Broken Access Control in Grape with Mongodb

Broken Access Control in a Grape API backed by Mongodb often occurs when authorization checks are applied inconsistently or omitted at the resource level. Grape endpoints that map directly to database operations can expose records if the route parameter (e.g., an :id) is used to construct a Mongodb query without validating that the requesting subject has permission to access that specific document.

Consider a Grape endpoint designed to fetch a user profile by ID. If the implementation builds a Mongodb query using the ID from the request without confirming the requesting user owns that document, an attacker can modify the ID to access other users’ data. This is a classic Broken Access Control / BOLA (Broken Level Access) pattern. Inadequate scoping of the query (for example, missing a user_id filter) means the database returns data it should not, and Grape may return it with a 200 status, effectively leaking information.

Another scenario involves privilege escalation where an attacker modifies a request to include an is_admin flag or role array that Grape does not validate before passing query filters to Mongodb. If the backend relies only on client-supplied parameters to construct the query, the attacker can escalate privileges. Even when Grape resources are organized with namespaced helpers, if authorization logic is not consistently applied across nested routes, the attack surface remains large. Because Grape is a REST-focused framework, developers may assume route-level protections are sufficient, but without per-document authorization in the Mongodb query, access controls break down.

Using real-world attack patterns such as IDOR (CVE-2021-29601-like logic flaws) and excessive data exposure, this misalignment between API routing and database filtering becomes critical. The absence of a zero-trust mindset — verifying permissions at the data layer rather than only at the route level — means that even authenticated requests can retrieve or modify data they should not. This is especially risky when endpoints return full documents from Mongodb without masking sensitive fields or enforcing field-level authorization before serialization.

To fix Broken Access Control in Grape with Mongodb, enforce user-level scoping in every query by embedding the subject’s identifier directly into the filter. This ensures that even if an attacker manipulates route parameters, the database will not return records outside their scope.

Example: a scoped endpoint that retrieves a user’s profile only by appending the user_id to the query filter:

require 'mongo'
require 'grape'

client = Mongo::Client.new([ '127.0.0.1:27017' ], :database => 'myapp')

class ProfileResource < Grape::Entity
  expose :id
  expose :email
  expose :display_name
end

class API < Grape::API
  format :json

  helpers do
    def current_user
      # Assume authentication sets current_user with :id and :role
      @current_user ||= { id: 'usr_123', role: 'user' }
    end

    def authorized_user?(user_id)
      current_user && current_user[:id] == user_id
    end
  end

  resource :profiles do
    desc 'Get current user profile with strict user_id scoping'
    params do
      requires :profile_id, type: String, desc: 'The profile ID from the URL'
    end
    get ':profile_id' do
      requested_id = params[:profile_id]
      # Enforce ownership: only return if IDs match
      unless authorized_user?(requested_id)
        error!({ error: 'Forbidden' }, 403)
      end

      # Scoped Mongodb query — includes user_id in the filter
      profile = client[:profiles].find({ _id: requested_id, user_id: current_user[:id] }).first
      error!({ error: 'Not found' }, 404) unless profile
      ProfileResource.represent(profile)
    end

    desc 'List profiles with role-based scoping'
    get do
      if current_user[:role] == 'admin'
        # Admin can query all profiles, but still filter by optional query params
        base_query = client[:profiles]
      else
        # Non-admins only see their own or shared entries
        base_query = client[:profiles].where(user_id: current_user[:id])
      end
      base_query.to_a.map { |doc| ProfileResource.represent(doc) }
    end
  end
end

Key remediation principles:

• Always include the user identifier (or role-derived constraints) in the Mongodb filter to enforce row-level security.
• Validate route parameters against the authenticated subject before using them in queries.
• Avoid returning sensitive fields unless explicitly authorized; consider projection to limit exposure.
• Use parameterized queries and avoid string interpolation to prevent injection and logic bypass.

By combining Grape route helpers with disciplined Mongodb filters, you reduce the risk of IDOR and privilege escalation. This approach aligns with OWASP API Top 10 controls and maps to compliance frameworks such as SOC2 and GDPR, where access restrictions are required. The middleBrick dashboard can surface misconfigurations of this nature during scans, and the Pro plan enables continuous monitoring so that regressions are flagged early.