Fix BOLA (Broken Object Level Authorization) in Grape

BOLA (Broken Object Level Authorization), formerly known as IDOR, remains the primary attack vector for data exfiltration in RESTful APIs. In Grape, this vulnerability surfaces when an endpoint accepts a user-controlled identifier and retrieves the corresponding database object without verifying if the authenticated user has the requisite permissions to access that specific instance.

The Vulnerable Pattern

resource :projects do
  desc 'Return a project'
  params do
    requires :id, type: Integer, desc: 'Project ID'
  end
  get ':id' do
    # VULNERABLE: Fetches project directly from global scope
    # An attacker can iterate IDs to leak any project in the DB
    Project.find(params[:id])
  end
end

The Secure Implementation

The fix eliminates the authorization bypass by replacing global lookups with scoped queries. By chaining the find operation to the 'current_user' association, the database engine enforces isolation at the query level. If an attacker attempts to access a resource ID they do not own, the query returns nil (triggering a 404), effectively masking the existence of the resource and preventing unauthorized data access. For complex permission logic, integrate a policy engine like Pundit within the Grape helper to handle granular 'can?' checks.

resource :projects do
  helpers do
    def project
      # SECURE: Scope the lookup to the authenticated user's projects
      @project ||= current_user.projects.find_by!(id: params[:id])
    rescue ActiveRecord::RecordNotFound
      error!('Project not found', 404)
    end
  end
desc ‘Return a project’
params do
requires :id, type: Integer
end
get ‘:id’ do
# Access is restricted via the scoped helper
project
end
end