What Is a Vulnerability Scan? Complete Overview

Think of yourself owning a house. You have a sturdy front door with a fairly strong lock and are feeling quite safe with them. But one day, you get to know from a friend that you’ve left a window open in your basement for several months now. Either you were unaware of it being there or you simply forgot about it.

And this is precisely what a vulnerability scan is. This is the process of going through your IT environment, including the network, servers, and applications, and finding out which of the windows and doors are left open.

Why This Matters Right Now

Consider the following statistics. In 2023 alone, over 29,000 new vulnerabilities have been reported in the CVE (Common Vulnerabilities and Exposures) database. That’s more than 80 new weaknesses per day. What’s even scarier? A whopping 60% of all data breaches can be linked to vulnerabilities that had a patch developed and available before but weren’t applied.

It means that breaches aren’t usually caused by something new and groundbreaking. Most of them happen because of exploiting well-known vulnerabilities that were not addressed on time.

Which leads us to the conclusion that if you operate a digital environment, whether you’re a freelance developer with a project, a growing startup, or an enterprise of some kind, you should understand how vulnerable you are. Vulnerability assessment is the primary step towards obtaining such information. It’s not an optional measure, but rather the cornerstone of any reasonable cybersecurity strategy. The need for vulnerability assessments is dictated by compliance requirements of such regulations as PCI DSS, HIPAA, ISO 27001, etc.

The Basics: How a Vulnerability Scan Works

To put it simply, a vulnerability scan is basically an automated procedure that looks for security weaknesses in computer systems, networks, and software applications. Here is how it is done:

Probes and Detects: The scanning software connects to your computer systems and scans for things like installed software versions, open ports, and configuration settings.
Compares Against a Database: Whatever is found by the scanning process is compared to extensive databases that keep on growing with information about existing vulnerabilities. There are two major databases: CVE (Common Vulnerabilities and Exposures), which gives naming for all publicly disclosed vulnerabilities, and the NVD (National Vulnerability Database), which provides additional information about the CVEs.
Prepares a Report: Based on the results of the scan, a report is generated, listing all the vulnerabilities that have been discovered and giving a severity rating based on the CVSS (Common Vulnerability Scoring System).

Vulnerability Scan Versus Penetration Test: A Vital Difference

Let me give you an example of something that confuses people all the time. When it comes to distinguishing between vulnerability scans and penetration testing, people have trouble telling the difference. Yet it is very important.

Features	Vulnerability Scan	Penetration Test
What it does	Automatically detects and documents known vulnerabilities	Uses vulnerabilities to demonstrate their existence and what an attacker could do
Method	Automated, wide, and rapid	Manual, focused, and intensive.
Analogy	It informs you that the basement window is unlocked.	It attempts to unlock the window and then walk into it.
Pros	Cost-effective, fast, and can be repeated	Very accurate (fewer false positives), uncovers intricate attack vectors, discovers logic flaws
Cons	Could give false positives, doesn’t exploit, so cannot prove that a vulnerability is truly exploitable	More costly, slower, offers a snapshot of the present and needs special expertise

The Takeway: You require both types of testing. Vulnerability scans are just like your regular health check-ups, where they identify the most obvious problems at a relatively low cost. Penetration testing is like your specialist test, where you look for the more complicated issues that cannot be detected by a simple vulnerability scan.

The Various Types of Vulnerability Scans

No, not all scans are made the same. It varies depending on what you want to look at, and your point of view. Below are some of the common scans you will need.

By Access: Authenticated or Unauthenticated Scan

One of the major things you should be aware of.

Authenticated Scans (Credentialed Scans)

It gets its login credentials (such as username and password). It would help if you had a security guard walk around inside your premises to get a clear picture. Due to having inside access, it is able to find something that a third party can’t, such as missing patches to your internal server, and outdated software libraries to your custom application.

Unauthorized Scans (Non-Authorized):

The scanner does not have any special permissions; it examines your systems externally, just like an attacker. This is great for analyzing the external attack surface; in other words, what someone can see from the public internet. It is a “think like an attacker” point of view.

By Location: Internal vs. External

External Scans: These scans are done externally from your network and show you what is visible on the Internet. This is the approach you would take for compliance with regulations such as PCI DSS. This scan will reveal to you all the vulnerabilities that could be seen by anyone out there on the Internet.

Internal Scans: These are performed internally and will show you all that is vulnerable should an attacker breach into your network or should you have malicious users working for you.

By Target: Different Instruments For Different Purposes

For various security purposes, specific scanners are used:

Network Scans: The most popular kind. Scan for open ports, vulnerable services, and firewall holes.
Web Application Scans (Dynamic Application Security Testing): Such scanners analyze working web applications and find typical coding vulnerabilities such as SQL injection, cross-site scripting (XSS), and broken authentication.
Container Scans: Due to the increasing use of containerization of software (such as with Docker), scanners now can detect vulnerabilities in the base image of the container, software libraries, and configuration files.
Cloud Scans: This type of scanner analyzes cloud environment security (AWS, Azure, GCP) and searches for vulnerabilities in storage bucket configurations (for example, S3), IAM (Identity and Access Management) roles, and security group configurations.

The Process for Vulnerability Scans: A Step-By-Step Guide

This isn’t a “click the button and get the report” procedure. There are a number of steps to be followed when conducting a mature vulnerability scanning program.

1. Scope and Asset Identification

You cannot scan something you do not know about. The first thing is to know all the devices, applications, and systems that might pose a threat. This means both the obvious ones (the servers and the workstations) and the hidden ones (a staging server here, a forgotten database instance there, and an unmanaged SaaS environment elsewhere).

2. Selecting and Configuring the Scanner

This will depend on the scope of your testing and what tools will best suit it. It entails choosing which scan you would use (authenticated or unauthenticated) and how to configure the scan. Important configurations include:

Rate Limiting: To ensure the scanner does not affect the performance of your network.
Maintenance Windows: To schedule the scans at a time when it is unlikely to disrupt user experience.
Sensitive System Exclusions: In case of very old and delicate systems, you may configure the scanner to skip them or perform a “safe check.”

3. Discovery and Fingerprinting

First of all, the scanner identifies active hosts, open ports, and running services. Essentially, it involves the scanner “looking” around, creating its impression or “fingerprint” of the asset.

4. Detection Stage

Time to get down to business. The scanner matches the fingerprints collected from services with the intelligence it has about vulnerabilities from its databases (CVE, NVD). It does scanning for known unpatched vulnerabilities, weak configurations, old protocols, and common misconfigurations.

5. Scoring and Prioritization

This is where the real wizardry happens. A good modern scanner will do much more than simply listing things for you. They would use the CVSS v4.0 standard to give a baseline rating for the severity of a vulnerability. However, good scanners do even better.

They will enhance the score with:

EPSS (Exploit Prediction Scoring System): This is an estimate of the probability of a vulnerability being exploited in the real world. A vulnerability that has a high CVSS and a low EPSS is less critical than a vulnerability that is medium and has a high EPSS .
CISA KEV (Known Exploited Vulnerabilities): This is a government list of vulnerabilities that have been exploited by the adversary. If a vulnerability appears in this list, it takes precedence over everything else, regardless of the CVSS score assigned to it theoretically.

6. Reporting and Remedy

The scan will provide you with a report, but that’s not all. It normalizes the information provided, removes duplicates (when an issue is found on multiple systems), and can even create tickets within your project management tool (such as Jira) for your IT and DevOps teams to handle.

7. Verification and Trending

The last thing you need to do is repeat the scan once you have applied your fixes. This will verify whether your remediation was effective. At the same time, you’ll be able to track how your program performs in terms of MTTD and MTTR.

Common Issues With Vulnerability Scanning

Vulnerability scanning is an effective practice; however, one must acknowledge its pitfalls too. Here is what to expect.

False Positives: Automated scanners are far from infallible. They may identify a non-existent vulnerability, leading to a false positive alert. Manual evaluation of results can take up the majority of your time if done on a large scale. Do not simply accept the results at face value; verify the most relevant findings.

False Negatives: Much worse than false positives are false negative vulnerabilities that exist yet are overlooked by the scanner. This is possible because of a number of reasons, including the absence of necessary database entry in the scanner, the presence of custom software code that the scanner cannot analyze, or the blocking of the scan by the network device, such as IPS. Authenticated scans are more reliable because of having access to the system and hence not being affected by firewalls and other network protection tools.

Potential for Systems to Be Brought Down: There are some aggressive types of scans that will be able to bring down the system or significantly slow it down during the process. This happens particularly to legacy systems. Test the scans in a non-production environment first and use safe checks and rate limiters on production systems.

Too Much Information: One single scan conducted throughout a network may generate tens of thousands of vulnerabilities. This is a typical situation when one is trying to drink from a fire hose. If you try to solve everything, then you will solve nothing. The system should have some method of prioritizing. In this situation the most effective strategy would be:

Exploitability: Is there an exploit?
Exposure: Internet-Facing?
Asset Criticality: Does it contain sensitive data or perform an important business function?

More Insights into Selecting the Right Tools

There are countless vulnerability scanners out there, from free and open-source software to costly enterprise solutions. It all boils down to what suits you best in terms of budget, technical expertise, and security needs.

Open Source Alternatives (“DIY Method”):

Nmap: The ultimate network reconnaissance and host enumeration tool. It is not a vulnerability scanner by default but can be used as the first step in any scan flow.
OpenVAS (Open Vulnerability Assessment System): A full-fledged vulnerability scanner that represents the open source alternative to Nessus and similar commercial solutions. OpenVAS is free software but requires more effort to learn how to use it .
OWASP ZAP (Zed Attack Proxy): An open-source web application security scanner. One of the most popular and highly appreciated scanners for web developers to test their applications for vulnerabilities.
Sqlmap: A dedicated open-source vulnerability scanner that automates the SQL injection vulnerability testing process. This is another example of a tool focused on just one particular type of vulnerability.
Burp Suite Community Edition: An open-source web application security testing suite aimed at security researchers and developers who need to test their web applications manually. It includes the functionality to intercept and modify HTTP traffic .

Commercial Platforms (“The Enterprise Way”):

Nessus (by Tenable): Industry-standard tool. A robust and versatile vulnerability scanner with an extensive plugin library.
VM (Vulnerability Management) by Qualys: Popular cloud-based solution that provides vulnerability scanning along with asset discovery and compliance reporting.
Nexpose (by Rapid7): Yet another popular commercial product, highly compatible with the Metasploit penetration testing framework.

Further Consideration: Turning Scans into Action

Vulnerability scans are merely data. Vulnerability assessments are the process through which those findings become actionable. Here’s how to ensure the process works for you.

Criticality Verification: For each high-severity finding, consult your asset list. What’s the impact of this system being taken offline? How sensitive is the data stored by this system? A finding related to a high-profile system that powers a customer-facing revenue stream is more critical than the exact same finding on an isolated test server.
Compensating Control Review: Is there any way to mitigate the threat posed by a vulnerability without patching the system? For instance, if you have an outdated system with a vulnerability that you cannot patch, can it be isolated from the rest of the network? Can a Web Application Firewall (WAF) block the attack? .
Ownership and Timeframes: Each finding should have an assigned owner a person in charge of solving the problem and a deadline for solving it. Security issues that get listed in a PDF report are never addressed. Issues that go into the standard project queue are solved.
Re-scan: Solving the problem and marking the ticket as “done” is not enough. You need to re-scan the system and verify whether the issue has been solved.

Practical Takeaways and Next Steps

Now that we’ve covered the basics, how should a beginner get started?

Find Free/OSS Tools: If there’s no budget for it, start immediately. Download Nmap for network scanning and either OpenVAS or OWASP ZAP for vulnerability assessments. While they may lack the slick interface that comes with a paid tool, they provide tremendous value.
Conduct Your First External Scan Without Authentication: Think like an attacker for a moment – what does your company look like on the internet? You could even start with free online tools such as securityheaders.com and assess your site configuration.
Map out the Environment: Using the results from your scan, compile a complete list of assets. You cannot secure what you do not know. Document every server, website, and cloud environment. Include information about its purpose and owner.
Focus on “Low-Hanging Fruit”: Your first scan will be overwhelming to say the least. Prioritize your findings based on critical findings. Consider:
- Known Exploited Vulnerabilities (KEV): Verify whether you have identified any KEVs from the list maintained by CISA.
- Internet-Facing Systems: Prioritize systems which are accessible through the public internet.
- Default Credentials: This is a very frequent issue to overlook.
Set a Cadence: Ensure that scanning becomes an integral part of your processes. A weekly or monthly scanning process is ideal for most companies to begin with. What matters is being consistent.

Resources and Continuing Education

CISA Known Exploited Vulnerabilities Catalog: It is your one-stop shop for what is currently being exploited. Refer to it regularly.
NIST National Vulnerability Database (NVD): The leading national vulnerability database maintained by the US government. All your research needs can be fulfilled here.
OWASP: An organization offering incredible free educational material for secure coding of web applications. OWASP Top Ten is a great place to begin.
Tools: For open-source tools, check out Nmap and OWASP ZAP. For commercial tools, if you can afford it, Tenable Nessus is your best bet.

Conclusion

The vulnerability scans serve as the cornerstone of your security policy. In essence, the vulnerability scans are the tool by which you answer the question, “What am I vulnerable to, and where should I begin to address those vulnerabilities?” You won’t become impenetrable. But you’ll lock up well over 90% of all entry points being used against you.

The key here is to just get started. Perform the scan. Review the results. Fix something. Scan again. This is how you build your security program. Be inquisitive, be aware, and be learning.

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