Imagine this situation: A user logs in successfully to your application, but upon loading their dashboard, they see someone else’s data. Why does this happen? The authentication worked, the session is

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IDOR (Insecure Direct Object Reference) vulnerabilities occur when APIs fetch resources by ID without verifying the requester owns or is permitted to access that resource. Authentication alone is insufficient — authorization must also be enforced. Using Next.js App Router API routes as examples, the tutorial walks through: identifying the vulnerable pattern (fetching by URL param with no ownership check), adding session validation via NextAuth's getServerSession, adding object-level authorization by comparing session user ID to the requested ID, and the safest design pattern — a /api/me endpoint that derives user identity from the session rather than a URL parameter, eliminating the attack surface entirely.

How to Prevent IDOR Vulnerabilities in Next.js API Routes

How to Design Safer Endpoints ( /api/me )