API Security Best Practices
Your API is the front door to your data. Here's how to make sure only the right people get in.
APIs are the backbone of modern software — and the most common attack surface. A single misconfigured endpoint can expose customer data, allow unauthorized actions, or enable denial-of-service attacks. API security isn't an add-on; it's a fundamental design concern.
This guide covers practical API security for custom software: authentication, authorization, input validation, rate limiting, and the common vulnerabilities that cause real-world breaches. No abstract theory — just actionable practices for production APIs.
Authentication: Proving Identity
Authentication answers "who are you?" The three common approaches for APIs are API keys (simple, good for server-to-server), JWT tokens (stateless, good for user authentication), and OAuth 2.0 (delegated authorization, good for third-party integrations).
API keys are the simplest but least secure for user-facing APIs. They're appropriate for server-to-server communication where both sides are trusted. For user authentication, use JWT tokens issued after login, with short expiration times (15-30 minutes) and refresh token rotation.
Never send credentials in URL parameters — they end up in server logs and browser history. Always use HTTPS. Always hash and salt passwords with bcrypt or Argon2 (not MD5, not SHA-256 alone). This sounds obvious, but credential handling mistakes are still among the most common API vulnerabilities.
API Keys
Best for server-to-server. Rotate regularly, scope per-service, and never commit to version control.
JWT Tokens
Best for user auth. Short-lived access tokens (15 min) with refresh token rotation.
OAuth 2.0
Best for third-party integrations. Delegate authorization without sharing credentials.
Authorization: Controlling Access
Authorization answers "what are you allowed to do?" Even with valid authentication, users should only access their own data and perform actions their role permits. This seems obvious but is the source of more vulnerabilities than any other category.
Implement authorization at the business logic layer, not just the route layer. Checking "is this user an admin?" at the route is not enough — you also need "does this admin have permission to access this specific resource?" Broken object-level authorization (BOLA) is the #1 API security vulnerability according to OWASP.
Use role-based access control (RBAC) as a minimum. For complex scenarios, attribute-based access control (ABAC) or policy engines (like Open Policy Agent) provide more granular control.
Input Validation and Injection Prevention
Never trust client input. Every piece of data from an API request — body, query parameters, headers, and URL parameters — must be validated before processing. Use schema validation (Zod, Joi, or JSON Schema) to define what valid input looks like and reject everything else.
SQL injection, NoSQL injection, and command injection are all caused by the same root problem: unsanitized user input reaching a query or command. Use parameterized queries (never string concatenation), ORM/query builders, and input sanitization to prevent injection attacks.
Validate content types, file uploads, and request sizes. A file upload endpoint without size limits is a denial-of-service vulnerability. An endpoint that accepts any content type is an injection vector.
Rate Limiting and Abuse Prevention
Every API endpoint needs rate limiting. Without it, a single client can overwhelm your server, scrape your data, or brute-force authentication. Implement rate limiting at multiple levels: per-IP, per-user, and per-endpoint.
Common rate limiting patterns: token bucket (allows bursts), sliding window (smooth distribution), and fixed window (simplest to implement). Redis is the typical backing store for distributed rate limiting.
Log and alert on rate limit violations. Repeated rate limiting from a single source often indicates automated abuse — credential stuffing, data scraping, or vulnerability scanning.
Frequently Asked Questions
What's the most common API security vulnerability?
Should we use API keys or JWT tokens?
How often should we rotate API keys?
Do we need a WAF for API security?
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