Cache Poisoning in Sinatra

Cache poisoning in Sinatra applications typically exploits the framework's flexible routing and response handling to inject malicious content into shared caches. Unlike traditional web applications, Sinatra's minimalist design means developers often implement caching manually using Rack middleware or HTTP headers, creating opportunities for attackers to manipulate cache keys and serve poisoned content to other users.

The most common Sinatra-specific attack pattern involves manipulating query parameters or headers that influence response content but aren't properly included in cache keys. For example, an endpoint that returns different content based on a 'user_id' parameter but caches responses using only the URL path:

get '/profile' do
  user_id = params[:user_id] || current_user.id
  user = User.find(user_id)
  cache_control :public, :max_age => 300
  user.to_json
end

An attacker can request '/profile?user_id=1' to populate the cache, then have other users receive the attacker's data when they access their own profiles. The cache key generation defaults to the request path without considering query parameters that affect the response body.

Another Sinatra-specific vulnerability arises from the framework's template rendering system. When using ERB or other template engines with caching enabled, attackers can sometimes inject template variables that persist across requests:

get '/dashboard' do
  @user = current_user
  cache_control :public, :max_age => 600
  erb :dashboard
end

If the dashboard template includes user-controllable content without proper escaping, and the rendered output is cached, an attacker could inject malicious HTML or JavaScript that serves to all subsequent visitors.

Session-based cache poisoning also occurs when Sinatra applications use in-memory caches like Memcached or Redis without proper isolation. A poisoned session ID or authentication token can cause the cache to serve malicious content to legitimate users:

configure do
  use Rack::Session::Cookie, secret: ENV['SESSION_SECRET']
  set :cache, Dalli::Client.new
end

get '/settings' do
  user = User.find(session[:user_id])
  settings.cache.set("user_settings_#{session[:user_id]}", user.settings)
  user.settings.to_json
end

If an attacker can predict or manipulate session IDs, they can poison the cache with settings that affect other users who later receive the same cached response.

Detecting cache poisoning in Sinatra applications requires examining both the code patterns and runtime behavior. Start by analyzing your route handlers for caching implementations that don't account for all variables affecting the response:

# Vulnerable pattern - cache key doesn't include user_id
get '/data/:id' do |id|
  data = DataModel.find(id)
  cache_control :public, max_age: 300
  data.to_json
end

# Safer pattern - includes variable in cache key
get '/data/:id' do |id|
  cache_key = "data_#{id}"
  data = settings.cache.fetch(cache_key) { DataModel.find(id) }
  cache_control :public, max_age: 300
  data.to_json
end

middleBrick's scanning engine specifically identifies Sinatra applications by detecting the framework's characteristic patterns: the use of 'get', 'post', 'put', 'delete' methods, the 'configure' block, and the 'settings' object. The scanner then analyzes how caching is implemented across your API endpoints.

The scanner tests for cache poisoning by making requests with varying parameters and checking if responses are served from cache inappropriately. For example, it might:

• Request an endpoint with specific query parameters
• Modify those parameters slightly
• Check if the modified request returns the original cached response
• Verify if sensitive data from one user appears in another user's response

middleBrick's LLM security module also checks for AI-specific cache poisoning scenarios in applications using language model endpoints, testing for prompt injection that could affect cached model responses.

Runtime detection involves monitoring cache hit rates and analyzing response variations. Tools like Rack::Cache provide detailed logging about cache operations, while custom middleware can track which request attributes influence cached responses:

class CacheKeyAnalyzer
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
    # Track which parameters affect the response
    sensitive_params = ['user_id', 'session_id', 'auth_token']
    cache_key = request.path
    
    sensitive_params.each do |param|
      if request.params[param]
        cache_key += "_#{request.params[param]}" if request.params[param]
      end
    end
    
    env['rack.cache.key'] = cache_key
    @app.call(env)
  end
end

Remediating cache poisoning in Sinatra requires a multi-layered approach that combines proper cache key generation, input validation, and response isolation. The most effective strategy is to ensure cache keys include all variables that affect the response content:

# Instead of generic caching
get '/profile' do
  user_id = params[:user_id] || current_user.id
  user = User.find(user_id)
  cache_control :public, max_age: 300
  user.to_json
end

# Use specific cache keys
get '/profile' do
  user_id = params[:user_id] || current_user.id
  cache_key = "profile_#{user_id}"
  user = settings.cache.fetch(cache_key) { User.find(user_id) }
  cache_control :public, max_age: 300
  user.to_json
end

For applications using Rack::Cache, customize the cache key generation to include relevant headers and parameters:

configure do
  use Rack::Cache,
    metastore: 'memcached://localhost:11211/meta',
    entitystore: 'memcached://localhost:11211/body',
    default_ttl: 300
end

before do
  # Include user-specific headers in cache key
  cache_key_headers = ['X-User-ID', 'Authorization']
  cache_key = request.path
  
  cache_key_headers.each do |header|
    cache_key += "_#{request.env[header]}" if request.env[header]
  end
  
  env['rack.cache.key'] = cache_key
end

Implement strict input validation for parameters that influence cached responses. Use strong parameter filtering and type checking:

get '/data/:id' do |id|
  # Validate that id is a positive integer
  halt 400, 'Invalid ID' unless id =~ /\A[1-9][0-9]*\z/
  
  cache_key = "data_#{id}"
  data = settings.cache.fetch(cache_key) { DataModel.find(id) }
  cache_control :public, max_age: 300
  data.to_json
end

For template-based responses, use Sinatra's built-in protection against XSS and ensure cached content is properly escaped:

get '/dashboard' do
  @user = current_user
  cache_control :public, max_age: 600
  # Use escaped output in templates
  erb :dashboard, locals: { user: @user }
end

Consider implementing cache segmentation based on user roles or permissions. Different user types should have isolated cache entries:

helpers do
  def role_based_cache_key(base_key)
    role = current_user&.role || 'guest'
    "#{base_key}_#{role}_#{current_user&.id || 'anon'}"
  end
end

get '/reports' do
  cache_key = role_based_cache_key('reports')
  reports = settings.cache.fetch(cache_key) { generate_reports }
  cache_control :public, max_age: 600
  reports.to_json
end

For applications with high security requirements, disable public caching entirely and use private, user-specific caching mechanisms:

# Disable public caching
before do
  headers 'Cache-Control' => 'private, no-store, no-cache'
end

# Use session-based caching instead
get '/sensitive-data' do
  cache_key = "user_data_#{session[:user_id]}"
  data = settings.cache.fetch(cache_key) { fetch_sensitive_data }
  data.to_json
end