Fix Logic Flow Bypass in Blitz.js

Blitz.js RPC layer is a prime target for logic flow bypasses. Developers often mistake the 'Zero-API' abstraction for 'Zero-Security'. If you aren't strictly enforcing session-to-resource ownership at the resolver level, an attacker can manipulate input parameters (Insecure Direct Object Reference) to bypass intended business logic and escalate privileges or modify unauthorized data.

The Vulnerable Pattern

import { resolver } from '@blitzjs/rpc';
import db from 'db';
export default resolver.pipe(
async ({ id, …data }) => {
// CRITICAL: No authorization middleware used.
// Attacker can pass any ‘id’ in the JSON-RPC payload to update any user.
const user = await db.user.update({
where: { id },
data,
});
return user;
}
);

The Secure Implementation

The vulnerability stems from trusting the client-side payload (`id`) as the source of truth for the database query. In the vulnerable snippet, the mutation ignores the server-side session context, allowing any authenticated (or even unauthenticated) user to target any record. The secure implementation uses `resolver.authorize()` to verify the session and then performs a manual identity check. By comparing the requested `id` against `ctx.session.userId`, we close the logic gap, ensuring users can only interact with their own data regardless of the input payload.

import { resolver } from '@blitzjs/rpc';
import db from 'db';
import { AuthorizationError } from 'blitz';
export default resolver.pipe(
resolver.authorize(), // Step 1: Ensure session exists
async ({ id, …data }, ctx) => {
// Step 2: Enforce strict ownership flow
// Never trust the ‘id’ from the client payload for identity
if (id !== ctx.session.userId) {
throw new AuthorizationError(‘Logic Bypass Detected: Identity Mismatch’);
}
const user = await db.user.update({
  where: { id: ctx.session.userId }, // Use session-bound ID
  data,
});
return user;
}
);