Insecure Design in Grape with Mongodb

Insecure Design occurs when application architecture and data-flow decisions do not enforce authorization and validation at every layer. When a Grape API uses MongoDB as the data store, several design patterns common in Ruby services can unintentionally expose data or enable privilege escalation.

Grape encourages building resource-centric endpoints with minimal automatic safeguards. If endpoints are defined without explicit scoping of data ownership, it is possible to construct routes where an identifier provided by the client (e.g., a project ID) is directly used to query MongoDB without confirming that the requesting subject has rights to that resource. Because MongoDB queries are built programmatically, it is easy to concatenate user input into a hash that becomes the query filter, and if keys like project_id or user_id are omitted from the filter, the query may return records belonging to other users.

A second design weakness arises from how MongoDB’s flexible schema interacts with Grape’s parameter coercion. Developers may store roles, permissions, or tenant identifiers as fields in each document. If the application does not enforce a rule that every query includes a tenant or role filter, a search intended to be scoped to the current tenant can inadvertently match documents from other tenants. This becomes a Broken Object Level Authorization (BOLA) / Insecure Direct Object Reference (IDOR) pattern: the object exists, the ID is predictable, and the endpoint lacks a mandatory ownership or access check.

Additionally, insecure design choices can appear in how indexes and query shapes are defined. For example, creating an index that supports performance but does not align with the authorization filter can make it easier to accidentally perform a collection scan, increasing the risk of data exposure through enumeration. If the API also lacks rate limiting at the design level, attackers can iterate over identifiers more quickly than intended, combining BOLA with a lack of rate control.

Because middleBrick scans test the unauthenticated attack surface and include BOLA/IDOR and Property Authorization checks, it can surface these design-level gaps by correlating endpoint behavior with the OpenAPI specification and runtime responses. The scanner does not fix the code, but its findings and remediation guidance help developers adjust the design so that authorization is embedded in the data-access layer rather than treated as an optional parameter.

To secure a Grape API that uses MongoDB, ensure every query explicitly includes tenant, ownership, or user scope, and validate input against a strict schema before building the filter. Below are concrete patterns and code examples.

1. Always include a user or tenant scope in the filter

Never build a MongoDB query from client-supplied IDs without also injecting the authorized subject’s identifier. Use a before block to resolve the current user and merge it into the query hash.

require 'mongo'

class AuthorizedBase
  def initialize(collection, current_user)
    @collection = collection
    @current_user = current_user
  end

  def scoped_collection
    @collection.find('user_id' => @current_user.id)
  end
end

class ProjectsResource
  include Grape::API

  before do
    # Assume current_user is set by an authentication strategy
    @user_scope = { 'user_id' => current_user.id }
  end

  desc 'Get a project by ID, ensuring tenant scope'
  params do
    requires :id, type: String, desc: 'Project ID'
  end
  get 'projects/:id' do
    project = mongo_collection.find(
      { '_id' => BSON::ObjectId.from_string(params[:id]) }.merge(@user_scope)
    ).first
    error!('Not found', 404) unless project
    project
  end

  private

  def mongo_collection
    client = Mongo::Client.new(['127.0.0.1:27017'], database: 'myapp')
    client[:projects]
  end
end

2. Validate and whitelist fields to prevent query injection via schema manipulation

Do not directly pass user params into a MongoDB hash. Validate and whitelist allowed fields, and use a document schema library or manual checks to avoid injection through nested operators.

def safe_project_params(params)
  allowed = %w[name description]
  filtered = params.slice(*allowed)
  filtered.transform_values! { |v| v.to_s.strip }
  filtered
end

post 'projects' do
  attrs = safe_project_params(declared(params))
  result = mongo_collection.insert_one(
    attrs.merge('user_id' => current_user.id, 'created_at' => Time.now.utc)
  )
  { id: result.inserted_id }.to_json
end

3. Enforce authorization at the data access layer, not route level

Design service classes that accept a scope and return only authorized documents. This keeps business logic consistent across endpoints and reduces the risk of forgetting to add a filter in a new route.

class ProjectPolicy
  def initialize(collection, user)
    @collection = collection
    @user = user
  end

  def find(id)
    @collection.find(
      { '_id' => BSON::ObjectId.from_string(id), 'user_id' => @user.id }
    ).first
  end

  def list
    @collection.find('user_id' => @user.id).to_a
  end
end

# In resource
get 'projects/:id' do
  policy = ProjectPolicy.new(mongo_collection, current_user)
  project = policy.find(params[:id])
  error!('Forbidden', 403) unless project
  project
end

4. Combine with middleBrick findings for continuous improvement

Use the middleBrick CLI to scan your endpoints and review findings related to BOLA, Property Authorization, and Input Validation. The tool reports per-category breakdowns and prioritized remediation guidance that you can map to the code patterns above. With the Pro plan, you can enable continuous monitoring so that new endpoints are automatically scanned and alerted if they deviate from secure design patterns.