Mitigating Broken Object Level Authorization (BOLA) in API gateway endpoints in Custom Ruby Implementations

Understanding Broken Object Level Authorization (BOLA) in API Gateways

Broken Object Level Authorization (BOLA), also known as Insecure Direct Object References (IDOR) in the context of APIs, is a critical security vulnerability. It occurs when an API endpoint allows a user to access or manipulate resources they are not authorized to interact with. This often happens when an API directly exposes identifiers for internal objects (like database primary keys) and fails to properly verify the requesting user’s permissions against the specific object being accessed.

In a typical API gateway architecture, requests first hit the gateway, which then routes them to the appropriate backend microservice. BOLA vulnerabilities can manifest at either the gateway level (if authorization logic is partially implemented there) or, more commonly, within the individual microservices themselves. This post focuses on mitigating BOLA within custom Ruby implementations of backend services, assuming an API gateway is in place but might not enforce granular object-level checks for every request.

Common BOLA Attack Vectors in Ruby APIs

The most prevalent attack vector involves manipulating resource IDs in API requests. Consider an endpoint designed to retrieve a user’s profile:

Scenario: Unprotected Resource ID Access

A common, insecure implementation might look like this in a Ruby on Rails application:

# app/controllers/api/v1/users_controller.rb
module Api
  module V1
    class UsersController << ApplicationController
      # GET /api/v1/users/:id
      def show
        user = User.find(params[:id])
        # Problem: No check if the *current* user is allowed to see *this specific* user's data
        render json: user, status: :ok
      end

      # PUT /api/v1/users/:id
      def update
        user = User.find(params[:id])
        if user.update(user_params)
          render json: user, status: :ok
        else
          render json: { errors: user.errors }, status: :unprocessable_entity
        end
      end

      private

      def user_params
        params.require(:user).permit(:name, :email)
      end
    end
  end
end

In this example, if a user is authenticated (e.g., via a JWT token processed by ApplicationController), they might be able to request or modify the profile of *any* user by simply changing the :id parameter in the URL. The application finds the requested user but doesn’t verify if the authenticated user has the right to access or modify *that specific user’s record*.

Implementing Robust Authorization in Ruby Backend Services

The core principle is to always associate resource access with the authenticated user’s identity and their defined permissions. This involves two key steps:

• Identify the authenticated user: Ensure your ApplicationController or a similar base controller correctly identifies the logged-in user from the request (e.g., from a JWT payload, session cookie, or API key). This user object should be readily available, often via a helper method like current_user.
• Enforce object-level checks: Before performing any action (read, write, delete) on a resource, verify that the current_user is authorized to perform that action on *that specific resource instance*.

Strategy 1: Inline Authorization Checks

The most straightforward approach is to add explicit checks within your controller actions. This is suitable for simpler applications or specific endpoints.

# app/controllers/api/v1/users_controller.rb
module Api
  module V1
    class UsersController << ApplicationController
      before_action :authenticate_user! # Assume this sets current_user
      before_action :set_user, only: [:show, :update, :destroy]
      before_action :authorize_user_access, only: [:show, :update, :destroy]

      # GET /api/v1/users/:id
      def show
        render json: @user, status: :ok
      end

      # PUT /api/v1/users/:id
      def update
        if @user.update(user_params)
          render json: @user, status: :ok
        else
          render json: { errors: @user.errors }, status: :unprocessable_entity
        end
      end

      # DELETE /api/v1/users/:id
      def destroy
        @user.destroy
        head :no_content
      end

      private

      def set_user
        @user = User.find(params[:id])
      rescue ActiveRecord::RecordNotFound
        render json: { error: "User not found" }, status: :not_found
      end

      def authorize_user_access
        # The core BOLA mitigation:
        # Only allow users to access/modify their OWN profile.
        # This logic might be more complex in real-world scenarios (e.g., admins).
        unless current_user.id == @user.id
          render json: { error: "Unauthorized access to this user's profile" }, status: :forbidden
        end
      end

      def user_params
        params.require(:user).permit(:name, :email)
      end
    end
  end
end

In this refined example:

• authenticate_user! ensures a user is logged in.
• set_user fetches the target user and handles not found errors.
• authorize_user_access is the critical piece. It compares the current_user.id with the @user.id. If they don’t match, it returns a 403 Forbidden status, preventing unauthorized access.

Strategy 2: Using an Authorization Gem (e.g., Pundit)

For more complex authorization rules, especially in larger applications, using a dedicated gem like Pundit is highly recommended. Pundit encourages writing authorization logic in separate policy classes, making controllers cleaner and authorization rules more maintainable.

First, add Pundit to your Gemfile:

# Gemfile
gem 'pundit'

Then, run bundle install.

Generate a policy for your User model:

rails generate pundit:policy User

This creates app/policies/user_policy.rb. Now, implement the authorization logic within the policy:

# app/policies/user_policy.rb
class UserPolicy << ApplicationPolicy
  # If you are using a different scope for users, you can override the scope method
  # class Scope << Scope
  #   def resolve
  #     scope.all
  #   end
  # end

  # Users can only see their own profile
  def show?
    user == record
  end

  # Users can only update their own profile
  def update?
    user == record
  end

  # Users can only destroy their own profile
  def destroy?
    user == record
  end
end

In your controller, you’ll use Pundit’s helpers:

# app/controllers/api/v1/users_controller.rb
module Api
  module V1
    class UsersController << ApplicationController
      before_action :authenticate_user!
      before_action :set_user, only: [:show, :update, :destroy]
      after_action :verify_authorized, except: [:index, :create] # Optional: ensures policies are used
      after_action :verify_policy_scoped, only: [:index, :create] # Optional: for scoped collections

      # GET /api/v1/users/:id
      def show
        authorize @user # Pundit checks the 'show?' method on UserPolicy
        render json: @user, status: :ok
      end

      # PUT /api/v1/users/:id
      def update
        authorize @user # Pundit checks the 'update?' method on UserPolicy
        if @user.update(user_params)
          render json: @user, status: :ok
        else
          render json: { errors: @user.errors }, status: :unprocessable_entity
        end
      end

      # DELETE /api/v1/users/:id
      def destroy
        authorize @user # Pundit checks the 'destroy?' method on UserPolicy
        @user.destroy
        head :no_content
      end

      private

      def set_user
        @user = User.find(params[:id])
      rescue ActiveRecord::RecordNotFound
        render json: { error: "User not found" }, status: :not_found
      end

      def user_params
        params.require(:user).permit(:name, :email)
      end
    end
  end
end

With Pundit:

• authenticate_user! still handles authentication.
• set_user fetches the target user.
• authorize @user is the key Pundit call. It infers the policy class (UserPolicy) and the action (show, update, or destroy based on the controller action) and executes the corresponding method (e.g., UserPolicy.new(current_user, @user).show?). If authorization fails, Pundit raises an exception that can be caught globally to return a 403 Forbidden.

Handling API Gateway Interactions

If your API gateway performs some level of authentication (e.g., validating JWTs) but doesn’t enforce granular object-level authorization, your backend services must still implement these checks. The gateway might pass user identity information (like a user ID or roles) in request headers. Ensure your backend services correctly parse and utilize this information.

# Example: Assuming gateway passes user ID in X-User-Id header
# app/controllers/application_controller.rb
class ApplicationController << ActionController::API
  protected

  def current_user
    @current_user ||= User.find_by(id: request.headers['X-User-Id'])
  end

  def authenticate_user!
    unless current_user
      render json: { error: "Authentication required" }, status: :unauthorized
    end
  end
end

The authorization logic within the controllers (either inline or via Pundit) remains the same, using the current_user derived from the gateway-provided information.

Testing for BOLA Vulnerabilities

Thorough testing is crucial. This involves both automated tests and manual penetration testing.

Automated Testing (RSpec Example)

Your test suite should include scenarios where an authenticated user attempts to access resources belonging to other users.

# spec/requests/api/v1/users_spec.rb
require 'rails_helper'

RSpec.describe "Api::V1::Users", type: :request do
  let!(:user_a) { create(:user, id: 1, name: "Alice", email: "[email protected]") }
  let!(:user_b) { create(:user, id: 2, name: "Bob", email: "[email protected]") }
  let(:api_key_a) { "token_for_alice" } # Assume authentication mechanism

  before do
    # Mock authentication to return user_a
    allow_any_instance_of(ApplicationController).to receive(:current_user).and_return(user_a)
    allow_any_instance_of(ApplicationController).to receive(:authenticate_user!).and_return(true)
    # If using headers:
    # request.headers['X-User-Id'] = user_a.id.to_s
  end

  describe "GET /api/v1/users/:id" do
    context "when accessing own profile" do
      it "returns the user's profile" do
        get api_v1_user_path(user_a.id)
        expect(response).to have_http_status(:ok)
        json_response = JSON.parse(response.body)
        expect(json_response['email']).to eq(user_a.email)
      end
    end

    context "when accessing another user's profile" do
      it "returns forbidden" do
        get api_v1_user_path(user_b.id)
        expect(response).to have_http_status(:forbidden) # Or :unauthorized depending on implementation
        json_response = JSON.parse(response.body)
        expect(json_response['error']).to eq("Unauthorized access to this user's profile") # Or similar message
      end
    end
  end

  describe "PUT /api/v1/users/:id" do
    let(:update_params) { { user: { name: "Alicia" } } }

    context "when updating own profile" do
      it "updates the user's profile" do
        put api_v1_user_path(user_a.id), params: update_params
        expect(response).to have_http_status(:ok)
        user_a.reload
        expect(user_a.name).to eq("Alicia")
      end
    end

    context "when updating another user's profile" do
      it "returns forbidden" do
        put api_v1_user_path(user_b.id), params: update_params
        expect(response).to have_http_status(:forbidden)
        json_response = JSON.parse(response.body)
        expect(json_response['error']).to eq("Unauthorized access to this user's profile")
      end
    end
  end
end

Manual Penetration Testing

Use tools like Postman, Insomnia, or Burp Suite to:

• Authenticate as a regular user.
• Identify API endpoints that accept resource IDs (e.g., /users/:id, /orders/:order_id, /accounts/:account_number).
• Attempt to modify the ID in the request URL or body to access/manipulate resources belonging to other users.
• Check responses for 200 OK (indicating success, a vulnerability) or 403 Forbidden / 401 Unauthorized (indicating proper protection).
• Test edge cases: non-existent IDs, malformed IDs, and IDs that might belong to different resource types.

Conclusion

BOLA is a pervasive and dangerous vulnerability. By consistently implementing explicit authorization checks at the object level within your Ruby backend services, whether through inline logic or robust gems like Pundit, you can significantly strengthen your API’s security posture. Always remember that authentication (who you are) is distinct from authorization (what you can do), and both are critical for secure API design.