API Security Best Practices
APIs are the backbone of modern applications, and they are prime targets for attackers. A poorly secured API can expose sensitive data, enable account takeover, or bring down your entire service. This guide covers the essential security practices every API should implement.
Authentication: Proving Identity
Every API must verify who is making the request. Common approaches include API keys (simple but limited), OAuth 2.0 bearer tokens (standard for user-facing APIs), and mutual TLS (for service-to-service communication). Never use basic authentication over plain HTTP — always require HTTPS. Use the Password Generator to create strong API keys and secrets.
Authorisation: Enforcing Permissions
Authentication is not enough — you must also check that the authenticated user has permission to perform the requested action. Broken Object Level Authorisation (BOLA) is the number one API vulnerability in the OWASP API Security Top 10: attackers change IDs in requests (e.g., /api/users/42 → /api/users/43) to access other users' data. Always verify ownership on every request, not just at the endpoint level.
Input Validation
Validate all input on the server side — type, length, format, and range. Reject unexpected fields. Use schema validation (JSON Schema, OpenAPI) to enforce the expected request shape. Never trust client-side validation alone. Validate your API's JSON request and response schemas with the JSON Formatter to ensure they conform to the expected structure.
Rate Limiting and Throttling
Rate limiting prevents abuse — brute-force attacks, credential stuffing, and denial of service. Implement rate limits per user, per IP, and per endpoint. Return 429 Too Many Requests with a Retry-After header. Use sliding window or token bucket algorithms. Key endpoints like login and password reset should have stricter limits than read-only endpoints.
The OWASP API Security Top 10
The OWASP API Security Top 10 identifies the most critical API risks: API1 Broken Object Level Authorisation, API2 Broken Authentication, API3 Broken Object Property Level Authorisation, API4 Unrestricted Resource Consumption, API5 Broken Function Level Authorisation, API6 Unrestricted Access to Sensitive Business Flows, API7 Server Side Request Forgery, API8 Security Misconfiguration, API9 Improper Inventory Management, API10 Unsafe Consumption of APIs.
Transport Security
Always use HTTPS (TLS 1.2+). Disable older TLS versions and weak cipher suites. Use HSTS headers to prevent downgrade attacks. For service-to-service communication, consider mutual TLS (mTLS) where both client and server present certificates. Use the Hash Generator to compute checksums when verifying certificate fingerprints.
Error Handling and Logging
Never expose stack traces, database errors, or internal details in API error responses. Return generic error messages with appropriate HTTP status codes. Log all authentication failures, authorisation violations, and validation errors for monitoring and incident response. Include correlation IDs in logs for tracing requests across services.
Security Headers and CORS
Configure CORS to allow only your frontend domains — never use Access-Control-Allow-Origin: * with credentials. Set security headers: X-Content-Type-Options: nosniff, X-Frame-Options: DENY, and Content-Security-Policy. Remove headers that reveal server technology (X-Powered-By, Server). Use the cURL Converter to test your API endpoints and verify that security headers are present in responses.