Cross Site Request Forgery in Sinatra with Mongodb

Cross Site Request Forgery (CSRF) in a Sinatra application that uses MongoDB arises when an authenticated endpoint performs state-changing operations without verifying that the request intentionally originated from the user’s own interface. In Sinatra, routes that rely on session cookies for authentication but do not enforce anti-CSRF tokens allow an attacker to trick a logged-in user into submitting forged requests from another site. Because MongoDB is typically used as a database backend, forged requests can directly invoke database operations such as updates or deletions if the route logic does not validate intent.

Consider a Sinatra route that updates a user’s email by parsing JSON from the request body and passing it to MongoDB without confirming the request source:

post '/users/update_email' do
  user_id = session[:user_id]
  data = JSON.parse(request.body.read)
  collection = MongoClient.database[:users]
  collection.update_one({ '_id' => BSON::ObjectId(user_id) }, { '$set' => { 'email' => data['email'] } })
  { status: 200, message: 'updated' }.to_json
end

If this route lacks a CSRF token check, an attacker can host a malicious page that sends a POST to /users/update_email with a crafted JSON payload. Because the browser automatically includes session cookies, the request is authenticated by Sinatra, and MongoDB executes the update using the user’s privileges. Common attack vectors include forms on external sites, image tags triggering GET-based state changes, or JavaScript initiated via third‑party widgets. The risk is amplified when Sinatra relies on cookie‑based sessions without SameSite attributes or CSRF tokens, and when MongoDB operations are bound directly to the authenticated session without additional authorization checks.

Additionally, if any part of the application exposes preview or debug endpoints that return raw MongoDB query results to the browser, attackers can refine forged requests by inspecting side effects or error messages. The combination of Sinatra’s lightweight routing, MongoDB’s direct write capabilities, and missing CSRF controls creates a clear path for unauthorized state manipulation.

To mitigate CSRF in Sinatra with MongoDB, implement strict origin and anti‑CSRF token validation for any state‑changing route, and ensure MongoDB operations are conditioned on verified intent.

Use anti‑CSRF tokens in forms and verify them in Sinatra

Generate a per‑session token, store it server‑side or in an httpOnly cookie, and require it for POST/PUT/DELETE actions involving MongoDB writes.

# Sinatra app setup
enable :sessions
set :session_secret, ENV['SESSION_SECRET']

before do
  session[:csrf_token] ||= SecureRandom.hex(32)
end

post '/users/update_email' do
  unless params[:csrf_token] == session[:csrf_token]
    halt 403, { error: 'Invalid CSRF token' }.to_json
  end
  user_id = session[:user_id]
  data = JSON.parse(request.body.read)
  collection = MongoClient.database[:users]
  result = collection.update_one(
    { '_id' => BSON::ObjectId(user_id) },
    { '$set' => { 'email' => data['email'] } }
  )
  if result.modified_count == 1
    { status: 200, message: 'email updated' }.to_json
  else
    halt 400, { error: 'update failed' }.to_json
  end
end

In the above, the token is validated before any MongoDB call, ensuring that forged requests without the correct token are rejected at the framework layer.

Set SameSite and Secure cookie attributes for session cookies

Configure session cookies to reduce the likelihood of cross‑origin transmission:

configure do
  set :sessions, true
  set :session_options, {
    httponly: true,
    secure: true,
    same_site: :strict
  }
end

same_site: :strict prevents the browser from sending cookies in cross‑site requests, which complements anti‑CSRF tokens.

Validate the request origin for state‑changing methods

For APIs, check the Origin or Referer header when the client does not use forms:

post '/api/records/:id' do
  origin = request.env['HTTP_ORIGIN']
  allowed_origin = 'https://your-app.com'
  halt 403 unless origin == allowed_origin
  id = params[:id]
  collection = MongoClient.database[:records]
  collection.update_one({ '_id' => BSON::ObjectId(id) }, { '$set' => JSON.parse(request.body.read) })
  { status: 200 }.to_json
end

Combine this with per‑action authorization checks to ensure the authenticated user has permission to modify the specific MongoDB document.

Use MongoDB client‑side validation and least‑privilege credentials

Define schema rules and use MongoDB’s field-level validation to reject unexpected fields, and connect Sinatra with a MongoDB user that has only the necessary write permissions for the targeted collections. This limits the impact of any successful CSRF attempt.