Vulnerability Discovery Fundamentals

Vulnerability scanning is a core part of cybersecurity assessments. Whether you're testing a small network or a large enterprise environment, scanners help identify weaknesses before attackers find them. This article breaks down how vulnerability scanners work, the types of scans you can perform, and the key considerations to keep in mind for accurate results.

While every scanner has its own features, the underlying process is generally the same. An automated vulnerability scan usually follows four main steps:

1. Host Discovery

The scanner first determines which systems are online and reachable. If a host doesn’t respond, the scanner marks it as inactive.

2. Port Scanning

Next, the scanner looks for open ports. Open ports reveal which services are exposed and potentially exploitable.

3. OS, Service, and Version Detection

The scanner identifies the operating system and enumerates running services along with their version numbers. This information is crucial for matching potential vulnerabilities.

4. Matching Results to a Vulnerability Database

The gathered data is compared against known vulnerability repositories such as the National Vulnerability Database (NVD) or CVE listings. Some commercial tools even attempt partial exploitation to confirm whether a vulnerability is real—though this can sometimes affect system stability.

Each identified vulnerability typically has a CVE ID, a standardized identifier used across the industry. However, a CVE doesn’t tell you how dangerous the issue is. That’s where the Common Vulnerability Scoring System (CVSS) comes in.

CVSS scores range from 0 to 10 and categorize severity levels such as:

• Low
• Medium
• High
• Critical

Modern scoring follows CVSS v3.1, which improves clarity and accuracy when rating risk.

Vulnerability scans are not perfect.

• False positives occur when a scanner flags an issue that isn’t actually exploitable. This may happen due to incorrect version detection or patches that were backported into older software versions.
• False negatives happen when the scanner completely misses an existing vulnerability. This often occurs with complex issues or misconfigured scans.

A balanced approach—combining automated tools with manual review—helps minimize both problems.

Manual Scanning

• Time-consuming
• Excellent for detecting logical or complex vulnerabilities
• Useful when evaluating unusual or custom systems

Automated Scanning

• Fast and scalable
• Ideal for large networks
• Quickly highlights easily detectable vulnerabilities
• Essential when working under time constraints

A penetration tester typically uses both methods to get the most accurate picture of a system’s security posture.

Vulnerability scans are usually categorized based on location and access level.

External Scans

Performed from outside the organization’s network—similar to how an attacker on the internet would view the system. Targets often include:

• Public-facing servers
• Web applications
• DMZ assets

Sometimes clients provide the list of IP addresses, but in other cases, the tester must identify exposed systems themselves.

Internal Scans

These scans occur inside the organization’s network, either on-site or through VPN access. They help identify the weaknesses an attacker might exploit after breaching the perimeter.

Unauthenticated Scans

The scanner evaluates a target without logging in. This reveals:

• Exposed services
• Open ports
• Publicly accessible weaknesses

But it cannot identify local security flaws, such as missing patches or insecure configurations.

Authenticated Scans

The scanner logs into the system using valid credentials. This provides deep visibility into:

• Installed software
• Missing security updates
• Weak configurations

Authenticated scans are much more comprehensive and are standard in most professional assessments.

1. Scanning Duration

Large scans take time—especially external scans where network latency and routing need to be considered.

2. Target Accessibility

Before scanning, verify that:

• Firewalls allow access
• VPN routes are correctly configured
• Internal segmentation won’t block or distort scan traffic

Network devices like routers, firewalls, or IPS systems may filter or drop scanning probes, leading to incomplete results.

3. Rate Limiting

Some environments throttle high-volume scanning traffic. When rate limits activate, scanners may:

• Miss live hosts
• Fail to detect running services
• Produce incomplete results

Most scanners allow you to adjust timing, delays, and parallelization to counter this.

4. Network and System Impact

Vulnerability scanning is noisy. It can:

• Generate heavy traffic
• Slow down networks
• Cause unstable services
• In rare cases, crash fragile systems

Always coordinate with the client and adjust scan intensity to avoid disruptions.

Vulnerability scanning is a powerful tool—but it’s not magic. Automated scanners excel at broad coverage, while human judgment helps validate results and uncover deeper issues. By understanding how scanners work, choosing the right type of scan, and planning carefully, you can deliver accurate, meaningful security assessments.