External penetration testing is a critical cybersecurity practice that helps organisations defend their internet-facing assets. By simulating the actions of a real-world attacker, external penetration tests reveal vulnerabilities in your web applications, networks, and other externally accessible systems.

This post delves into the methodology, popular tools, associated costs, and step-by-step procedures of external penetration testing. Discover how to protect your critical data and maintain a strong security posture.

💡This Guide is part of our extenstive guide on penetration testing.

What is External Penetration Testing?

External penetration testing, also known as external network penetration testing, is a cybersecurity assessment that simulates the actions of a malicious actor attempting to breach an organisation’s defences from the outside. This controlled test identifies and exploits vulnerabilities in internet-facing systems, networks, web applications, and other externally accessible assets.

External penetration tests often fall into the following categories:

• Black Box: The penetration tester has no prior knowledge of the target systems, mirroring a real-world attack scenario.
• White Box: The tester receives extensive knowledge of the target environment, enabling a more focused assessment.
• Grey Box: Testers possess some limited knowledge, offering a balance between realism and efficiency.

Targets of External Pentesting

• Web Applications: Common targets due to their frequent use of sensitive data and exposure to the web.
• Networks: Firewalls, routers, and other network devices are tested for misconfigurations and vulnerabilities.
• Wireless Networks: Assessment of security protocols, encryption, and unauthorised access points.
• Cloud Infrastructure: Security testing of cloud-based systems and services.
• Mobile Applications: Assessment of vulnerabilities on mobile platforms.

Why is External Penetration Testing Important?

There are several compelling reasons why external pen testing makes a compelling business case:

• Protects Sensitive Data: External pentesting helps identify vulnerabilities that could lead to data breaches, safeguarding confidential customer and business information.
• Compliance: Regulations such as PCI-DSS and GDPR often mandate regular penetration testing to ensure adherence to security standards.
• Mitigates Risk: By proactively identifying and addressing vulnerabilities, organisations reduce the likelihood and impact of successful cyberattacks.
• Protects Brand Reputation: A significant data breach can damage a company’s reputation. External penetration testing helps minimise that risk.

External vs. Internal Penetration Testing

External penetration tests focus on the perimeter defences, while internal tests assume a threat actor is already within the network. Both play vital roles in a comprehensive security strategy. 

Internal penetration tests cover various asset categories such as active directory, servers, workstations/desktops, segmentations, network devices and peripherals. A common myth that must be answered relates to measuring the attack surface of Internet-facing assets only. This is untrue. External penetration testing can only measure Internet-facing asset threats and associated risks. Internal threat cases are deployed to identify vulnerabilities using internal penetration testing in corporate environments, active directories, and network devices.

How long does external penetration testing take?

As a general guideline, expect the following:

• Small-scale Tests: A few days to a week is often sufficient for limited-scope engagements.
• Medium-scale Tests: Most common duration, typically spanning one to two weeks.
• Complex Environments: Extensive tests in large or highly complex environments may take several weeks or longer.

Cost of an external penetration test

An external penetration test cost is based on the number of assets to be assessed; for a small to medium-sized company, an external test costs anywhere between £2500 and £5000 for external penetration testing. For a large business, it’s customised pricing based on a few factors, i.e. assets, frequency, and related scope factors.

The effort and cost estimation for an external penetration test is highly dependent on the complexity and scope of the engagement. Here’s a more granular look at the factors influencing the overall duration:

• Number of Assets: More IP addresses, web applications, and other target systems naturally demand a longer testing timeframe.
• Depth of Assessment: A comprehensive assessment covering deep exploitation and post-exploitation phases will require more time than a basic vulnerability scan.
• Tester Experience: Seasoned penetration testers may work more efficiently due to their honed skills and knowledge.

When to Perform an external penetration test?

An external pen test must be performed annually or after significant changes.

• After Major Changes: Anytime there’s a significant infrastructure update, new application deployment, or major configuration change, consider a test.
• Frequency: At a minimum, external penetration tests should be conducted annually. Regulations like PCI-DSS or specific industry standards may mandate more frequent testing.

External Network Penetration Testing Methodology

Our external network penetration testing methodology includes a detailed approach with multiple steps in each stage.

1. Planning & Scoping

• Define Objectives: Clearly articulated goals in collaboration with the client (e.g., identify critical weaknesses, assess compliance, etc.).
• Target System Identification: A precise list of IP addresses, domain names, web applications, and other assets within the test’s boundaries.
• Rules of Engagement: Establish acceptable attack techniques and any off-limit actions or systems.

2. Reconnaissance

• Passive Information Gathering: DNS lookups, search engine queries, social media, company website.
• Active Information Gathering: Port scans and probes identify operating systems and running services.

3. Vulnerability Scanning

• Use of Automated Tools: Scans to identify a wide range of known vulnerabilities.
• Prioritisation: Focus on vulnerabilities with high potential for exploitation

4. Exploitation

• Chaining Vulnerabilities: Combining multiple weaknesses for increased impact.
• Privilege Escalation: Attaining higher access levels within the compromised system.
• Data Exfiltration: Mimicking attacker behaviour to test sensitive data exposure.

5. Post-Exploitation & Persistence

• Lateral Movement: Exploration of the network for further access points and assets.
• Establishing Backdoors: Creating mechanisms for long-term, covert access.
• Pivoting: Using a compromised system to facilitate attacks targeting other networks.

6. Reporting

• Documentation: Detailed records of activities, findings, and evidence gathered.
• Explicit Language: Technical information is presented accessibly, even for non-experts.
• Risk Prioritisation: Highlighting vulnerabilities posing the most significant threat.

What are the common vulnerabilities identified during external pentesting?

External penetration tests often expose weaknesses in an organisation’s internet-facing systems and applications. Here’s a breakdown of some of the most frequently encountered issues:

• Network Footprint of Internet-Facing Assets: Misconfigurations or unnecessary services on firewalls, routers, and other network devices can expose them, providing attackers with potential entry points.
• Validation of Security Controls and Weaknesses: External tests put your firewalls, intrusion detection systems, and other security measures to the test, helping determine their effectiveness in a real-world attack scenario.
• Cleartext Transmission of Data: The sending of login credentials, financial information, or other sensitive data without encryption puts it at risk of being intercepted by malicious actors on the network.
• Insecure Patch Management: Outdated systems and software with known vulnerabilities provide attackers with readily exploitable weaknesses to gain unauthorised access.
• Redundant and Ignored Endpoints: Unnecessary or forgotten devices, servers, and cloud instances connected to your network expand the potential attack surface and increase the risk of an overlooked vulnerability.
• Metadata Exposure, Data Leakages, Lack of Secure Hardening: Sensitive information revealed within metadata, accidental leakages of data, and inadequate security configurations create opportunities for attackers to gather intelligence or gain direct access to systems.

What does an external penetration testing report contain?

A well-structured external penetration testing report is a vital tool for decision-makers. It provides essential insights and actionable guidance. Here’s what to expect within the key sections:

• Executive Summary: A concise, non-technical overview summarising critical findings and their potential business impact. This prioritises the most essential items for senior management.
• Detailed Findings: A technical breakdown of each vulnerability, including its location, description, potential consequences of exploitation, and the Common Vulnerability Scoring System (CVSS) score to indicate severity.
• Remediation Guidance: Clear, prioritised recommendations on how to fix identified vulnerabilities. This should include step-by-step instructions and reference appropriate patches, configuration changes, or compensating controls.

External penetration testing tools

There is no one list of tools used in external penetration testing. A few baseline tools are used to identify potential vulnerabilities. However, further advanced checks are performed based on Operating systems, service-specific checks, or protocols-based utilities and tools are utilised. These tools include:

• Network Scanners: Nmap remains essential for network mapping and port scanning. Nessus, Nexpose, Qualys, and OpenVAS are go-to options for in-depth vulnerability scanning. Vulnerability scanning is a necessary input for penetration testing.
• Specialised Tools: Metasploit is a robust exploitation framework. Cobalt Strike and Wireshark are vital for advanced attack simulation and network traffic analysis.

Why use Cyphere as your external penetration testing company?

Cyphere offers CREST-accredited external penetration testing services that surpass industry standards. Our methodology delivers the following benefits:

• Quality-focused: Meticulous testing processes minimise false positives, providing highly accurate insights.
• Stakeholder Debriefing: Clear explanations for all stakeholders, promoting understanding.
• Remediation Support: Guidance extends beyond identification, offering solutions for effective risk mitigation.

Difference between external vulnerability scan and penetration testing

Here are the key differences between a vulnerability scan and an external pen test:

• Depth-wise, a vulnerability scan identifies potential vulnerabilities where a penetration test attempts to exploit vulnerabilities to demonstrate the extent of the attack lifecycle.
• Scope-wise, vulnerability scans are broader as they target many targets, whereas penetration is more focused on high-risk areas due to a concentrated approach. 
• Time-wise, a vulnerability scan is less time-intensive, and a pentest is more time-consuming due to manual effort. 
• A vulnerability scan costs a fraction of the cost of a pen test. 
• A vulnerability scan delivers high-level checks across many hosts in less time and, therefore, can be used continuously. A penetration test provides close to an actual attack simulation. 

Future Trends in External Penetration Testing

With the changing landscape of technology, the introduction of cloud, SaaS offerings and perimeter-less organisations are changing how security measures are assessed. The most likely changes shortly are expected around these areas:

• Increased Automation: Automated tools will streamline routine tasks, allowing penetration testers to focus on complex attacks and creative problem-solving.
• Cloud Security Emphasis: Expect a growing focus on testing cloud-based infrastructure, configurations, and the unique vulnerabilities of cloud environments.
• Continuous Testing: Shift toward continuous assessment models instead of periodic single tests, providing more real-time risk awareness.
• AI: The introduction of AI is coming up around a few sub-domains of cyber security, i.e. assessing and analysing large data sets, event log analysis, and assessments.

External penetration testing checklist

Pre-Engagement

• Scope: Define public IP ranges, domains, subdomains, web applications, external-facing servers (mail, VPN, etc.), and cloud assets (if applicable).
• Rules of Engagement: Clarify permitted actions (e.g., port scanning, denial of service testing), off-limits tactics, timing restrictions sensitive to business operations, and communication channels for escalation.
• Access: Determine if the tester needs a VPN or dedicated testing environment and what external ‘source’ IP addresses will be used (to avoid being blocked by firewalls).

Reconnaissance

• DNS Enumeration: Map domains/subdomains, identify mail servers (MX records), and uncover potential entry points.
• Whois Lookups: Gather registrant information, potential points of contact, and sometimes overlooked network blocks associated with the organisation.
• Open-Source Intelligence (OSINT): Search for leaked credentials, exposed data, employee information on social media, technology stacks mentioned in job postings, etc.
• Shodan/Censys Searches: Identify devices, services, and potential vulnerabilities exposed to the internet.

Vulnerability Identification

Infrastructure Vulnerabilities:

• Outdated software versions (web servers, firewalls, VPN gateways, mail servers)
• Open ports and misconfigured services (unnecessary services, weak protocols, default settings)
• Known software vulnerabilities (scan against CVE databases)

Web Application Vulnerabilities:

• OWASP Top 10 (Injection, XSS, Broken Authentication, etc.)
• Business logic flaws
• Misconfigured web server settings

Cloud Vulnerabilities (if in scope):

• Exposed storage buckets (S3, etc.)
• Weak permissions and access control (IAM)
• Configuration flaws in cloud-based services

Exploitation

• Prioritise: Focus on vulnerabilities posing the highest risk of compromise, unauthorised access, or data exfiltration.
• Infrastructure Exploitation: Target network devices and external-facing servers, leveraging known exploits, weak passwords, and configuration issues.
• Web Application Exploitation: Attempt exploits on web applications (SQL injection, XSS, etc.) to gain unauthorised access or sensitive data.
• Social Engineering (if in scope): Phishing, pretexting, or other methods targeting external-facing employees to gain credentials or network access.

Reporting

• Technical Report: Detail vulnerabilities, exploit paths, evidence, attack impact, CVSS scores, and prioritised remediation guidance.
• Executive Summary: Clear explanation for management on the risks, potential business impact, and high-level recommendations.

Post-Test

• Remediation Support: Work with IT teams to address vulnerabilities, secure web applications, and strengthen external defences. Make sure the stakeholders understand triage and remediation prioritisation.
• Lessons Learned: Review the test to improve the external pentesting process and adapt to evolving techniques.

Important Considerations

• Evolving Threat Landscape: Stay updated on the newest exploits and attack vectors targeting external infrastructure.
• Legal Considerations: Obtain explicit authorisation (for example, the Computer Misuse Act in the UK) and ensure compliance with relevant regulations and contractual obligations.

FAQs

What is the objective of external pentest?

The main objective of an external penetration test is to measure the effectiveness of security controls on Internet-facing assets such as firewalls, web servers, email servers, and other services.

How often should we conduct external penetration testing?

Perform external penetration testing at least annually. Compliance regulations or significant changes to your infrastructure might require more frequent tests.

Do external penetration tests cause disruptions?

Well-planned penetration tests minimise the risk of disruptions. Testers work with you to schedule testing and avoid interfering with critical business operations.

What information do you need to conduct an external penetration test?

We’ll need a list of target assets (IP addresses, domain names, web applications), the scope of allowable testing techniques, and any off-limits systems.

Can you help us fix the vulnerabilities you find?

Yes! We actively support prioritised remediation guidance. We can also provide further consultation and support to help you effectively address identified weaknesses.