Broken Access Control in Rails with Mongodb

Broken Access Control is a top-10 API and web risk (OWASP API Top 10 #1, A01:2023), and the Rails + Mongodb stack introduces specific patterns that can weaken authorization boundaries if not handled carefully. In Rails, developers often rely on model-level scopes and controller filters to enforce what a subject can read or modify. When the persistence layer is Mongodb, the risk arises from a mismatch between ActiveRecord-style expectations and Mongoid’s query behavior, and from unintentionally exposing record-level logic through parameterized endpoints.

Consider a typical Rails controller using Mongoid models where an action scopes records by a user identifier, for example /api/users/:user_id/profile. If the controller builds a query by directly interpolating the route parameter into a Mongoid criteria without validating ownership, an attacker can change :user_id to another valid ObjectId and access data belonging to other users. This is a classic Broken Object Level Authorization (BOLA) / Insecure Direct Object Reference (IDOR) pattern. Unlike SQL-based ORMs where constraints can be enforced at the database row level, Mongodb does not enforce ownership unless queries explicitly encode the subject context, making it easy for a developer to forget a scoping condition.

Moreover, Rails parameter filtering and strong parameters do not enforce authorization; they only control mass assignment. If a controller action accepts a broader set of fields from untrusted input and updates a Mongoid document without checking which attributes are allowed to be modified, an attacker can modify sensitive fields such as is_admin or role. This is the BFLA / Privilege Escalation vector. The presence of nested attributes and polymorphic relations in Mongoid can further obscure the effective permissions of a subject, especially when the developer relies on partial updates via patch semantics (JSON Merge patch) without validating field-level permissions.

Another subtle issue is record visibility in collections with compound indexes or when using string-based identifiers. If indexes are not aligned with authorization queries, or if string-based ids are concatenated into query selectors without type-safe casting, it can lead to unexpected matches or injection-style behaviors when user-controlled input is interpreted as part of a document key. Because Mongodb stores data in a schematype, inconsistent typing between stored values and query inputs can bypass intended filters. Without explicit checks that bind the authenticated subject to the document’s ownership field in every query, the effective access control model becomes leaky.

Finally, API output can inadvertently expose references or internal identifiers that enable horizontal privilege escalation. For example, returning a full document that includes foreign keys or tenant identifiers can allow an attacker to manipulate those values client-side and probe other resources. In a Rails + Mongodb setup, this often happens when developers rely on as_json or to_json without filtering fields, or when serializers include sensitive relations. The key takeaway is that authorization must be enforced at the query level and at the serialization layer, not only at the controller action level.

To mitigate Broken Access Control with Mongodb in Rails, bind every data access to the current subject explicitly and validate inputs before constructing queries. Prefer server-side scoping in the data access layer so controllers only request what the subject is allowed to see.

Example: a scoped profile endpoint that ensures users can only access their own document using Mongoid with explicit ownership checks.

# app/models/user.rb
class User
  include Mongoid::Document
  field :uid, type: String
  field :role, type: String, default: 'user'

  # scope for authorization: only the subject’s own record
  def self.owned_by(user_id)
    where(_id: ::Moped::BSON::ObjectId.from_string(user_id))
  end
end

# app/controllers/api/v1/profiles_controller.rb
class Api::V1::ProfilesController < ApplicationController
  before_action :set_user_from_id, only: [:show]

  def show
    # The query is scoped to the authenticated subject
    profile = User.owned_by(current_user.id).first
    render json: profile, serializer: ProfileSerializer
  end

  private

  def set_user_from_id
    # Ensure the route parameter is a valid ObjectId before querying
    ObjectId(params[:user_id]) rescue render(json: { error: 'invalid id' }, status: :bad_request)
  end
end

For attribute-level authorization, avoid mass assignment on updates. Use strong parameters together with explicit field filtering and verify that updates do not change sensitive flags.

# app/controllers/api/v1/users_controller.rb
class Api::V1::UsersController < ApplicationController
  before_action :set_user, only: [:update]

  def update
    permitted = user_update_params
    # Remove any fields that should never be user-writable
    permitted.except!(:role, :is_admin, :permissions)

    if @user.update(permitted)
      render json: @user, serializer: UserSummarySerializer
    else
      render json: { errors: @user.errors }, status: :unprocessable_entity
    end
  end

  private

  def set_user
    @user = User.owned_by(current_user.id).first
  end

  def user_update_params
    params.require(:user).permit(:name, :email, :preferences, :avatar_url)
  end
end

When implementing tenant or organization boundaries, embed the tenant_id in ownership checks to prevent horizontal escalation across tenants. This is especially important when using string identifiers rather than ObjectId for tenant references.

# app/models/tenant_document.rb
class TenantDocument
  include Mongoid::Document
  field :tenant_id, type: String
  field :data, type: Hash

  scope :for_tenant, ->(tenant_id) { where(tenant_id: tenant_id) }
end

# In controller
class Api::V1::DocumentsController < ApplicationController
  before_action :set_tenant

  def index
    documents = TenantDocument.for_tenant(@tenant.id)
    render json: documents
  end

  private

  def set_tenant
    @tenant = Tenant.find_by(subdomain: request.subdomain)
  end
end

Enforce input validation and type casting before using user input in queries to avoid type confusion. Explicitly cast strings to ObjectId where appropriate and reject unexpected keys at the parameter layer.

Finally, filter serialization output to remove internal references that could aid in privilege escalation. Combine scoping with attribute filtering so that even if an attacker enumerates identifiers, they cannot infer relationships or escalate laterally.