Penetration testing – ultimate guide

Penetration testing UK ultimate guide himage

In this guide, we cover what penetration testing is, why it’s essential, the different types of tests, how the process works, what to expect afterwards, industry-specific use cases, cost considerations, and how penetration testing supports compliance frameworks like PCI DSS and ISO 27001. We also explore the legal and ethical dimensions of testing and explain how Securious can support your organisation with expert, tailored services. 

What is Penetration Testing? 

Penetration testing, often referred to as pentesting, is a proactive cyber security assessment that simulates real-world attacks on your systems, applications, or infrastructure. Carried out by skilled ethical hackers, these tests are designed to uncover security weaknesses that could be exploited by malicious actors.

The objective is not just to find vulnerabilities, but to understand how they could be chained together to compromise sensitive data, disrupt operations, or bypass defences.

Why Would I Want Penetration Testing?

  • Ensure your critical assets/data are secure by identifying vulnerabilities so you can protect your environment from malicious attacks by mitigating critical threats, which reduces the likelihood of a breach 
  • Evidence that your systems have been tested to demonstrate to customers, business partners and stakeholders that it takes security seriously
  • It will help you meet regulatory compliance requirements (like PCI DSS)

How much does Penetration Testing Cost?

Penetration testing isn’t one-size-fits-all. Costs are typically influenced by:

  • The type of test required (e.g. external, internal, web application)

  • The number of systems, hosts or applications in scope

  • The complexity of the environment being tested

  • Whether testing needs to align with a standard or audit (e.g. PCI DSS, ISO 27001)

  • How much context or assurance you already have about your setup

We have developed a quote generator, which is designed to reflect these factors at a high level, so you can understand what drives cost and what to expect before you commit to deeper technical discussions.

Click here to try it for yourself – it’s free, instant and does not require you to leave your contact details in order to see your result.

The Penetration Testing Process: Step-by-Step 

A well-executed penetration test follows a defined methodology: 

  1. Scoping and Planning

A detailed scoping session defines the objectives, targets, access level, and rules of engagement. Legal authorisation is secured, and any limitations (e.g. systems to avoid) are agreed. 

  1. Reconnaissance

Testers gather intelligence – both passive (e.g. WHOIS, DNS, OSINT) and active (e.g. port scanning, banner grabbing) – to build a picture of the environment. 

  1. Vulnerability Analysis

Tools and manual techniques are used to discover potential weaknesses. This step identifies known CVEs, misconfigurations, and logic flaws. 

  1. Exploitation

Ethical hackers attempt to exploit identified issues under safe and controlled conditions. This may involve privilege escalation, lateral movement, or data access. 

  1. Post-Exploitation and Impact Analysis

The testers assess what an attacker could achieve if the exploit were real – such as gaining admin access or exfiltrating sensitive data. 

  1. Reporting and Debrief

A detailed report is prepared, prioritising findings by risk, and outlining remediation advice. Securious includes a walkthrough debrief to help technical and non-technical stakeholders understand the results. 

What Happens During a Penetration Test?

We start with a scoping call, identifying the boundaries we’re working within. We’ll agree on the requirements and outcomes of the testing, before moving on to testing your systems. Our penetration testers follow a proven methodology with a series of simulated tests to identify any weaknesses in your defences – whether internally or externally. We adhere to an agreed set of rules of engagement before, during and after every penetration test.

Often it is the combination of a series of weaknesses in your systems that allows attacks, rather than a single vulnerability. That’s why our tests combine a series of lower-risk exploits in a particular sequence, to determine whether they would have any effect.

We test against the OWASP top 10 and detail the penetration test findings in a report that includes guidance around the vulnerability, impact, threat and the likelihood of a breach within your organisation. It highlights the potential risks and recommends where additional resources should be applied to protect your systems.

We will provide a comprehensive combined technical and summary report, detailing our findings and any remediation points, and will go through these at the planned debrief within five days of pentest completion.

What Does a Tester Need to Know Before Starting?

Typically, penetration testers will want to know at least the following:

  • The list of targets, in detail
  • What the targets are (Network Infrastructure, Web Application, API, etc.)
  • In the case of a web application or API, a rough approximation to the number of endpoints/pages
  • The sensitivity of the data on the systems in scope
  • Where the systems are situated
  • If there are any third parties (web hosts, managed service providers)
  • Who these third parties are, with written permission allowing testing
  • The level of access you would like the testers to have

How Often Should we Pentest?

Penetration testing should be performed on a regular basis. This ensures that you can detect and respond to any newly discovered threats or emerging vulnerabilities that could lead to a system compromise by attackers.

Furthermore, penetration tests should also be carried out whenever:

  • Significant changes to your infrastructure have taken place
  • Additional locations or branch offices are opened
  • You suspect or have fallen victim to an attack

What Happens After a Penetration Test? 

The real value of a pentest comes after the report is delivered. Remediation activities should be planned and prioritised based on risk. For critical vulnerabilities, immediate action may be required. Once fixes are applied, a retest can validate that issues have been resolved. Organisations should also review policies, improve user training, and consider complementary services like vulnerability management or security awareness training. 

What are the Different Types of Penetration Tests?

What is Black Box Testing?

Black box testing, in the context of penetration testing, is a method of vulnerability assessment whereby the tester attacks a target with the same level of knowledge and permissions as a genuine malicious actor might have. Typically, this means the test begins with the tester receiving no information on internal workings, and no credentials or permissions.

While black box testing can offer valuable insight by using the same methodologies that a real attacker might, it is important to understand that the expected result from such a test is unlikely to include details of the potentially-critical vulnerabilities that could be detected during a white box test.

What is White Box Testing?

Conversely, white box testing involves the provisioning of a tester with the very knowledge and permissions denied from black box testers. This provides the assessor with the ability to probe the target for vulnerabilities present within the application’s core; an area hidden from a black box tester.

Performing this type of testing allows organisations to more accurately understand the risks present in their applications and services by revealing vulnerabilities that are presented only to end users with permissions greater than nil, and real attackers who have managed to crack the outermost layer of security.

Crucially, white box testing is not exclusive of black box testing; meaning a white box assessment includes everything involved with a black box test, but also includes testing of otherwise unreachable areas.

What is Grey Box Testing? 

A hybrid approach, grey box testing involves partial knowledge of the system – such as internal documentation or user-level credentials – allowing the tester to simulate an attacker who has already gained a foothold inside the network. This often reflects real-world scenarios, where attackers may exploit social engineering or insider threats to access parts of a system. Grey box testing provides a balance between realism and coverage, revealing both external exposure and internal misconfigurations or privilege escalation paths. 

Internal vs External Testing 

Internal testing simulates threats from inside the organisation, such as a rogue employee or a compromised internal device. These tests evaluate security controls that protect against lateral movement, privilege escalation, and internal data access. External testing, in contrast, focuses on systems accessible via the internet – such as public-facing websites, VPNs, and cloud services. These tests aim to uncover vulnerabilities that could be exploited remotely by unauthenticated attackers. 

Web Application and API Testing 

These tests scrutinise the security of software systems, especially those that handle sensitive data or serve customers over the internet. Web application testing focuses on common flaws such as injection attacks, broken authentication, and insecure direct object references. API testing, meanwhile, assesses endpoint security, input validation, and proper authentication/authorisation flows. These types of tests are essential for modern SaaS platforms, ecommerce systems, and digital services that rely on web interfaces and third-party integrations. 

Common Vulnerabilities Found in Penetration Tests 

While every environment is unique, penetration testers frequently uncover issues such as: 

  • Misconfigured firewalls and access control lists (ACLs) 
  • Outdated software with known vulnerabilities 
  • Default credentials left unchanged 
  • Insecure session management in web applications 
  • Lack of input validation leading to SQL injection or XSS 
  • Weak cryptographic practices 

These vulnerabilities, especially when combined, can be leveraged by attackers to escalate privileges, exfiltrate data, or establish persistent access. 

Penetration Testing for PCI DSS Compliance 

PCI DSS Requirement 11.4 mandates regular testing of systems that store, process, or transmit cardholder data. Key expectations include: 

  • Annual testing or after significant changes 
  • Testing of both internal and external systems 
  • Validation of segmentation controls 
  • Use of industry-recognised methodologies (e.g. NIST, OWASP) 

Securious supports PCI DSS penetration testing with experts who understand the full compliance context. 

Penetration Testing for ISO 27001 

While ISO 27001 does not prescribe penetration testing, it strongly supports the standard’s risk management objectives. Testing aligns with Annex A controls related to: 

  • Technical vulnerability management (A.12.6.1 and A.12.6.2) 
  • Secure system engineering (A.14) 
  • Supplier service delivery management (A.15) 

Evidence from penetration tests strengthens your Statement of Applicability and audit readiness. 

Learn more about our penetration testing for ISO 27001 services by clicking here.

Penetration Testing vs Vulnerability Scanning 

Although they’re often mentioned together, penetration testing and vulnerability scanning are distinct practices. Vulnerability scanning uses automated tools to detect known security issues – such as missing patches or outdated software – across your systems. It’s fast, repeatable, and valuable for maintaining ongoing cyber hygiene. 

Penetration testing, by contrast, is a manual and methodical effort to exploit vulnerabilities and uncover complex attack paths that automated tools might miss. It goes beyond surface-level issues to examine how weaknesses could be combined and leveraged in real-world attacks. Penetration tests also provide context – such as risk ratings and business impact – that scanners cannot. The two approaches are complementary, not interchangeable. Regular scanning should be paired with periodic penetration testing to provide full-spectrum visibility. 

Legal and Ethical Considerations in Penetration Testing 

Authorisation is critical. Penetration testing must be explicitly approved by the organisation being tested, with a clear scope and formalised rules of engagement. Testing without consent – even with good intentions – is illegal under UK law, including the Computer Misuse Act. 

Ethical testers operate with transparency, professionalism, and minimal disruption. They avoid unnecessary system impact, protect sensitive information encountered during the test, and report findings responsibly. A well-written contract or statement of work should outline legal protections for both parties, define escalation protocols, and ensure that business operations remain unaffected. 

Organisations should also consider data handling obligations, such as those under GDPR, and ensure that any test involving customer data is appropriately controlled. 

Benefits of Continuous Security Testing 

In today’s fast-paced threat landscape, point-in-time testing may no longer be sufficient. Continuous security testing – also known as ongoing assurance – incorporates regular assessments into your security programme. This model aligns closely with DevSecOps, agile development, and cloud-native environments, where change is constant. 

Benefits include: 

  • Immediate identification of newly introduced vulnerabilities 
  • Reduced mean time to detect and remediate security issues 
  • Enhanced risk visibility for stakeholders and leadership 
  • Ongoing validation of security controls and patching processes 

Securious offers managed penetration testing services, enabling businesses to adopt a more proactive and scalable approach. Learn more by clicking here.

How Securious can help with penetration testing

Securious is a CREST-accredited penetration testing provider. We offer three core testing services to suit different goals: 

  • Penetration Testing for PCI DSS and ISO 27001: For organisations working towards certification or audit readiness. We help you meet key compliance requirements and produce evidence for assessors and QSAs. Click here to learn more.
  • One-Off and Project-Based Penetration Testing: For product launches, infrastructure changes or cyber security projects. Fast, flexible, and designed to fit your timelines. Click here to learn more.
  • Ongoing Penetration Testing Partnership: For businesses taking a proactive approach to security. Regular testing, trend analysis and expert support as your systems evolve. Click here to learn more.
  • Free, Instant Online Pentesting Quote Generator: Get your free, instant penetration testing quote – no contact details required. We’ll show you what a penetration test is likely to cost, based on your environment and requirements, in under 2 minutes. Click here to try it.