Defence in Depth

Defence in depth is the foundational strategy of network security architecture. It requires deploying multiple, overlapping layers of security controls so that if one layer fails, others continue to protect the network. No single control is expected to be perfect — the strength lies in the combination.

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The Concept of Layered Security

Defence in depth is borrowed from military strategy where multiple defensive lines slow and attrit an advancing enemy. In network security, each layer adds friction for the attacker:

Attacker
   │
   ▼
┌─────────────────────────────┐
│  Layer 1: Perimeter         │  ← Firewall, IPS, DDoS mitigation
│  ┌───────────────────────┐  │
│  │ Layer 2: Network      │  │  ← Segmentation, VLANs, ACLs
│  │ ┌─────────────────┐   │  │
│  │ │ Layer 3: Host    │  │  │  ← Endpoint protection, hardening
│  │ │ ┌─────────────┐  │  │  │
│  │ │ │ Layer 4: App │  │  │  │  ← WAF, input validation, patching
│  │ │ │ ┌─────────┐  │  │  │  │
│  │ │ │ │ Layer 5 │  │  │  │  │  ← Encryption, DLP, access control
│  │ │ │ │  DATA   │  │  │  │  │
│  │ │ │ └─────────┘  │  │  │  │
│  │ │ └─────────────┘  │  │  │
│  │ └─────────────────┘   │  │
│  └───────────────────────┘  │
└─────────────────────────────┘

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The Layers of Defence in Depth

Layer	Description	Controls
Physical	Physical access to buildings, rooms, and hardware	Locks, CCTV, badges, mantraps
Perimeter	The boundary between the external world and the network	Firewalls, IPS, DDoS protection, proxies
Network	Internal network structure and segmentation	VLANs, ACLs, micro-segmentation, NDR
Host	Individual servers, workstations, and devices	Antivirus, EDR, host firewalls, hardening
Application	Software running on the network	WAFs, secure coding, patching, input validation
Data	The information that the network exists to protect	Encryption, DLP, access controls, classification
Human	The people who use and manage the network	Security awareness training, phishing simulations, policies

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Why Single-Layer Security Fails

Relying on a single control creates a single point of failure:

Scenario	Single Control	What Goes Wrong
Firewall only	Perimeter firewall	Attacker bypasses via phishing — no internal controls
Antivirus only	Endpoint AV	Zero-day malware evades signatures — no network detection
VPN only	Encrypted tunnel	Compromised credentials grant full access — no segmentation
Encryption only	Data encryption	Insider with decryption keys exfiltrates data — no DLP

The Swiss Cheese Model

Layer 1        Layer 2        Layer 3        Layer 4
┌─────────┐   ┌─────────┐   ┌─────────┐   ┌─────────┐
│         │   │    O    │   │         │   │         │
│    O    │   │         │   │         │   │    O    │
│         │   │         │   │    O    │   │         │
│         │   │    O    │   │         │   │         │
└─────────┘   └─────────┘   └─────────┘   └─────────┘

Each layer has holes (vulnerabilities) — but the holes rarely
align across all layers. An attacker must find a path through
ALL layers simultaneously.

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Implementing Defence in Depth

Perimeter Layer