Cross Site Request Forgery in Sinatra with Basic Auth

Cross Site Request Forgery (CSRF) in Sinatra when using HTTP Basic Authentication can create a dangerous scenario where authenticated state is implicitly trusted based solely on credentials provided with each request. Basic Auth transmits a base64-encoded username:password pair in the Authorization header on every request. Because the browser may automatically include this header for requests to the same origin, an authenticated session can be leveraged by a malicious site to perform actions on behalf of the victim without their consent.

Consider a Sinatra application that protects sensitive endpoints with Basic Auth but does not implement anti-CSRF protections such as synchronizer tokens or same-site cookie policies (when cookies are also used for session state). If a victim logs into a vulnerable site that includes an image or script tag pointing to your Sinatra endpoint, the browser will automatically attach the stored Authorization header. Sinatra, seeing valid Basic Auth credentials, processes the request as if it were explicitly initiated by the victim. This becomes particularly risky for state-changing methods like POST, PUT, or DELETE that modify resources or invoke financial operations.

For example, imagine a Sinatra route that transfers funds using a simple Basic Auth guard:

require 'sinatra'

before do
  auth = request.authorization
  unless auth && auth['username'] == 'admin' && auth['password'] == 'secret'
    halt 401, { 'WWW-Authenticate' => 'Basic realm="Secure Area"' }.to_json
  end
end

post '/transfer' do
  # Dangerous: assumes Basic Auth alone is sufficient to verify intent
  amount = params[:amount].to_i
  account = params[:account]
  # ... perform transfer
  "Transferred #{amount} to #{account}"
end

An attacker can craft a malicious HTML page hosted elsewhere that submits a form to /transfer. If the victim’s browser has cached the Authorization header for the Sinatra origin, the request will succeed from the server’s perspective. Because Basic Auth does not bind the authenticated state to a CSRF token or a same-site policy, the server cannot distinguish a legitimate request from a forged one. This exposes applications to unauthorized actions even when credentials are transmitted over TLS.

The vulnerability is compounded when combined with other attack vectors such as insecure direct object references or improper input validation. middleBrick scans can detect missing anti-CSRF controls in unauthenticated scans and highlight the risk of forged requests in authenticated contexts.

To mitigate CSRF in Sinatra with Basic Auth, you must ensure that authenticated requests cannot be trivially forged by external sites. While Basic Auth protects the channel with credentials, it does not provide request integrity guarantees. You should combine transport security with anti-CSRF mechanisms and avoid relying on Basic Auth alone for state-changing operations.

One effective approach is to require an additional anti-CSRF token for any non-GET request, even when Basic Auth is present. This token should be unpredictable, tied to the user’s session or a per-request basis, and validated on the server. Below is a hardened Sinatra example that retains Basic Auth for initial authentication but requires a custom header token for destructive actions:

require 'sinatra'
require 'securerandom'

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

  def csrf_token_matches?(token)
    session[:csrf_token] == token
  end
end

before do
  auth = request.authorization
  if auth && auth['username'] == 'admin' && auth['password'] == 'secret'
    @current_user = auth['username']
  else
    halt 401, { 'WWW-Authenticate' => 'Basic realm="Secure Area"' }.to_json
  end
end

# Provide token to clients that need it (e.g., via a secure header or HTML meta)
get '/csrf-token' do
  content_type :json
  { csrf_token: session[:csrf_token] }.to_json
end

post '/transfer' do
  token = request.env['HTTP_X_CSRF_TOKEN']
  halt 403, { error: 'CSRF token missing' }.to_json unless token && csrf_token_matches?(token)

  amount = params[:amount].to_i
  account = params[:account]
  # ... perform transfer safely
  "Transferred #{amount} to #{account}"
end

For APIs consumed by browsers, ensure that same-site policies and CORS are configured tightly. Do not allow wildcard origins for authenticated endpoints. If you serve HTML forms, include the CSRF token as a hidden field or in a custom header. For programmatic clients, require the token in a non-authentication header like X-CSRF-Token. middleBrick’s unauthenticated scans can verify that such protections are present and flag endpoints that accept state changes over Basic Auth without CSRF mitigation.

Additionally, prefer using session-based authentication with secure, same-site cookies over Basic Auth for browser-facing interactions. Reserve Basic Auth for machine-to-machine scenarios where each request is explicitly authorized and embedded in clients. Combine these practices with input validation and parameterized access controls to reduce the impact of IDOR and BOLA risks that may coexist with CSRF.