Your Next.js API Route Is Leaking Diagnostics in Its 400 Responses

A data export endpoint dumps system diagnostics when it hits an invalid field. Feed it garbage, read the debug output, grab the flag. A data export feature lets you pick which profile fields to download. The UI only offers valid fields through checkboxes, so everything looks locked down. But the API behind it accepts arbitrary field names -- send it one it doesn't recognize, and instead of a clean error, it dumps full system diagnostics including internal feature flags. That's where the flag is. You'll bypass the frontend, hit the endpoint directly, and read what comes back. Lab setup Start the lab: npx create-oss-store@latest Or with Docker (no Node.js required): docker run -p 3000:3000 leogra/oss-oopssec-store The app runs at http://localhost:3000 . What you're targeting The app has a profile page at /profile with a Data Export tab. It lets users download their own data in JSON or CSV by selecting fields through checkboxes ( User ID , Email , Role , Address ID ) and clicking "Export Data". The UI looks safe -- you can only pick from a fixed set of valid fields, so there's no way to submit an invalid one through the browser. But that's just client-side validation. The endpoint behind it is POST /api/user/export , and it accepts a JSON body with two parameters: { "format" : "json" , "fields" : [ "id" , "email" , "role" ] } The fields value is an array of strings. The API checks each field against an allowlist. Valid fields? You get your data back. Invalid fields? The API throws an error -- and that error says way too much. Step-by-step exploitation 1. Log in You need an authenticated session. Use one of the seeded accounts: Email: alice@example.com Password: iloveduck Log in through the UI at /login , or grab a session cookie via curl: curl -c cookies.txt -X POST http://localhost:3000/api/auth/login \ -H "Content-Type: application/json" \ -d '{"email":"alice@example.com","password":"iloveduck"}' 2. Explore the Data Export tab Go to /profile and click the Data Export