Fix BFLA (Broken Function Level Authorization) in Meteor

Meteor's DDP protocol makes server methods globally accessible. BFLA (OWASP API5:2023) occurs when developers assume UI-level restrictions equal security. If your 'Meteor.methods' don't explicitly verify the caller's identity and permissions on the server, any authenticated (or even unauthenticated) user can invoke administrative functions by guessing the method name and firing a DDP message from the browser console.

The Vulnerable Pattern

Meteor.methods({
  'admin.deleteUser'(targetId) {
    // VULNERABILITY: No check to see if the caller is an admin.
    // Any user can run: Meteor.call('admin.deleteUser', 'victim-id-here')
    Meteor.users.remove(targetId);
  }
});

The Secure Implementation

The fix involves moving from implicit trust to zero-trust server-side validation. First, use 'this.userId' to identify the requester—this is populated by Meteor's session and cannot be spoofed by the client. Second, implement a Role-Based Access Control (RBAC) check using a package like 'alanning:roles' to verify the user's scope. Finally, ensure all arguments are sanitized with 'check()' to prevent NoSQL injection or type confusion attacks. Never expose sensitive logic in methods without these gates.

import { Roles } from 'meteor/alanning:roles';
Meteor.methods({
‘admin.deleteUser’(targetId) {
const callerId = this.userId;
// 1. Authentication check
if (!callerId) {
  throw new Meteor.Error('not-authorized', 'Must be logged in');
}

// 2. Authorization check (BFLA Fix)
const isAdmin = Roles.userIsInRole(callerId, ['admin'], Roles.GLOBAL_GROUP);
if (!isAdmin) {
  throw new Meteor.Error('forbidden', 'You do not have permission to perform this action');
}

// 3. Input validation
check(targetId, String);

return Meteor.users.remove(targetId);
}
});