Security by Design

OWASP Security Design Principles

Apply these during design -- retrofitting security is 10-100x more expensive.

#	Principle	Architect Action
1	Security by Design	Include security requirements in architecture documents
2	Security by Default	Ship restrictive defaults; require explicit opt-in for relaxed settings
3	Defense in Depth	Layer controls: WAF + input validation + output encoding + parameterized queries
4	Fail Secure	Deny access on error; closed-by-default network policies
5	Least Privilege	Scoped service accounts; time-limited tokens; minimum permissions
6	Compartmentalize	Network segmentation; separate databases per trust level
7	Separation of Duties	Separate deployment approval from code authorship
8	Economy of Mechanism	Minimize attack surface; simple, auditable security code
9	Complete Mediation	Check authorization on every request; no cached auth decisions
10	Open Design	Use published, peer-reviewed algorithms; no security-through-obscurity
11	Least Common Mechanism	Separate admin and user interfaces
12	Psychological Acceptability	Make the secure path the easy path; minimize user friction

STRIDE Threat Modeling

Apply STRIDE to every component in a Data Flow Diagram (DFD). Four questions drive every session:

1. What are we working on? (system model)
2. What can go wrong? (threat identification)
3. What are we going to do about it? (mitigation)
4. Did we do a good enough job? (review)

STRIDE Reference

Threat	Violated Property	Architectural Mitigation
Spoofing	Authentication	MFA, mutual TLS, certificate pinning, OAuth2+PKCE
Tampering	Integrity	Input validation, HMAC, parameterized queries, immutable infra
Repudiation	Non-repudiation	Tamper-evident logging (append-only), digital signatures, SIEM
Info Disclosure	Confidentiality	Encryption at rest+transit, least privilege, generic error messages
Denial of Service	Availability	Rate limiting, circuit breakers, auto-scaling, query complexity limits
Elevation of Privilege	Authorization	Least privilege, RBAC/ABAC, signed tokens verified server-side

STRIDE per DFD Element

DFD Element	Most Relevant Threats
External Entity	Spoofing
Process	All six STRIDE threats
Data Store	Tampering, Info Disclosure, Repudiation, DoS
Data Flow	Tampering, Info Disclosure, DoS
Trust Boundary	Spoofing, Tampering, Elevation of Privilege

Risk Response Options

Response	When	Example
Mitigate	Probable and impactful; controls feasible	Add MFA for spoofing on admin login
Eliminate	Remove feature/component entirely	Remove unused admin API endpoint
Transfer	Better managed by another party	Use managed IdP (Auth0, Cognito)
Accept	Low risk; mitigation cost exceeds impact	Accept DoS risk on internal status page

OWASP Top 10 -- Architectural Prevention

Focus on what the architect decides at design time, not implementation details.

A01: Broken Access Control (61% of breaches)

• Deny by default -- no endpoint open unless explicitly granted
• Centralized authorization service (OPA, Casbin, Cedar), not scattered checks
• Resource-level ownership -- queries scoped to authenticated user
• ABAC over simple RBAC for complex multi-tenant systems
• CORS with explicit origin allowlists -- never wildcards with credentials

A02/A05: Security Misconfiguration (rose to #2 in 2025)

• Infrastructure as Code with security scanning (tfsec, checkov) in CI/CD
• Hardened base images -- minimal containers with security baked in
• Configuration drift detection with automated alerting
• Environment parity -- same hardening across dev/staging/prod

A03: Injection (SQL #2 in CWE Top 25 2025)

• Parameterized queries everywhere -- reject PRs with string concatenation in SQL
• Treat ALL database-sourced data as potentially tainted (second-order injection)
• Allowlisting for dynamic query elements (table names, sort columns)
• Template sandboxing -- user input as DATA, never as template source

A04: Insecure Design (new in 2021)

• Mandate STRIDE analysis as gate for architecture reviews
• Abuse cases alongside every user story ("As an attacker, I want to...")
• State machines with explicit transitions for business logic
• Rate limiting built into architecture from day one

A06/Supply Chain (expanded in 2025)

• SCA scanning on every commit (Snyk, Dependabot)
• SBOM generation as build artifact
• Private package registry; block direct public pulls in production builds
• Lock files committed with integrity hash verification

A07: Authentication Failures

• Centralized IdP (Keycloak, Auth0, Cognito) -- no custom auth per service
• MFA required for sensitive data access and admin functions
• Progressive delays on failed attempts (exponential backoff, not permanent lockout)
• Session ID regeneration on every authentication state change

Secure Architecture Patterns

Zero Trust

Core: "Never trust, always verify" -- no implicit trust from network location.

Component	Implementation
Identity verification	OAuth2/OIDC for users; mTLS for services
Transport security	mTLS everywhere; service mesh (Istio, Linkerd)
Micro-segmentation	Network policies limiting service-to-service
Continuous verification	Re-authenticate and re-authorize every request
Least privilege access	Scoped tokens; just-in-time access

Input Validation Pipeline

User Input
  -> Validation (allowlist, type, length, format)
    -> Business Logic (parameterized queries, ORM)
      -> Output Encoding (context-aware: HTML, JS, URL, CSS)
        -> Client

Key: input validation is complementary, not primary defense. Output encoding prevents XSS. Parameterized queries prevent SQLi. Validation adds defense in depth.

Secrets Management

Pattern	Implementation
Centralized vault	HashiCorp Vault, AWS Secrets Manager, Azure Key Vault
Sidecar injection	Vault agent injects secrets into pods at runtime
Encrypted in repo	SOPS + KMS for GitOps workflows
Pre-commit scanning	gitleaks or TruffleHog blocks secrets before they reach git

Rules: never in source code | never in container images | rotate automatically | separate per environment | audit all access.

Required HTTP Security Headers

Header	Value	Purpose
Content-Security-Policy	Strict nonce-based	Prevent XSS
Strict-Transport-Security	max-age=63072000; includeSubDomains; preload	Force HTTPS
X-Content-Type-Options	nosniff	Prevent MIME sniffing
X-Frame-Options	DENY or SAMEORIGIN	Prevent clickjacking
Referrer-Policy	strict-origin-when-cross-origin	Control referrer
Permissions-Policy	Feature-specific	Limit browser APIs

Security Testing in CI/CD

Type	Tests	Pipeline Stage	Tools
SAST	Source code vulnerabilities	Every commit/PR	Semgrep, CodeQL, SonarQube
SCA	Dependency CVEs	Build stage	Snyk, Dependabot, Trivy
DAST	Running application	Staging	OWASP ZAP, Burp Suite
Secrets	Leaked credentials	Pre-commit + CI	gitleaks, TruffleHog

Strategy: SAST pre-commit (fast) -> SCA at build -> container scan -> DAST on staging -> block on critical/high findings.

API Security Checklist (OWASP API Top 10)

#	Threat	Architect Decision
API1	BOLA (~40% of API attacks)	Object-level authorization in service layer, not just routes
API4	Unrestricted resource consumption	Rate limiting per-object, not just per-endpoint
API5	BFLA	Separate admin and user API surfaces
API6	Sensitive business flow abuse	State machine enforcement, idempotency keys
API9	Improper inventory management	API versioning with sunset policies; deprecate old versions
API10	Unsafe API consumption	Never trust third-party API responses; validate and sanitize

GraphQL-Specific

• Disable introspection in production
• Set maximum query depth (10 levels) and complexity limits
• Limit batch size; exclude sensitive operations from batching
• Field-level authorization in resolvers

Defensive Coding Patterns (Cross-Reference for Crafters)

Risk	Vulnerable Pattern	Secure Alternative
SQL Injection	f"SELECT * FROM users WHERE name = '{name}'"	Parameterized: cursor.execute("... WHERE name = %s", (name,))
Deserialization	pickle.loads(untrusted)	json.loads(untrusted) or Pydantic schema validation
Command Injection	os.system(f"cmd {input}")	subprocess.run(["cmd", input], shell=False)
SSTI	Template(user_input)	env.from_string(trusted).render(data=user_input)
Mass Assignment	User(**request.json())	DTO with explicit fields, server-set for sensitive
TOCTOU Race	Read-check-write in separate steps	Atomic conditional update (UPDATE ... WHERE condition)
Prototype Pollution (JS)	Object.assign(target, untrusted)	Map, null-prototype objects, strict schema validation