Cross Site Request Forgery in Sinatra

Cross Site Request Forgery (CSRF) in Sinatra applications exploits the framework's default behavior of processing HTTP requests without verifying their origin. Sinatra's minimalist design means developers must explicitly implement CSRF protection, making it a common vulnerability in production applications.

The most frequent CSRF attack pattern in Sinatra involves state-changing operations like POST, PUT, DELETE requests. Consider a banking application where users can transfer funds:

post '/transfer' do
  from = params[:from_account]
  to = params[:to_account]
  amount = params[:amount].to_f
  
  # No CSRF verification
  Account.transfer(from, to, amount)
  redirect '/balance'
end

An attacker can create a malicious page that automatically submits this form when loaded:

<form action="https://banking.example.com/transfer" method="POST" id="csrf_form">
  <input type="hidden" name="from_account" value="victim_account">
  <input type="hidden" name="to_account" value="attacker_account">
  <input type="hidden" name="amount" value="1000">
</form>
<script>document.getElementById('csrf_form').submit();</script>

When the victim visits this page while authenticated to the banking app, their browser automatically submits the transfer request with their session cookies, completing the attack without their knowledge.

Another Sinatra-specific CSRF pattern involves API endpoints that accept JSON payloads. Sinatra's default JSON parser doesn't enforce CSRF tokens:

post '/api/update_profile' do
  data = JSON.parse(request.body.read)
  user = current_user
  user.update_attributes(data)
  user.save
end

Attackers can exploit this using JavaScript's fetch API to send crafted requests from malicious sites.

Detecting CSRF vulnerabilities in Sinatra requires examining both code patterns and runtime behavior. middleBrick's Sinatra-specific scanning identifies these issues through several techniques.

Code analysis looks for state-changing routes without CSRF protection. middleBrick parses Sinatra route definitions and flags patterns like:

get '/login' do
  erb :login
end

Without examining the corresponding view, middleBrick can't detect missing CSRF tokens in forms, but it will flag any POST/PUT/DELETE routes that don't explicitly require CSRF verification.

Runtime detection involves submitting test requests with forged origins. middleBrick's black-box scanner sends POST requests to Sinatra endpoints and analyzes responses. For CSRF vulnerabilities, it checks:

• Whether state-changing operations execute without origin verification
• If session cookies are accepted from cross-origin requests
• Whether anti-CSRF tokens are actually validated
• If the application has proper SameSite cookie settings

For Sinatra applications using Rack middleware, middleBrick examines the middleware stack to identify missing CSRF protection gems like rack_csrf or rack-protection.

The scanner also tests for common Sinatra CSRF bypasses, such as:

# Vulnerable: custom CSRF check that's easily bypassed
post '/update' do
  if params[:csrf_token] == 'static_value'
    # Process request
end

middleBrick's LLM security module additionally checks for AI-specific CSRF scenarios where model endpoints might be manipulated through crafted prompts or requests.

Remediating CSRF in Sinatra requires implementing proper token-based protection. The most straightforward approach uses Rack::Protection, which Sinatra includes by default but may be disabled:

require 'sinatra'
require 'rack/protection'

If Rack::Protection is disabled, enable it explicitly:

use Rack::Protection::AuthenticityToken

For manual CSRF token implementation in Sinatra:

enable :sessions

helpers do
  def csrf_token
    session[:csrf] ||= SecureRandom.hex(16)
  end

  def csrf_tag
    %().html_safe
  end
end

before do
  if request.post? || request.put? || request.delete?
    halt 403 unless params[:authenticity_token] == session[:csrf]
  end
end

In your forms, include the token:

<form method="POST" action="/update">
  <%= csrf_tag %>
  <input type="text" name="name">
  <button type="submit">Update</button>
</form>

For API endpoints that must accept cross-origin requests, implement double-submit cookies or synchronizer token patterns:

before do
  if request.post? && request.content_type == 'application/json'
    body = JSON.parse(request.body.read)
  end
end

Set secure SameSite cookies to provide defense-in-depth:

set :session_secret, ENV['SESSION_SECRET']
set :session_options, { secure: true, httponly: true, samesite: 'strict' }

For Sinatra applications using DataMapper or ActiveRecord, ensure CSRF protection works with your ORM's session management. The Rack::Protection middleware handles this automatically when properly configured.

middleBrick's CLI tool can verify your remediation:

npx middlebrick scan https://your-sinatra-app.com/api/protected-endpoint

The scanner will confirm whether CSRF protections are properly implemented and provide a security score with specific findings about any remaining vulnerabilities.