External Vulnerability Scanner — What It Tells You and How to Read It

An external vulnerability scanner identifies security weaknesses on internet-facing assets. It probes public IP addresses, domain names, and web services from an outside perspective. Use it for perimeter defense, compliance requirements, and proactive security checks. This tool helps discover open ports, misconfigurations, outdated software, and known vulnerabilities exposed to the internet.

What this tool/technique does and when to use it

An external vulnerability scanner simulates an attacker's view of your organization's internet-facing infrastructure. It uncovers flaws accessible from the public internet. Security teams use these scanners to audit websites, mail servers, VPN endpoints, and other publicly exposed services. Perform scans regularly to maintain a strong security posture. Deploy them before new services go live. Use them to validate remediation efforts after a security incident.

Step-by-step usage with real commands and output examples

Begin with reconnaissance. Identify your target's public-facing assets. Nmap is a versatile tool for initial port scanning and service detection.

Initial Reconnaissance with Nmap

Run a SYN scan to quickly find open ports. Add service version detection to identify running software. This helps narrow down potential vulnerabilities.

nmap -sS -sV -p 1-1000 example.com

This command performs a TCP SYN scan (`-sS`) and attempts to determine service versions (`-sV`) on the first 1000 common ports (`-p 1-1000`) for `example.com`. Replace `example.com` with your target domain or IP. Example output:

Starting Nmap 7.92 ( https://nmap.org ) at 2023-10-27 10:00 EDT
Nmap scan report for example.com (93.184.216.34)
Host is up (0.015s latency).
rDNS record for 93.184.216.34: example.com

PORT    STATE  SERVICE VERSION
22/tcp  closed ssh
80/tcp  open   http    nginx
443/tcp open   ssl/http nginx
5000/tcp closed upnp

Service detection performed. Please report any incorrect results at https://nmap.org/submit/ .
Nmap done: 1 IP address (1 host up) scanned in 0.56 seconds

The output shows open ports 80 and 443, both running Nginx. This information directs further scanning efforts. For a quick, browser-based port scan, Secably offers a free port scanner.

Web Server Scanning with Nikto

Nikto focuses on web server vulnerabilities. It checks for outdated server software, dangerous files, misconfigurations, and known issues.

nikto -h https://example.com

This command scans the web server at `https://example.com`. Example output:

- Nikto v2.1.6
---------------------------------------------------------------------------
+ Target IP:          93.184.216.34
+ Target Host:        example.com
+ Target Port:        443
+ Start Time:         2023-10-27 10:05:00 (GMT-4)
---------------------------------------------------------------------------
+ Server: nginx
+ The anti-clickjacking X-Frame-Options header is not present.
+ The X-XSS-Protection header is not defined. This header can prevent some browser XSS attacks.
+ No CGI Directories found, skipping CGI checks.
+ "robots.txt" found: /robots.txt
+ Allowed HTTP Methods: GET, HEAD, POST, OPTIONS
+ OSVDB-3092: /admin/: This might be an interesting directory.
+ OSVDB-3233: /icons/: This directory may contain sensitive information.
+ OSVDB-3268: /manual/: Apache manual found.
+ OSVDB-3092: /info.php: PHP info file found.
+ 0 items checked: 0 error(s) and 1 warning(s)
+ End Time:           2023-10-27 10:05:15 (GMT-4) (15 seconds)
---------------------------------------------------------------------------

Nikto identified missing security headers like X-Frame-Options and X-XSS-Protection. It also found potentially interesting directories. Secably provides a free website vulnerability scanner and an HTTP security headers checker for similar web-focused assessments. Our CMS vulnerability scanner helps identify issues in common content management systems.

Comprehensive Scanning with OpenVAS (Greenbone Community Edition)

OpenVAS, now Greenbone Community Edition, offers a more extensive vulnerability scan. It includes a vast database of vulnerability tests (NVTs). Installation and setup are more involved, typically requiring a dedicated virtual machine or Docker container. 1. Install Greenbone Community Edition: Follow official documentation for your preferred deployment method (e.g., Docker, virtual appliance). 2. Access the Web Interface: Navigate to the Greenbone Security Assistant (GSA) web interface, usually `https://localhost:9392` or your VM's IP. 3. Log In: Use your administrative credentials. 4. Create a New Target:

• Go to `Configuration > Targets`.
• Click the "New Target" icon.
• Enter the IP address or hostname of your target (e.g., `example.com`).
• Save the target.

5. Create a New Scan Task:

• Go to `Scans > Tasks`.
• Click the "New Task" icon.
• Give the task a name (e.g., "Example.com External Scan").
• Select the target created in the previous step.
• Choose a scan configuration, typically "Full and fast ultimate" for comprehensive external scanning.
• Click "Create".

6. Start the Scan:

• Select the newly created task.
• Click the "Start" button (play icon).

The scan runs in the background. It can take minutes to hours depending on target size and scan configuration. Once complete, click on the task to view the results. Example partial scan result (snippet from GSA report):

Vulnerability: SSL/TLS Server Supports Weak Cipher Suites
  Severity: High (7.5)
  CVE: CVE-2015-2808
  Solution: Configure the server to reject weak cipher suites.

Vulnerability: Web Server Directory Listing Enabled
  Severity: Medium (5.0)
  Solution: Disable directory listing for all web directories.

Vulnerability: Outdated Nginx Version Detected
  Severity: Medium (6.0)
  CVE: CVE-2021-23017
  Solution: Upgrade Nginx to the latest stable version.

Vulnerability: HTTP Security Headers Missing or Misconfigured
  Severity: Low (3.0)
  Solution: Implement X-Frame-Options, X-XSS-Protection, Content-Security-Policy.

This output provides detailed vulnerability information, including severity, CVE references, and specific remediation advice. Commercial tools like Nessus offer similar capabilities with enhanced reporting and support.

Common use cases with practical scenarios

External vulnerability scanning serves several critical functions in a security program.

Perimeter Security Audits

Regularly scan your organization's public IP ranges and domain names. Identify new services or misconfigurations exposed to the internet. For instance, a development team might accidentally deploy a test server with an open database port. An external scan quickly flags this exposure. This proactive approach helps manage your attack surface effectively.

Compliance Requirements

Many regulatory frameworks mandate external vulnerability scans. PCI DSS requires quarterly external scans by an Approved Scanning Vendor (ASV). HIPAA and ISO 27001 also recommend regular assessments. Use scan reports to demonstrate compliance. Document identified vulnerabilities and their remediation.

Mergers and Acquisitions Due Diligence

Before acquiring a company, assess its external attack surface. Perform an external vulnerability scan on the target company's public assets. Discover hidden risks or critical vulnerabilities that could impact your integration plan. This helps renegotiate terms or plan for immediate remediation post-acquisition.

Third-Party Vendor Assessment

When integrating with a third-party application or service, scan their public-facing components if contractual agreements permit. Understand their security posture before connecting your systems. For example, scan a vendor's API gateway for known vulnerabilities. This mitigates supply chain risks.

Post-Deployment Verification

After deploying a new application or service to production, conduct an external vulnerability scan. Verify that all security configurations are correct. Ensure no unintentional ports are open. Confirm no default credentials remain exposed. This is a final security check before full public access.

External Attack Surface Management (EASM)

External vulnerability scanning forms a core component of EASM. It involves continuous discovery and assessment of all internet-facing assets. Tools like OpenVAS integrate with asset management systems. This provides ongoing visibility into your external posture. External Attack Surface Management Explained for Security Practitioners details this concept. Secably offers an EASM platform, which includes continuous external scanning, as part of its pricing plans.

Troubleshooting common issues

Scanners can encounter various problems during operation. Understanding these issues helps streamline your scanning process.

Firewall Blocking Scan Traffic

Symptoms include incomplete scans or no open ports reported. Your target's firewall or an intermediate network device might block the scanner's IP.

• Solution: Check firewall logs on the target network. Whitelist the scanner's IP address. Ensure the scanner can reach the target on the intended ports. Test connectivity with `ping` and `telnet` first.

Rate Limiting or IP Blocking

Web Application Firewalls (WAFs) or intrusion prevention systems (IPS) can detect and block aggressive scanning attempts. The scanner's IP might get temporarily or permanently blocked.

• Solution: Configure the scanner for slower scan speeds. Use a distributed scanning approach with multiple source IPs. Consider using a proxy service, though this adds complexity. Some commercial scanners have built-in evasion techniques.

False Positives

Scanners sometimes report vulnerabilities that do not exist. This happens due to signature mismatches or incomplete service information.

• Solution: Manually verify each reported vulnerability. Use other tools or perform manual penetration testing steps. Tune scan policies to exclude specific checks known to cause false positives. Report false positives to the scanner vendor.

Incomplete Scans or Timeouts

Scans may stop prematurely or report "host unreachable" even when the host is online. This could stem from network instability, high latency, or scanner timeouts.

• Solution: Review scanner logs for specific error messages. Increase scan timeouts in the scanner configuration. Ensure stable network connectivity between the scanner and the target. Break down large scan targets into smaller segments.

High Resource Usage on Scanner Host

Running comprehensive scans can consume significant CPU, memory, and disk I/O on the scanner host. This impacts other services running on the same machine.

• Solution: Run scans during off-peak hours. Dedicate a virtual machine or physical server for scanning. Allocate sufficient resources (CPU, RAM) to the scanner. Distribute scans across multiple scanner nodes for large environments.

Pro tips for advanced usage

Maximize the effectiveness of your external vulnerability scanner with these advanced techniques.

Authenticated Scans (Internal Distinction)

While focusing on external scanning, understand that some external services require authentication. For example, a public-facing web application might have an admin login. Providing credentials to the scanner allows it to find vulnerabilities behind the login form. This provides a deeper assessment of the application's security. It simulates an attacker who has gained initial access.

Custom Scan Policies

Default scan policies are broad. Create custom policies to target specific vulnerabilities or services. For instance, if you only care about web application vulnerabilities, create a policy that focuses solely on HTTP/S checks. This reduces scan time and noise. It also allows for compliance-specific checks, like PCI DSS requirements.

Integration with CI/CD Pipelines

Automate external vulnerability scans for new deployments or significant code changes. Integrate the scanner into your Continuous Integration/Continuous Delivery (CI/CD) pipeline. Trigger a scan automatically after a successful build. This catches new exposures or regressions before they reach production. Fail the pipeline if critical vulnerabilities are found.

API Automation and Reporting

Many commercial external vulnerability scanners offer APIs. Use these APIs to automate tasks. Script scan initiation, retrieve results, and integrate with ticketing systems. Generate custom reports for stakeholders. This reduces manual effort and improves response times.

Baseline Management and Delta Reporting

Establish a security baseline for your external assets. Run a comprehensive scan and document all findings. Subsequent scans compare against this baseline. Focus on new vulnerabilities or changes in security posture. This "delta reporting" highlights immediate areas of concern. It helps track remediation progress over time.

Combining Tools for Deeper Insights

No single tool finds everything. Combine the strengths of different scanners. Use Secably's subdomain discovery tool to identify all public-facing subdomains. Then use Nmap for initial port and service discovery. Follow up with Nikto for web-specific checks. Finally, run a comprehensive scan with OpenVAS or Nessus. This multi-layered approach provides a more complete picture. Also, check SSL/TLS configurations with Secably's SSL/TLS certificate checker.

Continuous Monitoring

Schedule recurring scans. Set up daily, weekly, or monthly scans for critical assets. This ensures continuous monitoring of your external attack surface. New vulnerabilities emerge constantly. New configurations happen frequently. Continuous scanning helps detect these changes quickly. This aligns with modern attack surface management practices.