API security failures are rarely exotic. The recurring issues are missing authorization checks, excessive data exposure, and inadequate rate limiting — all straightforward to prevent and easy to overlook.
Authentication establishes who is calling; authorization establishes what they may do. Broken object-level authorization — where a valid token can access another user's record by changing an ID — is among the most common serious API vulnerabilities. Every endpoint that accepts an identifier must verify the caller owns or may access that object.
Do not return more than the client needs. Serializing an entire database record and letting the frontend display a subset means the extra fields are still on the wire and visible to anyone inspecting traffic. Define explicit response shapes.
Validate input at the boundary against a schema, and reject rather than coerce. Accepting unexpected fields and passing them into an update operation is how mass-assignment vulnerabilities occur.
Rate limit per authenticated identity, not only per IP address. Limits should apply to expensive operations specifically — search, export, and report generation — rather than uniformly across all endpoints.
Treat every API as versioned from the first release. Changing behaviour under clients that depend on it is a reliability problem as much as a security one, and versioning gives you a path to deprecate insecure patterns.
Log authentication failures, authorization denials, and unusual access patterns. These are the signals that reveal credential stuffing and enumeration attempts before they succeed.
Finally, keep the specification and the implementation in sync. A generated specification that reflects real routes and schemas prevents undocumented endpoints from quietly remaining exposed.