Penetration Testing for Critical Infrastructure: A Guide for Security Leaders 

Critical National Infrastructure (CNI) is the backbone of the UK’s economy, public safety and national security. From the power grids that keep the lights on and our devices charged, to the water treatment facilities that safeguard public health, these systems really do underpin our daily lives.  

As technology advances and operational technology (OT) becomes more interconnected with traditional IT systems, the risk of vulnerabilities that could be exploited increases. Threat actors, whether state-sponsored or cybercriminal groups, are targeting CNI with increasing frequency and sophistication. 

For IT Directors, CISOs and other senior security leaders responsible for safeguarding CNI systems, penetration testing provides an essential way to understand where your vulnerabilities are before adversaries can exploit them. When carried out in alignment with frameworks such as NIS2 and DORA, and performed by CHECK or CREST accredited providers, penetration testing delivers both compliance assurance and real world resilience. 

In this guide, we’ll discuss why penetration testing is critical for CNI operators in the UK and EU, the compliance obligations driving its adoption, the types of threats being faced, and how to choose the right provider. You will also gain practical insight into integrating testing into your ongoing security programme, so it delivers measurable value. 

Key Takeaways 

• Penetration testing for UK CNI is essential to meet regulatory and operational security requirements 
• CHECK accredited providers are the benchmark for government and CNI sector testing 
• Frameworks like NIS2 and DORA mandate robust testing and incident readiness for CNI operators 
• OT and IT convergence increases vulnerabilities, and regular testing can identify and address gaps early 
• Choosing a provider with compliance expertise ensures tests align with regulatory obligations 

Understanding Critical National Infrastructure in the UK 

Critical National Infrastructure (CNI) covers a number of critical industries that are integral to the running of the country’s infrastructure and services.  

What qualifies as Critical National Infrastructure? 

The UK government defines CNI as the assets, systems and networks that are essential for the functioning of society and the economy.  

These are categorised into 13 sectors: 

• Energy 
• Water 
• Transport 
• Communications 
• Emergency services 
• Government 
• Health 
• Finance 
• Food 
• Space 
• Civil nuclear 
• Chemicals 
• Defence 

The National Protective Security Authority (NPSA) and the National Cyber Security Centre (NCSC) provide guidance to help operators to secure these sectors against a wide range of threats, including cyberattacks, physical attacks and insider compromise. 

Why CNI is a prime target for cyber attacks 

CNI systems are high value targets for cybercriminals because of the potential impact on the country if they are successfully disrupted.  

A successful cyberattack on a water treatment facility or an energy distribution network, for example, could cause widespread outages, public health risks and significant economic loss.  

Adversaries may also target CNI to achieve political, military or financial objectives. Recent years have seen a rise in state-sponsored attacks that aim to destabilise operations, as well as ransomware campaigns that seek financial gain. 

Medusa Ransomware Attacks: Over 300 CNI Organisations Targeted 

A recent report revealed that over 300 UK critical infrastructure organisations were hit by Medusa ransomware attacks, raising significant concerns about cyber resilience in essential sectors.  

This widespread assault underscores the growing risk ransomware poses to CNI entities, highlighting the urgent need for proactive measures such as penetration testing and robust incident response strategies. 

Top threats to UK critical infrastructure – the case for proactive pen testing 

The UK government has stated that a major cyberattack on the UK’s Critical National Infrastructure is a matter of ‘when, not if’. The cyber incident that affected Telecoms firm, Colt Technologies, which was suspected to be a ransomware attack, highlights the growing threat to UK and European criticial infrastructure. Colt was forced to take systems offline, including its customer facing portal, and the incident has raised concerns about the resilience of the telecommunications sector. 

With new regulations being introduced and a move to ban the public sector and CNI organisations from paying ransomware payments, businesses within these industries must be engaging with proactive cyber security strategies to protect their operations.  

What is Penetration Testing for Critical Infrastructure? 

Due to its critical nature, not all providers are able to run security assessments on critical national infrastructure.  

The difference between standard pen testing and CNI focused testing 

Penetration testing for CNI goes beyond standard corporate network testing. It often involves assessing OT environments, industrial control systems (ICS) and SCADA networks that manage physical processes. These systems have unique constraints, such as requiring minimal downtime and running on legacy technologies that cannot easily be patched.

CNI focused pen testing must also align with strict regulatory and sector specific requirements, such as NIS2, to ensure safety and compliance. 

The role of CHECK and CREST accreditation 

The NCSC’s CHECK scheme approves companies and individuals to conduct authorised penetration testing of government and CNI systems. CHECK-accredited testers follow defined methodologies and meet rigorous security clearance requirements. CREST accreditation also validates a provider’s technical competence and adherence to recognised testing standards. For many CNI sectors, working with a CHECK or CREST accredited provider is not only recommended but expected. 

Regulatory Drivers for CNI Penetration Testing 

There are a number of bodies and regulations that the organisations operating in CNI must adhere to, which include mandates on pen testing for compliance.  

NIS2 Directive 

NIS2 expands on the original Network and Information Systems Directive to strengthen cyber resilience across the UK and EU’s critical sectors. It applies to operators of essential services and key digital infrastructure, including many CNI organisations.  

NIS2 requires security measures to be implemented based on risk assessments and mandates incident reporting within strict timelines. Regular security testing, including penetration testing, is a key component of demonstrating compliance. 

DORA (Digital Operational Resilience Act) 

DORA focuses on the financial services sector operating in or with the EU, which is considered part of CNI due to its importance to economic stability. It mandates that financial entities, including banks, payment service providers and critical third-party ICT suppliers, have robust operational resilience.  

Testing requirements include vulnerability assessments and penetration testing to validate the effectiveness of security controls and ensure operational continuity in the event of a cyber incident. 

UK specific compliance and guidance 

In addition to NIS2 and DORA, many CNI organisations in the UK are subject to UK-specific regulatory oversight. Ofgem, Ofwat, the Civil Aviation Authority, FCA, NHS Digital and other bodies have their own security requirements and reporting obligations.  

Penetration testing is often used to evidence compliance with sector regulations and to support frameworks such as ISO 27001 and the UK Cyber Assessment Framework. 

How Penetration Testing Strengthens CNI Defences 

Penetration testing can uncover vulnerabilities before they can be exploited by an attacker, this strengthens your security posture and meets compliance mandates by enabling fixes and mitigation before a breach can occur.   

Identifying vulnerabilities before adversaries do 

Penetration testing simulates real world attacks to identify vulnerabilities in both your IT and OT environments. By exposing weaknesses before they can be exploited, you can prioritise remediation efforts and reduce your risk profile. 

Protecting both IT and OT environments 

With IT and OT systems being increasingly interconnected, a compromise in one can have direct consequences for the other. Penetration testing that covers both of these environments ensures that any vulnerabilities across the entire attack surface are identified. 

Reducing the risk of regulatory fines and service disruption 

Failure to comply with frameworks like NIS2 and DORA can result in significant fines. Testing can help you to demonstrate that your organisation is taking proactive steps to meet security obligations. It also reduces the risk of costly downtime or public safety incidents caused by cyberattacks. 

Building incident response readiness 

Penetration tests can help you to assess your organisation’s detection and response capabilities. By identifying how quickly and effectively you can respond to simulated attacks, you can refine your incident response processes and improve resilience. 

CNI Threat Landscape 

With high profile attacks and increased pressure from nation-state attackers, there is a lot of focus on CNI as high value targets.  

Cloud and OT convergence risks 

Many CNI operators are integrating cloud technologies into their operations, including OT systems. While this improves flexibility and efficiency, it also expands the attack surface and introduces new vulnerabilities that must be tested. 

Supply chain security gaps 

CNI sectors must rely on complex supply chains, and therefore vulnerabilities in third-party systems can a huge risk as they could be exploited to gain access to critical networks. Penetration testing can include supply chain assessments to reduce this risk. 

Insider threats and privileged access abuse 

Insider threats, whether malicious or accidental, remain a major concern for CNI. Testing access controls and monitoring systems can help identify and mitigate insider risks. 

Real world case studies of CNI breaches 

As we’ve shown, recent incidents have shown how sophisticated cyberattacks can disrupt critical services. Analysing these cases provides valuable lessons for improving defences and ensuring that penetration testing addresses the right threats. 

In June 2024, a Russian cybercrime group known as Qilin targeted Synnovis, a pathology services provider used by NHS hospitals, such as King’s College Hospital and Guy’s and St Thomas’ NHS Foundation Trust. 

The attack disrupted blood test processing and resulted in the release of 400 GB of stolen data. 

A total of 170 patient incidents were reported, mostly classified as low harm. Critically, one patient’s death has been directly linked to delayed blood test results caused by this cyberattack, highlighting the real world consequences of breaches in healthcare infrastructure.  

Choosing the Right Penetration Testing Provider for CNI 

As we’ve seen, with the criticality of the systems, networks and applications that we’re dealing with as part of the CNI sector, the choice of who should conduct your pen testing is a very important one. There are a few key criteria that should be considered when making your choice.  

Essential selection criteria 

When selecting a provider, consider: 

• CHECK or CREST accreditation 
• Proven experience in your specific CNI sector 
• Capability to test both OT and IT environments 
• Understanding of NIS2, DORA and UK specific compliance requirements 

Questions to ask your prospective provider 

• How do you align tests with NIS2 and DORA? 
• Can you provide reporting in formats required by our regulator? 
• Do you offer remediation support and re-testing? 
• How do you ensure minimal disruption to live CNI systems during testing? 

Integrating Pen Testing into Your Ongoing Security Programme 

Penetration testing is an essential part of a layered cyber security strategy to protect your CNI business.  

Aligning tests with compliance audits 

Scheduling penetration tests in line with audit cycles can help streamline evidence gathering and reduce the time required to prepare for inspections. 

Continuous testing vs. annual testing for CNI 

While annual testing is a minimum standard, continuous penetration testing can provide ongoing assurance by identifying vulnerabilities as they emerge. This is particularly valuable in dynamic environments where systems are regularly updated. 

Combining pen testing with red teaming and threat intelligence 

Integrating penetration testing with red teaming exercises to test your security defence measures, and real time threat intelligence ensures that testing reflects current attack methods and provides more comprehensive security assessments throughout the year. 

Final Thoughts: Strengthening Your CNI Security Posture 

Penetration testing for critical infrastructure is far more than a compliance exercise. It is a proactive strategy for identifying and mitigating vulnerabilities that could disrupt essential services, compromise public safety or damage national security. By working with an accredited provider that understands your sector and regulatory obligations, you can build a security posture that is both compliant and resilient. 

Get in touch with DigitalXRAID today to discuss CNI penetration testing. 

FAQs about Penetration Testing for Critical Infrastructure 

What is critical national infrastructure in the UK? 

Critical national infrastructure includes the systems, assets and networks vital to the UK’s security, economy and public health, such as energy, water, transport and communications. 

Is penetration testing mandatory for CNI operators? 

While not universally mandated, frameworks like NIS2 and DORA require robust security testing, and UK regulators strongly recommend CHECK accredited testing for CNI. 

How often should CNI systems be penetration tested? 

Best practice for testing CNI systems is at least annually, with additional testing after major changes, system upgrades or significant threat developments. 

What is the difference between OT and IT penetration testing? 

IT testing focuses on corporate networks and systems, while OT testing examines industrial control systems, SCADA and other operational environments that manage physical processes. 

Why choose a UK CHECK accredited provider? 

CHECK accreditation ensures testing is performed by NCSC approved professionals who meet rigorous standards for government and CNI security testing.