Broken Access Control in Sinatra

Broken Access Control in Sinatra applications often stems from improper session management and inadequate authorization checks. Sinatra's minimalist nature means developers must explicitly implement security controls, creating opportunities for subtle vulnerabilities.

The most common manifestation is missing authentication checks on sensitive routes. Consider this vulnerable pattern:

get '/admin' do
  erb :admin
end

get '/users/:id' do |id|
  user = User.find(id)
  erb :profile, locals: { user: user }

Both routes lack authentication verification, allowing anyone to access admin functionality or view any user's profile by simply changing the ID parameter. This is classic IDOR (Insecure Direct Object Reference) specific to Sinatra's route parameter handling.

Session fixation is another Sinatra-specific concern. By default, Sinatra uses Rack::Session::Cookie without secure flags:

set :sessions, true

# Vulnerable to session fixation
get '/login' do
  session[:user_id] = params[:user_id]
  redirect '/dashboard'
end

Without proper session configuration, attackers can hijack sessions or fix session IDs before authentication.

Cross-route authorization bypasses occur when developers rely on client-side indicators rather than server-side verification:

get '/dashboard' do
  @is_admin = session[:user_id] == 1 # Hardcoded admin ID
  erb :dashboard
end

get '/admin' do
  # No authorization check - relies on @is_admin from dashboard
  erb :admin
end

This creates a privilege escalation path where users can directly access '/admin' without proper authorization.

Detecting Broken Access Control in Sinatra requires understanding its routing patterns and middleware stack. Manual testing should focus on:

• Modifying route parameters to access other users' data
• Accessing admin routes without authentication
• Testing session fixation by setting custom session cookies
• Verifying that authentication checks exist on every sensitive route

middleBrick's Sinatra-specific scanning identifies these patterns through black-box testing:

middlebrick scan https://your-sinatra-app.com --profile sinatra

The scanner tests for:

• Missing authentication on admin routes (Authentication category)
• IDOR vulnerabilities in user-specific endpoints (BOLA/IDOR category)
• Privilege escalation attempts on role-based endpoints (BFLA category)
• Session fixation and cookie security issues (Data Exposure category)

middleBrick's OpenAPI analysis is particularly effective for Sinatra apps using sinatra-contrib's doc generator:

require 'sinatra/base'
require 'sinatra/json'
require 'sinatra/contrib/docs

class API < Sinatra::Base
  register Sinatra::Doc
  
  doc do
    {
      description: 'Get user profile',
      params: [{name: 'id', type: 'integer'}]
    }
  end
  
  get '/users/:id' do |id|
    # Missing auth check here
    User.find(id).to_json
  end
end

The scanner cross-references the documented parameters with actual runtime behavior to detect authorization bypasses.

Effective remediation in Sinatra leverages its modular middleware architecture and before filters:

require 'sinatra/base'
require 'sinatra/reloader'

class App < Sinatra::Base
  configure do
    enable :sessions
    set :session_secret, ENV['SESSION_SECRET']
    set :protection, :session => true
  end

  helpers do
    def current_user
      @current_user ||= User.find(session[:user_id]) if session[:user_id]
    end

    def authenticate!
      halt 401, 'Unauthorized' unless current_user
    end

    def authorize_admin!
      halt 403, 'Forbidden' unless current_user&.admin?
    end
  end

  before '/admin/*' do
    authenticate!
    authorize_admin!
  end

  before '/users/:id' do |id|
    authenticate!
    user = User.find(id)
    halt 403, 'Forbidden' unless current_user == user || current_user&.admin?
  end

  get '/admin/dashboard' do
    erb :admin
  end

  get '/users/:id' do |id|
    @user = User.find(id)
    erb :profile
  end
end

This pattern ensures authentication and authorization checks are consistently applied. The before filters run before route handlers, preventing unauthorized access at the routing level.

For comprehensive protection, integrate Rack::Protection middleware:

use Rack::Protection::AuthenticityToken
use Rack::Protection::RemoteReferrer
use Rack::Protection::SessionHijacking

These middleware components provide additional defenses against session fixation, CSRF, and other attacks that enable broken access control exploitation.