Crlf Injection in Sinatra with Api Keys

Crlf Injection occurs when an attacker can inject carriage return (CR, \r) and line feed (\n) characters into an HTTP header, causing the header to be split and additional headers or response body content to be injected. In Sinatra, this risk is pronounced when API keys are handled via request headers and the application uses those keys to construct downstream HTTP requests or to set response headers without proper sanitization.

Consider a Sinatra service that accepts an API key in a custom header (e.g., X-API-Key) and uses it to authenticate requests to a backend or to personalize responses. If the application directly embeds the key or a value derived from it into a header without validation, an attacker can supply a key containing \r\n sequences. For example, a crafted key like abc123\r\nX-Extra: injected can cause Sinatra or an intermediary proxy to treat the injected CRLF as a header delimiter. This can lead to response splitting, header injection, or cache poisoning if the backend or proxy does not enforce strict header formatting.

When API keys are logged or echoed in responses or error messages, Crlf Injection can escalate the impact. An attacker-supplied key with injected CRLF can cause log forging or enable HTTP response splitting, which may facilitate cross-site scripting (XSS) in browsers that process the split response. Because API keys often bypass traditional web filters, the combination of user-controlled keys and unchecked header usage creates a path for exploiting Crlf Injection in Sinatra applications that do not sanitize or strictly validate header values derived from keys.

In an OpenAPI 3.0 context, if the API specification defines a header parameter for the API key without explicit pattern constraints or examples of safe usage, the runtime behavior may diverge from the intended contract. middleBrick’s OpenAPI/Swagger analysis (2.0, 3.0, 3.1) with full $ref resolution can highlight mismatches between declared header schemas and runtime behavior, exposing scenarios where Crlf Injection could be leveraged through malformed keys.

Active probing by tools that test input validation and header handling can surface Crlf Injection vectors early. For instance, submitting keys with encoded line breaks and inspecting whether the server preserves strict header separation helps identify unsafe concatenation or logging practices. This aligns with the broader set of 12 security checks in middleBrick, which run in parallel to assess authentication, input validation, and data exposure risks without requiring authentication or credentials.

To mitigate Crlf Injection when handling API keys in Sinatra, enforce strict validation and avoid direct embedding of key values into headers or logs. Below are concrete, safe patterns and code examples for Sinatra applications.

1. Validate and sanitize API key values

Ensure API keys conform to an expected format using a strict regular expression. Reject any key containing control characters, whitespace, or line break sequences.

# config/routes.rb or app.rb
require 'sinatra'
require 'securerandom'

# Accept only alphanumeric and safe symbols; reject \r, \n, and other control chars
API_KEY_PATTERN = /\A[a-zA-Z0-9\-_=]+\z/

helpers do
  def valid_api_key?(key)
    key.is_a?(String) && key.match?(API_KEY_PATTERN)
  end
end

before do
  provided_key = request.env['HTTP_X_API_KEY']
  halt 400, { error: 'Invalid API key format' }.to_json unless valid_api_key?(provided_key)
  # Proceed with authenticated request handling
end

# Example safe usage: do not embed the key directly in outgoing headers
get '/resource' do
  key = request.env['HTTP_X_API_KEY']
  # Use key for lookup or authorization, not for constructing response headers
  { status: 'ok' }.to_json
end

2. Avoid using API key values in header construction

When your Sinatra app makes downstream HTTP calls, do not copy the API key into headers that could be influenced by attacker input. Use a dedicated secret or token store instead.

# Example safe outbound request (using net/http)
require 'net/http'
require 'uri'

post '/forward' do
  key = request.env['HTTP_X_API_KEY']
  # Validate first
  halt 400, { error: 'Invalid key' }.to_json unless valid_api_key?(key)

  uri = URI('https://backend.example.com/api')
  http = Net::HTTP.new(uri.host, uri.port)
  http.use_ssl = true

  req = Net::HTTP::Post.new(uri)
  # Do NOT set req['X-API-Key'] = key; instead use a backend-specific credential
  req['Authorization'] = "Bearer #{ENV['BACKEND_TOKEN']}"
  req['Content-Type'] = 'application/json'
  req.body = { data: 'example' }.to_json

  response = http.request(req)
  { forward_status: response.code, body: response.body }.to_json
end

3. Sanitize logging and error handling

Ensure logs do not reflect raw keys with injected control characters. Truncate or hash keys before logging, and avoid including user input directly in error messages that could enable response splitting.

# Example safe logging
require 'logger'
logger = Logger.new(STDOUT)

before do
  provided_key = request.env['HTTP_X_API_KEY']
  if provided_key && !valid_api_key?(provided_key)
    logger.warn("Invalid API key attempt from #{request.ip}")
  else
    # Hash or truncate for safe logging; never log raw keys with embedded sequences
    safe_ref = Digest::SHA256.hexdigest(provided_key)[0,12] if provided_key
    logger.info("Request authenticated with key ref=#{safe_ref}")
  end
end

These patterns reduce the attack surface for Crlf Injection by ensuring API keys are strictly validated, never directly embedded in headers, and safely handled in logs. They complement the checks provided by middleBrick’s scans, which include input validation and header handling tests without requiring authentication.