Cache Poisoning in Grape (Ruby)

Cache poisoning in a Grape API built with Ruby occurs when an attacker tricks the service into storing and serving malicious cache entries, causing other users to receive attacker-controlled responses. This typically arises from unsafe use of request-derived values in cache keys or cache-control directives within Grape endpoints. Because Grape encourages explicit routing and parameter handling, developers may inadvertently include untrusted input such as subdomain, custom headers, or query parameters directly into caching decisions without validation or normalization.

Consider a Grape API that varies cache by a brand URL parameter to serve tenant-specific content. If the cache key is built directly from the parameter value without strict allowlisting, an attacker can request /api/products?brand=evil.com and cause the brand string to be stored as a cache entry. Subsequent requests for seemingly benign brands may receive the poisoned response if the cache lookup matches the tainted key or if cache normalization is inconsistent between runtime and storage layer. The unauthenticated attack surface of middleBrick scanning helps detect such weaknesses by testing how the API handles varied inputs and whether responses differ in unsafe ways across parameter values.

Ruby-specific nuances amplify the risk. For example, string interpolation and symbol conversion can lead to inconsistent cache-key representations, and certain Ruby cache stores may treat headers or subdomains in a case-sensitive or mutable way that differs from runtime expectations. If Grape routes rely on params[:subdomain] or headers like X-Client-Region to determine caching behavior without strict validation, an attacker can manipulate these values to poison entries across users or tenants. MiddleBrick’s checks for Input Validation and Property Authorization are designed to surface these exposures by correlating spec definitions with runtime behavior, highlighting where untrusted input reaches cache-affecting logic.

SSRF and Data Exposure checks are relevant because cache poisoning can sometimes be chained with request forgery to force the server to cache responses from internal or malicious endpoints, or to exfiltrate sensitive data via manipulated cache entries. In Ruby on Grape, failing to normalize or reject unexpected parameter values, headers, or hostnames allows attackers to manipulate cache behavior and persist malicious content across the application’s caching layer. By combining strict input validation with careful cache-key design and runtime verification, teams can reduce the likelihood of cache poisoning in Grape services.

To remediate cache poisoning in Grape with Ruby, enforce strict allowlisting and canonicalization of any request-derived data used in cache keys or cache-control decisions. Validate and normalize inputs before they influence caching, avoid direct use of user-controlled strings as cache keys, and ensure consistent representation across Ruby and the cache store. The following examples illustrate secure patterns.

1) Safe cache key construction with allowlist validation:

class ValidatedProductEndpoint < Grape::API
  version 'v1', using: :path
  format :json

  ALLOWED_BRANDS = %w[acme widget trustedco].freeze

  helpers do
    def safe_cache_key(brand, user_region)
      # Normalize and restrict brand to a known set
      normalized_brand = ALLOWED_BRANDS.include?(brand) ? brand : 'default'
      # Combine with region only after validation
      "products:brand=#{normalized_brand}:region=#{user_region}"
    end
  end

  get '/products' do
    brand = params['brand'].to_s.strip.downcase
    region = request.env['HTTP_X_REGION']&.to_s || 'unknown'
    key = safe_cache_key(brand, region)

2) Using Rack cache with controlled vary headers:

class SecureApi < Grape::API
  use Rack::Cache, 
      verbose: true,
      metastore: 'file:/tmp/rack/meta',
      entitystore: 'file:/tmp/rack/body'

  before do
    # Only vary by strict, validated headers
    header 'Vary', 'Accept-Encoding' if ENV['RACK_ENV'] == 'production'
    # Reject requests with unexpected host-derived inputs
    error!('Bad Request', 400) if request.host.end_with?('.attacker.com')
  end

  get '/items' do
    # No brand-based cache control; safe defaults
    cache_control :public, max_age: 3600
    { items: [] }
  end
end

3) Avoiding symbol conversion pitfalls and ensuring consistent serialization:

class HardenedCache < Grape::API
  helpers do
    def normalized_key(params)
      # Avoid symbols from user input; use strings consistently
      parts = [
        'v2',
        params.fetch('tenant_id', 'public'),
        params.fetch('locale', 'en')
      ]
      parts.map(&:to_s).join(':')
    end
  end

  get '/resource' do
    tenant = params['tenant_id'].to_s
    locale = params['locale'].to_s
    key = normalized_key('tenant_id' => tenant, 'locale' => locale)
    # Store and retrieve using the normalized key
    { key: key, data: 'safe_payload' }
  end
end

These patterns emphasize input validation, canonicalization, and avoiding direct inclusion of potentially mutable request values in cache logic. By combining these practices with middleBrick’s scans for BOLA/IDOR, Input Validation, and Data Exposure, teams can identify and remediate cache poisoning risks in Grape APIs.