Defence in depth: Building layers of security in a modern threat landscape

August 21, 2025

image: ©MF3d | iStock

Raymond Setchfield discusses building layers of security in a modern threat landscape within this defence-in-depth focus

A single security measure is no longer enough to protect an organisation from cyber threats. Attackers are persistent, sophisticated, and constantly evolving their methods. Defence in depth (DiD), a long-standing security principle, offers a comprehensive approach by implementing multiple layers of defence across an organisation’s systems, processes, and people.

This layered strategy aims to ensure that if one control fails, others remain in place to prevent or minimise the impact of a breach. It is not about relying on any single solution but about creating a security architecture where weaknesses in one area are compensated for by strengths in another.

The principle behind defence in depth

The concept originates from military strategy, where multiple defensive positions are used to slow down, detect, and repel attackers. In cybersecurity, the same principle applies: an adversary must overcome several layers before reaching valuable assets.

These layers are typically designed to:

Deter:
- Make an attack more difficult or unattractive.
Detect:
- Identify malicious activity as early as possible.
Delay:
- Slow down an attacker’s progress, giving time to respond.
Respond:
- Contain and mitigate the impact of an incident.

The layers of defence in depth

While the specific layers vary by organisation, most models include a combination of the following:

Physical security
- Physical access to systems remains an often-overlooked security risk. Measures such as secure entry points, CCTV, server room locks, and visitor management systems prevent unauthorised individuals from physically tampering with equipment or gaining access to sensitive areas.
Network security
- The network is a primary target for attackers. Segmentation, firewalls, intrusion detection and prevention systems (IDPS), and secure configurations can limit an attacker’s ability to move laterally if they
  gain a foothold. Network monitoring tools provide visibility into unusual traffic patterns that could signal an intrusion.
Endpoint security
- Laptops, desktops, and mobile devices are common entry points for attackers. Security measures such as anti-malware, device encryption, application whitelisting, and endpoint detection and response (EDR) tools help secure endpoints against both external and insider threats.
Application security
- Applications can be exploited through vulnerabilities if not properly secured. Secure development practices, regular patching, code reviews, and application firewalls help protect software from exploitation. The principle of least privilege should be applied to application accounts to limit the damage if they are compromised.
Data security
- Protecting the data itself is essential. Encryption at rest and in transit, access controls, and data loss prevention (DLP) solutions ensure that sensitive information remains protected even if an attacker breaches other defences. Data classification schemes help identify what needs the highest level of protection.
Identity and Access Management (IAM)
- Compromised credentials are a leading cause of breaches. Strong authentication methods such as multi-factor authentication (MFA), regular access reviews, and just-in-time access provisioning reduce the risk of unauthorised access.
User awareness and training
- Even the most advanced technical controls can fail if users are unaware of threats. Phishing simulations, regular security briefings, and clear reporting procedures empower staff to act as part of the defence strategy rather than as potential weak links.
Monitoring and incident response
- Effective logging, continuous monitoring, and well-tested incident response plans enable rapid detection and containment of threats. The faster a threat is identified; the less opportunity it has to cause significant harm.

The benefits of a layered approach

DiD provides several key benefits:

Resilience:
- Multiple layers reduce the likelihood of a single point of failure leading to a catastrophic breach.
Early detection:
- Monitoring across layers improves the chance of spotting suspicious activity before it escalates.
Adaptability:
- Layers can be updated or replaced independently as threats evolve.
Regulatory compliance:
- Many industry regulations require multi-layered security controls.

Common pitfalls to avoid

Implementing DiD comes with challenges. Over-reliance on technology is a common issue; tools are vital, but human awareness and sound policies are equally important. Poor integration between layers can cause inefficiencies or gaps, while neglecting maintenance leaves defences outdated and vulnerable. Flat network designs are another risk, as a lack of segmentation allows attackers to move quickly once inside.

Building an effective defence in depth strategy

It all begins with assessing the threat landscape to understand the risks specific to your industry, size, and operating environment. From there, it is essential to prioritise the protection of critical assets, focusing first on systems and data whose compromise would cause the greatest harm.

Layered controls should then be implemented, combining preventative, detective, and responsive measures across physical, technical, and administrative areas. Regular testing through penetration exercises, red teaming, and security audits ensures these measures remain effective. Finally, the strategy must adapt continually, as threats, business processes, and technologies evolve.

Defence in depth in the modern context

Modern threats such as ransomware, supply chain attacks, and cloud-based breaches highlight the importance of this approach. For example, if ransomware bypasses email filtering, it might still be stopped by endpoint protection. If that fails, network segmentation and backups can limit the damage and enable recovery.

In cloud and hybrid environments, DiD extends beyond the corporate network. Organisations must consider identity protection, API security, secure cloud configurations, and visibility across multi-cloud infrastructures.

The principle is also vital in supply chain risk management. A trusted supplier’s breach can be as damaging as a direct attack, so layered controls should extend to vetting and monitoring third-party access.

By embracing a multi-layered defence strategy, organisations position themselves not only to withstand attacks but also to detect and respond to them more effectively, ensuring business continuity and maintaining trust with customers and partners.