Cors Wildcard in Sinatra with Dynamodb

A CORS wildcard (Access-Control-Allow-Origin: *) in a Sinatra service that directly proxies or interacts with Amazon DynamoDB can unintentionally expose sensitive data and amplify authorization flaws. When * is used in response to credentialed requests (requests with cookies or authorization headers), browsers will allow cross-origin responses to be read by any site the user visits. If the Sinatra endpoint also performs DynamoDB queries using IAM credentials tied to the caller’s identity or returns item-level data, the wildcard enables a CORS-based data leakage path where a malicious site can read another user’s records via the permissive CORS policy.

Consider a Sinatra route that forwards an incoming user identifier to DynamoDB to fetch profile data. With a wildcard CORS header, any webpage can invoke that route cross-origin and collect the responses. Because the browser enforces CORS for cross-origin reads but not for same-origin requests, the attacker’s page can repeatedly call the Sinatra endpoint and harvest information that should be isolated to authenticated sessions. This pattern is especially risky when combined with BOLA/IDOR issues in DynamoDB where insufficient ownership checks allow one user’s identifier to retrieve another’s items.

The interaction with DynamoDB also means misconfigured resource policies or overly permissive IAM roles on the Sinatra backend can compound the risk. If the Sinatra app uses a shared AWS credential with broad table access, a leaked CORS wildcard does not just expose data returned by the app—it can enable downstream abuse where an attacker’s script drives the Sinatra service to query DynamoDB on their behalf at scale, increasing read throughput and cost while evading per-user rate limits that might otherwise protect the backend.

In practice, scanning an endpoint like this with middleBrick demonstrates the exposure: the scanner detects the Access-Control-Allow-Origin: * header on responses that include authenticated-appearing data and flags the CORS wildcard as a high-severity finding. The scan also surfaces DynamoDB-related findings such as missing ownership validation and excessive IAM permissions, mapping them to OWASP API Top 10 A01:2023 (Broken Object Level Authorization) and A05:2023 (Security Misconfiguration).

Remediation centers on two controls: tightening CORS to avoid wildcards for credentialed interactions, and enforcing strict DynamoDB authorization checks in Sinatra. For CORS, return a specific origin derived from a whitelist and only include credentials when necessary. For DynamoDB, validate ownership on every request using the authenticated subject and apply least-privilege IAM for the Sinatra role.

Below is a minimal Sinatra example that implements these controls. It uses the official AWS SDK for Ruby to query DynamoDB safely, checks the request origin against an allowlist, and enforces that the logged-in user can only access their own items by using the subject as a partition key value.

require 'sinatra'
require 'aws-sdk-dynamodb'
require 'json'

# Configure allowed origins; in production this should be read from config/secrets
ALLOWED_ORIGINS = ['https://app.example.com', 'https://admin.example.com'].freeze

def allowed_origin(request_origin)
  return nil unless request_origin && ALLOWED_ORIGINS.include?(request_origin)
  request_origin
end

# Least-privilege DynamoDB client (configured via environment or IAM role)
ddb = Aws::DynamoDB::Client.new(region: 'us-east-1')
TABLE_NAME = ENV['DYNAMODB_TABLE']

before do
  request_origin = request.env['HTTP_ORIGIN']
  origin = allowed_origin(request_origin)
  if origin
    headers 'Access-Control-Allow-Origin' => origin,
            'Access-Control-Allow-Methods' => 'GET, OPTIONS',
            'Access-Control-Allow-Headers' => 'Content-Type,Authorization',
            'Access-Control-Allow-Credentials' => 'true'
  else
    headers 'Access-Control-Allow-Origin' => ''
  end

  if request.options? # preflight
    halt 200
  end
end

get '/api/profile/:user_id' do
  content_type :json

  # Require authentication; in practice integrate with your auth layer
  user_identity = authenticated_user_id # e.g., from session or token
  raise Sinatra::NotFound unless user_identity

  # Enforce ownership: user can only fetch their own profile
  requested_id = params['user_id']
  halt 403, { error: 'Forbidden' }.to_json unless requested_id == user_identity

  # Query DynamoDB with the specific user_id as key
  resp = ddb.get_item({
    table_name: TABLE_NAME,
    key: {
      'user_id' => { s: requested_id }
    },
    consistent_read: true
  })

  item = resp.item
  if item.nil?
    halt 404, { error: 'Profile not found' }.to_json
  end

  # Return a sanitized subset; avoid returning raw credentials or internal fields
  {
    user_id: item['user_id'],
    display_name: item['display_name'],
    email: item['email']
  }.to_json
end

Key points in this example:

• CORS is limited to an allowlist and credentials are only reflected when the origin is approved, preventing wildcard leakage.
• Ownership is enforced by comparing the authenticated subject with the user_id path parameter before any DynamoDB call.
• The IAM role assigned to the Sinatra host should have a policy scoped to dynamodb:GetItem on the specific table and only for the principal tied to the application, avoiding broad table access.
• The response omits sensitive attributes and uses consistent reads to reduce the impact of stale reads in strongly consistent use cases.

If you use the middleBrick CLI (middlebrick scan <url>) or the GitHub Action to add API security checks to your CI/CD pipeline, these configuration issues will appear as distinct findings with remediation guidance, helping you enforce least privilege and avoid CORS-based data exposure in production.