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Fix Business Logic Errors in Blitz.js

In Blitz.js, the abstraction of the API layer often leads developers into a false sense of security. Business logic errors typically arise when mutations trust client-side inputs—like user IDs or roles—without validating them against the server-side session context. This opens the door for Insecure Direct Object References (IDOR) and unauthorized data manipulation via the RPC layer.

The Vulnerable Pattern

import { resolver } from 'blitz';
import db from 'db';
import * as z from 'zod';

const UpdateUser = z.object({ id: z.number(), bio: z.string() });


export default resolver.pipe(
resolver.zod(UpdateUser),
async ({ id, bio }) => {
// VULNERABILITY: Any authenticated (or even unauthenticated) user
// can pass any ‘id’ to update someone else’s profile.
const user = await db.user.update({
where: { id },
data: { bio },
});
return user;
}
);

The Secure Implementation

The vulnerable code is a textbook IDOR. Because the 'id' is part of the payload, an attacker can intercept the RPC call and swap their ID for a victim's ID. The fix involves two critical steps: 1) Using 'resolver.authorize()' to ensure the request is authenticated, and 2) sourcing the 'userId' directly from 'ctx.session'. By decoupling the target record's identity from the user-controllable input, you eliminate the possibility of cross-user data tampering.

import { resolver, AuthorizationError } from 'blitz';
import db from 'db';
import * as z from 'zod';

const UpdateUser = z.object({ bio: z.string() });


export default resolver.pipe(
resolver.zod(UpdateUser),
resolver.authorize(),
async ({ bio }, ctx) => {
// FIX: Retrieve the ID from the secure session context (ctx.session)
// instead of trusting the input object.
const userId = ctx.session.userId;


if (!userId) throw new AuthorizationError();

const user = await db.user.update({
  where: { id: userId },
  data: { bio },
});
return user;


}
);

System Alert • ID: 9042
Target: Blitz.js API
Potential Vulnerability

Your Blitz.js API might be exposed to Business Logic Errors

74% of Blitz.js apps fail this check. Hackers use automated scanners to find this specific flaw. Check your codebase before they do.

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