Cache Poisoning in Sinatra with Api Keys

Cache poisoning occurs when an attacker manipulates cached responses so that malicious content is served to other users. In a Sinatra application that uses API keys for access control, this risk arises when keys or key-related parameters are improperly handled by caching layers. If API keys are passed in query strings or headers and responses are cached based on the request URL alone, a cached response containing a valid key could be reused for a different user, exposing authorization boundaries.

Consider a Sinatra endpoint that proxies requests to a downstream service and caches results. If the route uses the API key directly as a query parameter, the cache key may include the full URL, including the key. An attacker who can trick a victim into making a single authenticated request might cause the victim’s key to be cached. Subsequent requests to the same URL without the key could then receive the cached response that includes sensitive data or elevated permissions tied to the victim’s key.

For example, a route that forwards an API key to a payment provider and caches the balance could inadvertently share one user’s financial data if the cache ignores the presence of the key. This violates the principle of separating user contexts and can lead to information disclosure across users. The issue is compounded when responses include sensitive headers or cookies that should not be cached, and the cache rules do not differentiate based on authorization metadata like API keys.

Additionally, if the Sinatra app caches error responses keyed only by URL, an attacker may induce errors that include key validation messages, causing those messages to be cached and served to other users. Such exposure can aid further attacks by revealing patterns or valid key formats. Proper cache configuration must ensure that responses containing or dependent on API keys are either not cached or cached with a cache key that incorporates the key or a user-specific context, preventing cross-user contamination.

To mitigate cache poisoning related to API keys in Sinatra, ensure that caching respects authorization boundaries and does not mix user contexts. Below are concrete code examples demonstrating secure handling of API keys.

1) Do not include API keys in cacheable URLs

Avoid passing API keys as query parameters. Instead, use headers or request bodies, and configure caching to ignore sensitive headers.

# Safe Sinatra route: API key passed via header, not query string
require 'sinatra'
require 'net/http'
require 'uri'

before do
  content_type :json
end

get '/account' do
  api_key = request.env['HTTP_X_API_KEY']
  halt 401, { error: 'API key missing' }.to_json unless api_key && valid_key?(api_key)

  uri = URI('https://api.example.com/v1/balance')
  http = Net::HTTP.new(uri.host, uri.port)
  http.use_ssl = true
  request = Net::HTTP::Get.new(uri.request_uri)
  request['X-API-Key'] = api_key

  response = http.request(request)
  # Ensure caching respects user context; do not cache sensitive responses
  if response.code.to_i == 200
    response.body
  else
    halt response.code.to_i, response.body
  end
end

def valid_key?(key)
  # Validate key format against a secure store or pattern
  key =~ /\A[a-zA-Z0-9]{32}\z/
end

2) Configure cache to vary by authorization context

If you must cache responses that depend on API keys, include the key or a user-specific fingerprint in the cache key to prevent cross-user leakage.

# Sinatra with a caching layer that varies by API key
require 'sinatra'
require 'securerandom'

# In-memory cache simulation; in production, use a robust cache with key normalization
CACHE = {}

get '/data' do
  api_key = request.env['HTTP_X_API_KEY']
  halt 401, { error: 'API key missing' }.to_json unless api_key && valid_key?(api_key)

  # Create a cache key that incorporates the API key or a hash of it
  cache_key = "data_v1_#{Digest::SHA256.hexdigest(api_key)}"
  if CACHE.key?(cache_key)
    CACHE[cache_key]
  else
    # Simulate fetching data that is key-specific
    data = { balance: 100, user_hash: cache_key }
    CACHE[cache_key] = data.to_json
  end
end

def valid_key?(key)
  key =~ /\A[a-zA-Z0-9]{32}\z/
end

3) Prevent caching of responses containing sensitive headers or keys

Ensure that responses with sensitive headers, cookies, or error messages are marked as non-cacheable.

# Sinatra: set cache-control headers to avoid storing sensitive responses
require 'sinatra'

before do
  # Do not cache responses that contain API keys or errors that may reveal key info
  cache_control :no_store, :must_revalidate
end

post '/login' do
  api_key = request.env['HTTP_X_API_KEY']
  halt 401, { error: 'Invalid key' }.to_json unless api_key && valid_key?(api_key)

  { token: 'session-token' }.to_json
end

def valid_key?(key)
  key =~ /\A[a-zA-Z0-9]{32}\z/
end

4) Validate and normalize inputs before caching

Never use raw user-supplied values in cache keys. Normalize and validate them to prevent key smuggling via encoding or case manipulation.

# Normalize API key before using in cache logic
require 'sinatra'
require 'digest'

helpers do
  def normalized_key(api_key)
    # Enforce lowercase hex format to avoid case-sensitive cache bypass
    Digest::SHA256.hexdigest(api_key.strip.downcase) if api_key
  end
end

get '/profile' do
  api_key = request.env['HTTP_X_API_KEY']
  normalized = normalized_key(api_key)
  halt 401, { error: 'Invalid key' }.to_json unless normalized

  cache_key = "profile_#{normalized}"
  # Safe caching using normalized key
  CACHE.fetch(cache_key) { { user: 'alice', key_fingerprint: cache_key }.to_json }
end

5) Use short-lived tokens and rotate keys

Short-lived API keys reduce the impact of accidental cache exposure. Implement key rotation and avoid long-term caching of key-dependent responses.