Crlf Injection in Sinatra with Dynamodb

Crlf Injection occurs when an attacker can inject a CRLF sequence (\r\n) into a header or a redirect value. In Sinatra, routes that directly use user input in HTTP response headers—such as Location for redirects or Set-Cookie—are susceptible. When a Sinatra app stores or retrieves data from DynamoDB and reflects user-controlled values into headers, the combination amplifies risk: data from DynamoDB may be trusted implicitly, and headers are set without validation or sanitization.

Consider a Sinatra route that reads a redirect URL from a DynamoDB item and issues a 302 redirect:

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

DYNAMO = Aws::DynamoDB::Client.new(region: 'us-east-1')

get '/redirect' do
  item = DYNAMO.get_item(table: 'Redirects', key: { id: params[:id] }).item
  redirect item['url'] if item
end

If the stored URL in DynamoDB (or the params[:id] used to fetch it) contains a CRLF sequence, an attacker can inject additional headers or split the response. For example, a URL like https://example.com\r\nSet-Cookie: session=attacker can cause the response to include an extra Set-Cookie header, leading to session fixation or HTTP response splitting. DynamoDB does not validate header-safe characters, so if an application stores or uses arbitrary strings as headers or redirect targets, the risk exists. The unauthenticated scan on middleBrick can surface this by flagging missing input validation on user-controlled data used in headers and detecting whether response splitting is feasible.

Additionally, if DynamoDB values are rendered in HTML context without escaping (e.g., in an error page or JSON output), CRLF injection may facilitate cross-site scripting (XST) when combined with header splitting. middleBrick’s checks for Input Validation and Property Authorization help identify whether user data flows into headers or unsafe outputs without sanitization.

To prevent Crlf Injection when working with DynamoDB in Sinatra, validate and sanitize any data used in HTTP headers, and enforce a strict allowlist for redirect URLs. Avoid reflecting raw DynamoDB attributes into headers; instead, map stored values to a controlled set of known-safe targets. Below are concrete, safe patterns.

1. Validate URLs before redirecting

Use a strict allowlist for redirect destinations. Do not trust DynamoDB-hosted URLs directly.

require 'uri'
require 'aws-sdk-dynamodb'
require 'sinatra'

DYNAMO = Aws::DynamoDB::Client.new(region: 'us-east-1')

ALLOWED_HOSTS = ['example.com', 'api.example.com']

def safe_redirect_url(url)
  uri = URI.parse(url)
  return false unless ['http', 'https'].include?(uri.scheme)
  return false unless ALLOWED_HOSTS.include?(uri.host)
  # Ensure no CRLF characters are present
  return false if url.include?("\r") || url.include?("\n")
  url
end

get '/redirect' do
  item = DYNAMO.get_item(table: 'Redirects', key: { id: params[:id] }).item
  if item
    target = safe_redirect_url(item['url'])
    redirect target if target
  end
  halt 400, 'Invalid redirect target'
end

2. Encode output when rendering user data

If DynamoDB data is used in HTML or JSON responses, escape newlines and special characters to prevent header splitting and injection.

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

DYNAMO = Aws::DynamoDB::Client.new(region: 'us-east-1')

get '/profile' do
  item = DYNAMO.get_item(table: 'Profiles', key: { user_id: params[:user_id] }).item
  if item
    # Ensure no CRLF in fields that may be rendered in headers or JSON
    bio = item['bio'].to_s.gsub(/[\r\n]+/, ' ').strip
    { user_id: item['user_id'], bio: bio }.to_json
  else
    halt 404
  end
end

3. Use a whitelist for header-safe values

When setting headers derived from DynamoDB, restrict values to known-safe patterns. For example, for a custom header like X-Item-Status, allow only predefined values.

VALID_STATUSES = %w[active inactive pending]

get '/item/:item_id' do
  item = DYNAMO.get_item(table: 'Items', key: { id: params[:item_id] }).item
  if item && VALID_STATUSES.include?(item['status'])
    response['X-Item-Status'] = item['status']
    { id: item['id'], status: item['status'] }.to_json
  else
    halt 400, 'Invalid status'
  end
end

These practices reduce the attack surface by ensuring that data from DynamoDB is never directly reflected into protocol-sensitive locations such as headers or redirects. middleBrick’s scans can verify that input validation is in place and that no user-controlled CRLF characters reach headers or URL fields.