Introduction

APIs are the backbone of modern applications, making API security crucial. Proper authentication and authorization are fundamental to protecting your API endpoints from unauthorized access.

Exposed or poorly secured APIs are among the most common attack vectors for data breaches. The OWASP API Security Top 10 lists broken object-level authorization and broken authentication as the top two API security risks , both of which stem from inadequate implementation rather than exotic attack techniques. Building API security correctly from the start is significantly easier than retrofitting it after an incident.

Authentication Methods

There are several authentication methods for APIs including API keys, OAuth 2.0, JWT tokens, and Basic Authentication. Each has its use cases and security considerations.

Choosing the right authentication method depends on who is calling your API and in what context. Server-to-server integrations are best served by API keys or client credentials OAuth flows , there's no user involved. User-facing applications should use OAuth 2.0 with the authorization code flow (with PKCE for SPAs and mobile apps) rather than storing user credentials in the client. Basic authentication over HTTPS is acceptable for internal tools but should never be used in public-facing APIs.

API Keys: Simple, good for server-to-server, but must be treated as secrets and rotated regularly
JWT (Bearer tokens): Stateless, self-contained, ideal for user authentication in stateless APIs
OAuth 2.0: Delegated authorization, ideal when third-party apps need access on behalf of users
mTLS (Mutual TLS): Strongest option for service-to-service authentication in zero-trust architectures

JWT Token Security

JSON Web Tokens (JWT) are widely used for API authentication. Implement proper token expiration, refresh tokens, and secure token storage. Always use HTTPS and validate token signatures.

JWTs are powerful but frequently misimplemented. A JWT contains three parts: a header specifying the algorithm, a payload with claims (user ID, roles, expiry), and a signature. The signature must be verified on every request , never trust JWT claims without verifying the signature. Use asymmetric algorithms (RS256, ES256) for tokens that may be verified by multiple services, so the signing private key doesn't need to be shared.

Set short expiration times on access tokens (15 minutes to 1 hour) to limit the damage of token theft
Never store JWTs in localStorage , use httpOnly cookies to protect against XSS attacks
Include only necessary claims in the payload , JWTs are base64 encoded, not encrypted, and visible to the holder
Implement a token blocklist for logout and revocation, stored in Redis for fast lookup

OAuth 2.0 Implementation

OAuth 2.0 is the industry standard for authorization. Understand the different grant types (authorization code, client credentials, etc.) and implement them correctly based on your use case.

OAuth 2.0 is a framework, not a protocol , it defines patterns without specifying implementation details, which creates room for insecure implementations. Always use the authorization code flow with PKCE (Proof Key for Code Exchange) for public clients (SPAs, mobile apps). Never use the implicit flow , it was deprecated in OAuth 2.1 due to security weaknesses. For first-party apps that own both the frontend and backend, the authorization code flow with PKCE remains the correct choice even though it requires more implementation effort.

Use PKCE for all public clients to prevent authorization code interception attacks
Validate the state parameter on OAuth redirects to prevent CSRF attacks
Restrict redirect URIs to exact, pre-registered values , never allow wildcard or open redirectors
Use short-lived authorization codes (10 minutes max) that can only be exchanged once

Rate Limiting and Throttling

Implement rate limiting to prevent abuse and DDoS attacks. Set appropriate limits based on user tiers and use token bucket or sliding window algorithms.

Rate limiting protects your API from both malicious abuse and unintentional overuse (a misconfigured client in an infinite loop can be as damaging as an attacker). Apply rate limits at multiple levels: per IP for unauthenticated endpoints, per API key or user for authenticated endpoints, and per endpoint for sensitive operations (like authentication) that warrant stricter limits. Return 429 Too Many Requests with a Retry-After header so legitimate clients can implement exponential backoff.

Apply stricter rate limits to sensitive endpoints: authentication, password reset, and account creation
Use the sliding window algorithm for smoother rate limiting that prevents burst exploitation at window boundaries
Implement distributed rate limiting with Redis for multi-instance API deployments
Whitelist IP ranges for trusted internal services to avoid rate limiting your own infrastructure

API Monitoring and Logging

Monitor API usage patterns, log all access attempts, and set up alerts for suspicious activities. Track authentication failures and implement account lockout mechanisms.

Comprehensive API logging is essential for both security incident response and performance optimization. Log every authenticated request with the user ID, endpoint, HTTP method, response status, and latency. This data enables you to detect anomalies (unusual access patterns, bulk data exports, repeated authentication failures) and reconstruct the sequence of events during a security incident. Store logs in an immutable, centralized system so they cannot be tampered with if an attacker gains access to your application servers.

Use structured logging (JSON format) to make logs machine-parseable by SIEM and log analysis tools
Set up real-time alerts for authentication failure spikes, which indicate brute-force or credential stuffing attacks
Implement distributed tracing (OpenTelemetry) to correlate requests across multiple API services
Retain security-relevant logs for at least 90 days to support incident investigation timelines