Security by Design

Also Known As

TL;DR

Explanation

Security by Design (SbD) means security is a first-class requirement alongside functionality — considered during threat modelling, system architecture, data flow design, and API contracts, not discovered during a pre-release penetration test. Core principles include: minimise attack surface, apply least privilege at every layer, use defence in depth, fail securely (default-deny), avoid security through obscurity, keep design simple (complexity breeds vulnerabilities), and separate duties. For PHP projects this means: choosing framework defaults that are secure out of the box, designing data flows that never touch user input without sanitisation, and reviewing every new feature through a threat-model lens.

Common Misconception

Why It Matters

Common Mistakes

• Security as a final phase ('we'll add security later') — architectural flaws cannot be patched.
• Designing for the happy path only — not considering what happens when inputs are malicious.
• No threat modelling in the design phase — security gaps are discovered in production.
• Prioritising features over security constraints in early sprints — security debt compounds quickly.

Code Examples

// Design that makes security impossible to add later:
class UserController {
    // All user data in one object — cannot restrict field visibility per role
    // No audit log hook points — cannot add logging without rewriting
    // Direct DB calls everywhere — cannot add encryption layer without touching all code
    // Feature shipped — security 'added later' means full rewrite
}

// Security by design — bake it in from the start

// 1. Default deny
class Policy {
    public function can(User \$user, string \$action, mixed \$resource): bool {
        return false; // deny by default — explicitly grant per action
    }
}

// 2. Fail securely — errors don't open access
function getUser(int \$id): User {
    return User::findOrFail(\$id); // throws, not returns null
}

// 3. Immutable value objects — can't enter invalid state
readonly class Email {
    public function __construct(public string \$value) {
        if (!filter_var(\$value, FILTER_VALIDATE_EMAIL))
            throw new \InvalidArgumentException("Invalid email: \$value");
    }
}

// 4. Least privilege — each component gets minimum permissions it needs

References

• ↗ https://owasp.org/www-project-secure-coding-practices-quick-reference-guide/
• ↗ https://cheatsheetseries.owasp.org/cheatsheets/Secure_Product_Design_Cheat_Sheet.html

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