are critical for getting an objective expert opinion about your security. Here's when you might consider investing in one:

• Annual Security Audits: Conducting a yearly penetration test is a best practice to maintain strong security. 

• Compliance Requirements: Different compliance frameworks require or strongly recommend that you produce a recent pentest and evidence that any findings were remediated.

• Expansion: If your company is a growing SaaS targeting larger or enterprise customers, expect to be asked for a pentest report sooner or later. These customers are trusting you with their data and expect your app to be secure. 

Limitations 

Pentests have their drawbacks:

• Costly: With the expertise and amount of manual labor required, pentests can get expensive.

• Time-Consuming: An engagement is typically one or two weeks. Add time on the front end to meet with the testers to define your scope and time after to review and remediate findings.

• Not Continuous: Compared to daily vulnerability scans, a pentest is only a snapshot in time of your security.

Which One Does Your Organization Need?

Like most things in IT, this question depends on your circumstances. Do you need either for compliance reasons? Do you sell SaaS software? Do you host publicly available sites? Do you host customer data or sensitive internal data? These questions might help decide whether you need an expensive pentest or not. 

This table outlines the core differences you'll need to be familiar with before making a decision. 

As a rule of thumb, if you maintain any servers in your org, there's no reason not to scan them (at a minimum) weekly with a good open source vulnerability scanner. Vulnerabilities on public-facing servers are low-hanging fruit for bad actors. Make their job a little harder by finding and patching those vulnerabilities!

Want to Learn Pentesting?

Vuln scans and pentests are both essential tools that fit specific needs. They both answer questions around how secure your systems are and what weaknesses might need to be fixed. Understanding what requirements your industry or business has around compliance is the first step in determining what type of testing your organization needs. Ultimately, both pentesting and vulnerability scans are a part of providing a constantly improving security posture for your organization.

Maybe while discussing pentesting, you thought, "People get paid to do that?!" Yes, legal ethical hacking is a great career path. It requires lots of experience as mentioned, and very specific training in attacking systems. If you want to explore this path, consider courses like the CompTIA PenTest+ or OSCP. We've covered preparing and passing the OSCP on the blog previously, so these are a great starting place.

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