Cross Site Request Forgery in Sinatra with Dynamodb

Cross Site Request Forgery (CSRF) in a Sinatra application that uses DynamoDB arises from a mismatch between browser behavior and server-side assumptions. In a typical web flow, browsers automatically include cookies (including session cookies) with any request to your domain, even if the request originates from a malicious site. If your Sinatra routes rely on cookie-based sessions and perform state-changing operations (POST, PUT, DELETE) without verifying intent, an attacker can craft an image or form on another site that triggers those routes.

When those routes directly call DynamoDB to mutate data (for example, updating user settings or initiating a purchase), the server-side code may not adequately distinguish between authenticated actions the user intended and those induced by the attacker. DynamoDB itself does not enforce CSRF protections; it processes requests that carry valid credentials (e.g., via AWS Signature Version4 or temporary tokens). Therefore, if your Sinatra app supplies those credentials and does not validate the request origin or require explicit anti-CSRF tokens, an attacker’s forged request can succeed with the privileges of the victim’s session.

This risk is especially pronounced when developers use server-side sessions stored in cookies (signed or encrypted) and couple them with DynamoDB conditional writes without ensuring the request’s provenance. For example, a route like /update-email that accepts POST data and writes directly to a DynamoDB table without origin checks or synchronizer token patterns can be exploited. Attackers do not need to compromise DynamoDB; they leverage the web app’s trust in cookie-based authentication combined with missing CSRF defenses to cause unauthorized writes.

Tools like middleBrick can surface these issues by scanning the unauthenticated attack surface and flagging missing CSRF protections in Sinatra endpoints that invoke DynamoDB, providing findings mapped to the OWASP API Top 10 and actionable remediation guidance.

To mitigate CSRF in Sinatra with DynamoDB, implement synchronizer token patterns and enforce strict origin checks. Below are concrete, working examples that combine Sinatra route design with the AWS SDK for Ruby to reduce risk.

1. Use anti-CSRF tokens for state-changing operations

Generate and validate per-session tokens for any POST/PUT/DELETE that call DynamoDB. Store the token server-side (e.g., in the session) and require it on form submissions or API calls.

# Gemfile: gem "sinatra", "~> 3.0"
# Gemfile: gem "aws-sdk-dynamodb", "~> 1.0"
require "sinatra"
require "aws-sdk-dynamodb"
require "securerandom"

enable :sessions

# Configure DynamoDB client (credentials sourced from environment or IAM)
dynamodb = Aws::DynamoDB::Client.new(region("us-east-1"))

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

  def verify_csrf(token)
    halt 403, "Invalid CSRF token\n" unless session[:csrf_token] && session[:csrf_token] == token
  end
end

# Serve a form with the CSRF token embedded
get "/profile" do
  <form action="/update-profile" method="POST">
    <input type="hidden" name="csrf_token" value="#{csrf_token}">
    <label>Display Name: <input type="text" name="display_name"></label>
    <button type="submit">Save</button>
  </form>
end

# Process the form with CSRF verification and DynamoDB write
post "/update-profile" do
  verify_csrf(params["csrf_token"])
  display_name = params["display_name"]
  # Example DynamoDB update
  dynamodb.update_item({
    table_name: "users",
    key: { user_id: { s: current_user_id } },
    update_expression: "SET display_name = :val",
    expression_attribute_values: { ":val": { s: display_name } }
  })
  "Profile updated successfully"
end

2. Validate Origin and Referer headers

While not sufficient alone, checking Origin/Referer headers adds a layer of defense-in-depth for same-site requests. Combine this with anti-CSRF tokens for robust protection.

before do
  # Allow requests from your own frontend origin only
  allowed_origin = "https://your-app.example.com"
  request_origin = request.env["HTTP_ORIGIN"]
  request_referer = request.env["HTTP_REFERER"]
  unless [request_origin, request_referer].any? { |r| r&.start_with?(allowed_origin) }
    halt 403, "Forbidden origin\n"
  end
end

3. Use DynamoDB condition expressions to enforce ownership

Ensure that updates affect only resources belonging to the authenticated user. This prevents tampering even if CSRF slips through.

dynamodb.update_item({
  table_name: "users",
  key: { user_id: { s: current_user_id } },
  update_expression: "SET display_name = :val",
  condition_expression: "user_id = :uid",
  expression_attribute_values: {
    ":val": { s: display_name },
    ":uid": { s: current_user_id }
  }
})

4. Prefer API-style endpoints with explicit authentication

For APIs consumed by JavaScript clients, require an explicit token (e.g., Bearer) and validate it before executing DynamoDB calls. This reduces reliance on cookie-based session handling for sensitive operations.

post "/api/profile" do
  auth = request.env["HTTP_AUTHORIZATION"]
  halt 401, "Missing token\n" unless auth&.start_with?("Bearer ")
  token = auth.split(" ").last
  halt 403, "Invalid token\n" unless valid_token?(token)
  # Proceed with DynamoDB write
end

By combining synchronizer tokens, origin validation, DynamoDB condition expressions, and explicit authentication, you significantly reduce the CSRF attack surface for Sinatra applications that rely on DynamoDB.