How do you design secure APIs against web vulnerabilities?

Long Answer

APIs are the backbone of modern web services, and poor security design exposes businesses to massive risk. My approach is layered: secure entry (authentication), controlled use (authorization), strict data handling (input validation), and systemic resilience against common threats.

1) Authentication

Authentication ensures callers are who they claim to be. I favor OAuth 2.0 and OpenID Connect for delegated identity, combined with JWT access tokens bound to scopes and expirations. For sensitive B2B or internal APIs, I add mTLS so both client and server authenticate via certificates. Short-lived tokens reduce replay risk; refresh tokens rotate securely. Strong password policies and MFA apply where direct user auth is required.

2) Authorization

Even after identity is verified, fine-grained authorization governs what actions are allowed. I apply RBAC (roles like “admin”, “editor”) combined with ABAC (attributes like department, device, time). APIs enforce least privilege by granting only minimal required scopes. Multi-tenant systems partition data strictly by tenant IDs and ensure queries cannot leak across boundaries. For sensitive endpoints, I enforce step-up authentication (e.g., re-auth for payments).

3) Input validation

Every external input is hostile until proven safe. I implement schema-based validation (e.g., JSON Schema, Joi, Yup) to enforce data types, lengths, and allowed values. Whitelisting is safer than blacklisting. All dynamic inputs flow into parameterized queries or ORM bindings to block SQL injection. On the output side, I encode responses to prevent reflected XSS. File uploads undergo MIME checks, virus scanning, and size limits.

4) Protecting against common vulnerabilities

I align API security with the OWASP Top 10:

• Injection (SQL, NoSQL, LDAP): always use prepared statements.
  
• Broken Auth/Session: use secure cookies (HttpOnly, SameSite), rotate keys, and expire tokens.
  
• XSS: encode/escape outputs, sanitize inputs.
  
• CSRF: enforce CSRF tokens for state-changing requests, check Origin/Referer.
  
• Rate Limiting & DoS: apply API gateways (Kong, Apigee, NGINX) with per-IP/user quotas, exponential backoff, and circuit breakers.
  
• CORS: default-deny origins, allow only trusted domains, and restrict credentials.
  
• Error Handling: return generic error messages to clients, log details securely for developers.
  

5) Secure API design patterns

I adopt zero-trust principles: each API call authenticates independently. Secrets (API keys, credentials) are stored in vaults like HashiCorp Vault or AWS Secrets Manager, never hardcoded. Endpoints enforce HTTPS/TLS 1.2+, HSTS headers, and disable weak ciphers. Pagination and max limits prevent data over-exposure. Sensitive data is encrypted at rest (AES-256) and in transit (TLS).

6) Monitoring, logging, and testing

Security without visibility is blind. I log access attempts, failed auth, and abnormal usage. Structured logs feed into SIEM systems (Splunk, ELK, Datadog). Alerts trigger on spikes, anomalies, or suspicious patterns. Testing includes unit tests, integration tests, and fuzzing for malformed inputs. Regular penetration testing and threat modeling exercises keep APIs resilient. Automation enforces dependency patching and secret scanning in CI/CD pipelines.

7) Governance and compliance

Finally, secure APIs must align with compliance standards: GDPR, HIPAA, PCI-DSS. Data minimization ensures only required fields are collected and stored. Audit trails are immutable. Access control policies are documented and reviewed periodically.

In essence, secure API design is not a single tactic but a multi-layer strategy combining strong identity, strict data validation, least-privilege authorization, protection against OWASP threats, and continuous monitoring. This transforms APIs from attack surfaces into resilient, trusted platforms.

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Table

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Common Mistakes

• Using API keys without expiration or rotation, making them easy targets.
  
• Relying only on role-based checks without context-aware rules.
  
• Accepting input without schema validation, exposing SQL/NoSQL injection risks.
  
• Returning verbose error messages that leak stack traces or schema info.
  
• Misconfigured CORS allowing * with credentials, opening cross-site attacks.
  
• Granting unlimited token approvals or global admin scopes by default.
  
• Not applying rate limiting, allowing brute force or denial of service.
  
• Logging sensitive data like passwords or tokens in plaintext.
  
  

Sample Answers

Junior:
“I secure APIs with OAuth2.0 tokens for auth and check user roles for permissions. I validate all inputs with schemas and sanitize strings. I use parameterized queries and add rate limits to reduce brute force risk.”

Mid:
“I combine OIDC for authentication with RBAC + tenant checks for authorization. Inputs go through JSON Schema validation. I enforce HTTPS/TLS, set strict CORS, and apply CSRF tokens on state changes. Alerts are configured for suspicious traffic and failures.”

Senior:
“My strategy is layered: OAuth2.0/OIDC + mTLS, ABAC/RBAC with least privilege, strict schema validation, and zero-trust design. I mitigate OWASP Top 10 risks with parameterized queries, CSRF tokens, rate limiting, and strict error handling. Monitoring flows into SIEM with automated alerting, and CI/CD enforces secret scanning and patch compliance.”

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Evaluation Criteria

Strong candidates show awareness of layered security: authentication, authorization, input validation, and OWASP defenses. They should mention OAuth2.0/OIDC, JWTs, RBAC/ABAC, schema validation, CSRF, rate limiting, and CORS. Advanced answers include mTLS, secret vaults, logging/monitoring, penetration testing, and compliance alignment. Red flags: vague answers (“use SSL”), ignoring input validation, or proposing only API keys. The best answers highlight principle of least privilege, zero trust, and continuous monitoring—not one-time fixes.

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Preparation Tips

• Build a small Node.js/Express API and add JWT-based authentication with refresh tokens.
  
• Implement RBAC and test least privilege scenarios.
  
• Add Joi or Yup validation middleware; attempt SQL injection to confirm it fails.
  
• Enable CORS correctly: only trusted origins, no wildcards with credentials.
  
• Configure rate limiting and API gateway quotas.
  
• Run OWASP ZAP against your API to see vulnerabilities.
  
• Integrate Sentry or Datadog for error monitoring.
  
• Document an escalation process if repeated failed logins occur.
  
• Practice a 60-second explainer connecting auth, validation, and OWASP defense.
  
  

Real-world Context

A fintech startup once exposed an API with unlimited token scopes. Attackers drained funds by exploiting overbroad approvals. After migrating to least-privilege scopes and Permit2 patterns, risk was reduced dramatically. An e-commerce firm suffered SQL injection from unvalidated parameters; schema validation and parameterized queries solved the issue. A healthcare SaaS provider misconfigured CORS, exposing patient data; tightening policies and enforcing mTLS closed the hole. Another media platform reduced downtime by routing all API logs into SIEM and setting automated anomaly alerts, catching brute-force attempts within minutes. These real-world stories prove that combining auth, validation, and layered defenses prevents catastrophic incidents.

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Key Takeaways

• Use OAuth2.0/OIDC or mTLS for strong authentication.
  
• Apply RBAC + ABAC for least-privilege authorization.
  
• Enforce strict schema validation and parameterized queries.
  
• Mitigate OWASP Top 10 risks: CSRF, XSS, SQLi, DoS.
  
• Monitor continuously and align with compliance standards.
  
  

Practice Exercise

Scenario:
You are designing a payments API for a SaaS product that must handle thousands of daily transactions securely.

Tasks:

1. Implement OAuth2.0 with JWTs (short-lived) for authentication. Add MFA on sensitive endpoints.
   
2. Apply RBAC/ABAC so admins, users, and partners have scoped actions. Test tenant isolation.
   
3. Enforce JSON Schema validation on all inputs; run injection attempts to confirm rejection.
   
4. Secure against CSRF (tokens), XSS (output encoding), and SQLi (parameterized queries).
   
5. Configure rate limiting (per IP and per user) and logging to SIEM.
   
6. Set strict CORS with trusted origins.
   
7. Document a runbook: if anomalies or SEV1 issues occur, escalation → on-call engineer → security lead → incident call.
   
8. Run penetration testing before go-live and enforce CI/CD scans for secrets and dependencies.
   
   

Deliverable:
A secure, resilient API with strong auth, scoped authorization, validated inputs, OWASP protections, and a documented escalation path—ready for production at scale.