How to Perform a Network Security Audit

A network security audit is one of the most valuable exercises any UK business can undertake to understand and improve its cyber security posture. Yet many organisations — particularly small and medium-sized businesses — have never conducted one, either because they believe it is too complex, too expensive, or simply unnecessary for a business of their size.

The reality is that network security audits are essential for businesses of all sizes. The UK government's Cyber Security Breaches Survey consistently shows that around 39% of UK businesses identified a cyber attack in the most recent reporting year, with the figure rising to 69% for medium-sized businesses. An audit helps you understand where your vulnerabilities lie before an attacker exploits them, rather than discovering weaknesses in the aftermath of a breach.

This guide provides a comprehensive, practical framework for conducting a network security audit, covering the key areas you need to assess, the tools and techniques involved, and how to turn your findings into actionable improvements.

What Is a Network Security Audit?

A network security audit is a systematic evaluation of your organisation's network infrastructure, policies, and practices to identify vulnerabilities, assess risks, and verify compliance with relevant standards. Unlike a penetration test — which simulates an attack to see how far an intruder could get — an audit takes a broader view, examining everything from firewall configurations and access controls to user behaviour and policy documentation.

Think of it as a comprehensive health check for your network. A penetration test is like a stress test for your heart — it tests one specific aspect under extreme conditions. An audit is a full medical examination — it checks everything systematically, identifies areas of concern, and produces recommendations for overall improvement.

For UK businesses, a network security audit also supports compliance with frameworks such as Cyber Essentials, Cyber Essentials Plus, ISO 27001, and GDPR. Many of these frameworks either require or strongly recommend regular security assessments, and an audit provides the evidence needed to demonstrate due diligence.

Phase 1: Preparation and Scope Definition

Before you begin the technical assessment, you need to define the scope and objectives of your audit. A poorly scoped audit will either be too narrow to be useful or so broad that it becomes unmanageable. Start by answering these key questions.

What are you trying to achieve? Common objectives include identifying vulnerabilities before an attacker does, verifying compliance with a specific framework (Cyber Essentials, ISO 27001), preparing for a client or regulatory audit, or establishing a baseline security posture for ongoing improvement.

What is in scope? Define the boundaries of your audit. For most SMEs, a comprehensive audit should cover your entire network — firewalls, switches, wireless infrastructure, servers, cloud services, endpoints, and remote access solutions. If resources are limited, you might focus on your perimeter defences and critical systems first, then expand in subsequent audits.

Who will conduct the audit? You have three options: internal IT staff, your managed service provider, or an independent third-party auditor. Each has advantages. Internal staff know your environment intimately but may have blind spots. Your MSP understands your setup but has an inherent conflict of interest in auditing their own work. An independent auditor provides objectivity but needs time to understand your environment. For the most rigorous results, an independent audit is recommended, particularly for compliance purposes.

Phase 2: Asset Discovery and Inventory

You cannot secure what you do not know about. The first technical phase of any audit is discovering and cataloguing every device, service, and connection on your network. This inventory serves as the foundation for every subsequent assessment.

Use network scanning tools to discover all connected devices. Tools such as Nmap, Advanced IP Scanner, or your existing network management platform can identify every IP address in use, the device type, operating system, and open ports. Compare the results against your existing asset register — any discrepancies indicate either undocumented devices (which are a security risk) or decommissioned devices that are still connected (which are also a security risk).

Pay particular attention to shadow IT — devices and services that have been deployed without the knowledge or approval of your IT team. This might include personal laptops connected to the corporate Wi-Fi, consumer-grade NAS devices plugged into the network, or cloud services that individual staff members have signed up for using their work email. Shadow IT is one of the most common sources of security vulnerabilities in UK businesses.

Phase 3: Firewall and Perimeter Assessment

Your firewall is the primary barrier between your internal network and the internet, and its configuration is one of the most critical elements of any security audit. A misconfigured firewall can render all other security measures ineffective.

Begin by reviewing your firewall ruleset. Every rule should have a documented business justification. Rules that allow inbound traffic from the internet should be minimal and specific — only the ports and protocols that your business genuinely requires should be permitted. Any rules that allow broad access (such as "any-any" rules) should be flagged as high-risk findings requiring immediate remediation.

Check for rules that have become obsolete. As services change and applications are decommissioned, the corresponding firewall rules are often left in place. These orphaned rules create unnecessary attack surface. Every open port that is no longer needed is a potential entry point for an attacker. A well-maintained firewall should be reviewed quarterly, with unused rules removed promptly.

Verify that your firewall firmware is up to date. Firewall vendors regularly release security patches to address discovered vulnerabilities, and running outdated firmware is one of the most common weaknesses found during audits. Check the vendor's support lifecycle — if your firewall hardware is no longer receiving security updates, it needs to be replaced regardless of how well it appears to be functioning.

Examine your NAT configuration, VPN settings, and any DMZ architecture. If you host any internet-facing services (such as a web server, email server, or remote access portal), verify that these are properly isolated from your internal network. A compromised internet-facing service should not provide an attacker with direct access to your core infrastructure.

Phase 4: Wireless Network Security

Wireless networks are one of the most frequently exploited attack vectors, and many UK businesses have wireless configurations that fall short of security best practices. Your audit should examine every aspect of your wireless infrastructure.

Verify that all wireless networks use WPA3 or, at minimum, WPA2 Enterprise authentication. If you are still using WPA2 Personal (with a shared pre-shared key), this is a significant finding. Shared keys are easily compromised — when a staff member leaves, the key should be changed, but in practice this rarely happens. WPA2 Enterprise (802.1X) authenticates each user individually against your directory service, providing far stronger security and accountability.

Check for rogue access points. These are unauthorised wireless devices that someone has connected to your network, either maliciously or through ignorance. A staff member might plug in a consumer wireless router to improve coverage in their area, inadvertently creating an unsecured entry point to your network. Use wireless scanning tools to detect any access points that are not part of your managed infrastructure.

Review your guest Wi-Fi configuration. Guest networks should be completely isolated from your corporate network, with no ability to access internal resources. They should use a separate VLAN, have bandwidth limits to prevent abuse, and ideally require registration or acceptance of terms before providing access.

Phase 5: Access Controls and Identity Management

Who has access to what on your network, and how is that access controlled? This phase examines your authentication mechanisms, authorisation policies, and identity management practices.

Review your Active Directory or Azure AD configuration. Check for accounts that should have been disabled — former employees, temporary contractors, and test accounts that were never removed. Stale accounts are a significant security risk, as they can be compromised without anyone noticing. The ICO has issued guidance emphasising that organisations should have robust joiners, movers, and leavers processes to ensure access is granted and revoked appropriately.

Examine password policies. Are you enforcing sufficient complexity, length, and rotation requirements? The NCSC now recommends a minimum password length of 12 characters, with an emphasis on length over complexity. More importantly, check whether multi-factor authentication (MFA) is enabled for all users, particularly for remote access, administrative accounts, and cloud services. MFA remains one of the single most effective security controls available, yet many UK businesses have not fully deployed it.

Check for excessive privileges. Are staff members operating with administrator accounts for their daily work? Do departmental managers have access to systems and data that are not relevant to their role? The principle of least privilege dictates that users should only have the minimum access necessary to perform their job. Excessive privileges increase the blast radius of any account compromise.

Phase 6: Endpoint and Patch Assessment

Every workstation, laptop, server, and mobile device on your network is a potential entry point for an attacker. This phase assesses the security posture of your endpoints and the effectiveness of your patch management processes.

Verify that every endpoint has current, active endpoint protection. This should be a business-grade solution — not consumer antivirus — with centralised management, real-time threat detection, and ideally endpoint detection and response (EDR) capabilities. Check that definitions are current, real-time scanning is enabled, and tamper protection is active to prevent malware from disabling the security software.

Assess your patch management posture. How quickly are security patches applied after release? Microsoft releases patches on the second Tuesday of each month (Patch Tuesday), and critical vulnerabilities should be patched within 14 days as recommended by the NCSC and required by Cyber Essentials. Use your RMM or management tools to generate a report showing the patch status of every device — you may be surprised to find machines that are months behind on updates.

Check for unsupported software. Windows 10 reaches end of life in October 2025, and any machines still running older versions of Windows (or other end-of-life software) represent a significant vulnerability. Unsupported software no longer receives security patches, meaning that newly discovered vulnerabilities will never be fixed. These machines should be upgraded or replaced as a matter of urgency.

Phase 7: Reporting and Remediation Planning

The value of a network security audit lies not in the assessment itself, but in what you do with the findings. A comprehensive audit report should categorise findings by risk level — critical, high, medium, and low — with clear remediation recommendations and suggested timelines for each.

Critical findings — those representing an immediate risk of compromise — should be addressed within days. These might include exposed RDP ports, default administrator credentials, unpatched critical vulnerabilities, or disabled security tools. High-risk findings should be remediated within 30 days. Medium-risk findings within 90 days. Low-risk findings can be addressed as part of routine maintenance and ongoing improvement.

Present the findings to business stakeholders in business terms, not technical jargon. Instead of saying "we found CVE-2024-38112 unpatched on three servers," explain "three of our servers have a known vulnerability that attackers are actively exploiting worldwide, and our current exposure could lead to a complete network compromise." Decision-makers need to understand the business risk to approve the resources needed for remediation.

Schedule a follow-up audit — typically three to six months after the initial assessment — to verify that remediation actions have been completed and are effective. Security is not a one-time exercise; it requires continuous attention and regular reassessment as your environment evolves and new threats emerge.