security-by-design

name	security-by-design
description	Defense-in-depth security principles — OWASP Top 10 prevention, input validation, secure error handling, encryption, least privilege
license	MIT

Security-by-Design Skill

Context

Applies to all code handling user input, state persistence, external assets, audio, networking, authentication, authorization, or sensitive configuration. Also applies to infrastructure config, CI/CD pipelines, and security-critical changes.

Aligned with Hack23 AB's ISMS — see Information Security Policy and Secure Development Policy.

Rules

1. Assume Breach — design so a single failure cannot cascade; limit blast radius
2. Defense in Depth — multiple independent layers (validate → sanitize → encode → CSP)
3. Least Privilege — minimum permissions for tokens, workflows, actions, agents
4. Fail Securely — fail-closed; generic error to users, detailed log server-side
5. Never Trust Input — validate, sanitize, and encode every external value (client and server)
6. Secure by Default — disable unused features; explicit opt-in for risky ones
7. Encrypt Everything Sensitive — AES-256 at rest, TLS 1.3+ in transit, SHA-256+ hashing
8. No Secrets in Code — env vars and secrets.* only; rotate on exposure
9. Audit Everything — log auth, authorization, security events; never log secrets/PII
10. Minimize Attack Surface — remove dead features, unused endpoints, unused deps
11. Test Security Controls — cover happy + failure paths (≥ 95 % coverage on security code)
12. Patch Promptly — Critical ≤ 7 d, High ≤ 30 d, Medium ≤ 90 d (Vulnerability Management)
13. Document Decisions — threat models, ADRs, ISMS policy citations in comments/PRs

OWASP Top 10 Quick Mapping

OWASP 2021	Enforcement in this project
A01 Broken Access Control	No client-side authorization; render-based capabilities only
A02 Cryptographic Failures	TLS only, no custom crypto, no plaintext storage of sensitive data
A03 Injection	Validate + encode; never dangerouslySetInnerHTML without DOMPurify
A04 Insecure Design	Threat-model per Threat Modeling Policy; STRIDE for new features
A05 Security Misconfiguration	Security headers (SECURITY_HEADERS.md), secure defaults
A06 Vulnerable Components	npm audit, Dependabot, license gate, SBOMQS ≥ 7.0
A07 Identification/Auth Failures	Not applicable (no user auth in this template) — document when added
A08 Software/Data Integrity	SLSA L3 attestations, SHA-pinned Actions, signed releases
A09 Logging/Monitoring Failures	GitHub audit + Scorecard + CodeQL alerts reviewed
A10 SSRF	Avoid arbitrary URL fetches from untrusted input

Compliance Controls

• ISO 27001:2022 — A.8.25 secure development, A.8.28 secure coding, A.8.29 security testing
• NIST CSF 2.0 — PROTECT (PR.DS, PR.PS, PR.IR), DETECT (DE.CM), RESPOND (RS.AN)
• CIS Controls v8.1 — 2 (software inventory), 16 (app security), 18 (pen testing)

Examples

✅ Input Validation + Encoding

/**
 * Validate plain-text user input.
 * ISMS: Secure Development Policy §Phase 2 — Secure Coding
 */
function validatePlainText(input: unknown, max = 100): string {
  if (typeof input !== 'string') throw new TypeError('Input must be a string');
  const trimmed = input.trim();
  if (trimmed.length === 0) throw new RangeError('Input is required');
  if (trimmed.length > max) throw new RangeError('Input too long');
  return trimmed;
}

/** Encode untrusted text before inserting it into HTML. */
function escapeHtml(s: string): string {
  return s
    .replace(/&/g, '&')
    .replace(/</g, '&lt;')
    .replace(/>/g, '&gt;')
    .replace(/"/g, '&quot;')
    .(, );
}

✅ Secure Error Handling

function handleRequest(): { success?: true; error?: string } {
  try {
    // do work
    return { success: true };
  } catch {
    // In the browser, avoid console.error with raw details — users can see it.
    // Send structured telemetry to restricted server logs instead.
    return { error: 'An error occurred. Please try again.' };
  }
}

✅ Numeric / Boundary Validation

/** ISMS: SDP §Phase 2 — Input Validation */
export function saveHighScore(score: number): void {
  if (!Number.isFinite(score) || score < 0 || score > 1_000_000) {
    throw new RangeError('Invalid score value');
  }
  localStorage.setItem('highScore', String(Math.floor(score)));
}

❌ Anti-Patterns

// BAD: hardcoded secret
const API_KEY = "<API_KEY>";

// BAD: leaking stack trace to user
function handleError(e: Error) { return { error: e.stack }; }

// BAD: no input validation — XSS
document.body.innerHTML = userContent;

// BAD: trusting typeof-less JSON.parse output
const data = JSON.parse(input) as User; // no validation/narrowing!

Validation Checklist

• Every external input is validated + typed + bounded
• Every output to HTML is encoded (or DOMPurify-sanitized)
• No secrets/credentials/tokens in code, logs, or error messages
• Errors fail closed; user message is generic
• Security-sensitive paths have ≥ 95 % test coverage
• New dependencies passed npm audit + license check
• Policy cited in comments/PR for security-relevant changes
• Threat model updated if attack surface changed