Broken Access Control in Nestjs with Mongodb

Broken Access Control in a Nestjs application using Mongodb often arises when authorization checks are performed in application code after data has already been retrieved, or when query filters rely on user-supplied identifiers without proper validation and scoping. Nestjs does not automatically enforce object-level permissions on data returned from Mongodb; it is the developer’s responsibility to ensure that every resolver, service, or gateway applies the correct authorization logic before passing data to the client.

At a practical level, a developer might define a Mongodb repository in Nestjs and call Model.find() with an _id taken directly from request parameters. If the parameter is used without confirming that the authenticated user has rights to that specific document, an attacker can change the ID to access another user’s data. Because Mongodb queries are constructed dynamically in JavaScript/TypeScript, missing filters or incomplete $in checks can allow broader data exposure than intended.

The combination of Nestjs’s dependency injection and Mongoose or native Mongodb drivers can inadvertently encourage patterns where permissions are assumed rather than enforced. For example, a service layer might expose a method like getUserProfile(userId: string) and rely on the caller to ensure the caller’s session matches the userId. Without an explicit ownership or role check tied to the Mongodb query, the method returns data regardless of permissions, resulting in Insecure Direct Object References (IDOR) or Broken Object Level Authorization (BOLA).

Additionally, field-level or property-level authorization can be incomplete. A user may be allowed to see a subset of fields (e.g., non-sensitive profile data) but the Mongodb projection is missing or misconfigured, causing the API to return full documents that include roles, permissions, or internal flags. This misconfiguration is common when iterating quickly and forgetting to restrict projection in find or aggregate pipelines, effectively exposing more information than the access control model intends.

Because middleBrick scans unauthenticated attack surfaces, it can detect endpoints where IDs are used without scoped filters, where responses leak sensitive fields, and where rate limiting is absent, all of which amplify the impact of Broken Access Control. The scanner maps findings to OWASP API Top 10, highlighting BOLA/IDOR and Property Authorization issues with severity and remediation guidance to help teams tighten authorization in the Nestjs + Mongodb stack.

Remediation centers on enforcing authorization at the database query level and consistently applying the principle of least privilege to both data access and response content. The following examples assume an authenticated user context available as userId and use Mongoose for clarity; equivalent filters can be applied with the native driver.

1. Scope queries by ownership or role

Always include a user or role filter in every Mongodb operation. Avoid passing raw IDs without verifying access.

import { Injectable } from '@nestjs/common';
import { InjectModel } from '@nestjs/mongoose';
import { Model } from 'mongoose';
import { UserDocument } from './user.schema';

@Injectable()
export class ProfilesService {
  constructor(@InjectModel('User') private readonly userModel: Model) {}

  async getProfileSafe(userId: string, requesterId: string) {
    // Ensure the requester can only fetch their own profile unless admin
    const isAdmin = await this.userModel.exists({ _id: requesterId, roles: 'admin' });
    const filter = { _id: userId };
    if (!isAdmin) {
      filter['ownerId'] = requesterId;
    }
    return this.userModel.findOne(filter).exec();
  }
}

2. Use strict projection to limit returned fields

Explicitly define which fields are returned to prevent accidental exposure of sensitive data.

async getPublicFields(userId: string) {
  return this.userModel
    .findById(userId, 'username email avatar createdAt')
    .lean()
    .exec();
}

3. Validate and sanitize input IDs

Ensure IDs conform to expected formats and are not used to traverse relationships unintentionally.

import { Types } from 'mongoose';

function isValidId(id: string): boolean {
  return Types.ObjectId.isValid(id);
}

async safeFindById(id: string, requesterId: string) {
  if (!isValidId(id)) {
    throw new Error('Invalid identifier');
  }
  return this.getProfileSafe(id, requesterId);
}

4. Apply property-level authorization at query construction

For sensitive fields, enforce visibility rules directly in the filter or aggregation pipeline.

async getLimitedUserRecord(recordId: string, viewerId: string) {
  return this.userModel.aggregate([
    { $match: { _id: new Types.ObjectId(recordId), ownerId: viewerId } },
    { $project: {
        username: 1,
        email: 1,
        isActive: 1,
        passwordHash: 0,
        apiKeys: 0,
        __v: 0
      }
    }
  ]).exec();
}

5. Centralize authorization logic

Use guards or decorators to avoid repeating checks across endpoints. Combine with middleware to ensure every request validates scope before data is fetched.

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';
import { Observable } from 'rxjs';

@Injectable()
export class OwnershipGuard implements CanActivate {
  constructor(private readonly userService: UserService) {}

  canActivate(context: ExecutionContext): boolean | Promise<boolean> | Observable<boolean> {
    const request = context.switchToHttp().getRequest();
    const userId = request.user.id;
    const targetId = request.params.id;
    return this.userModel.exists({ _id: targetId, ownerId: userId });
  }
}

By combining strict query filters, explicit projections, input validation, and centralized guards, teams can mitigate Broken Access Control in Nestjs with Mongodb. These practices align with OWASP API Top 10 BOLA/IDOR guidance and help ensure that even if an ID is guessed or leaked, unauthorized access is prevented at the persistence layer.