Cache Poisoning in Sinatra with Hmac Signatures

Cache poisoning occurs when an attacker causes a cache to store a malicious response that is served to other users. In Sinatra applications that use Hmac Signatures to validate requests, a misconfiguration or oversight in how the signature is computed and verified can enable an attacker to poison cached responses.

Consider a Sinatra endpoint that caches responses based on the request URL and selected query parameters. If the application signs only a subset of request attributes (for example, the path and a static secret) but uses the cache key on a broader set of inputs that include attacker-controlled values, an attacker can vary non-signature inputs to cause distinct cache entries. The Hmac Signature may still validate because the signed portion remains unchanged, while the cache differentiates responses by unsigned parameters. This mismatch allows an attacker to inject a poisoned response into the cache under a key that appears valid to the application and subsequent users.

For example, imagine a route like /api/items that supports a category query parameter. The Sinatra app generates an Hmac Signature over the path and a secret, stores the response in the cache keyed by the full URL including category, and later serves cached content without re-validating that the signed inputs align with the cache key. An attacker can request /api/items?category=safe to get a response cached under that key, then cause the application to treat later requests for category=malicious as a cache hit for the safe response if the signature verification does not include the category parameter. The vulnerability is compounded when the cache is shared across users or is public, enabling widespread exposure of tainted content.

Real-world parallels include issues observed in systems where cache keys diverge from the data covered by integrity checks, leading to SSRF-like exposure or unauthorized data manipulation. While this is not a direct deserialization or injection flaw, it falls under the broader impact patterns seen in SSRF and data exposure checks, where trust boundaries between validation and storage are not consistently enforced.

To mitigate cache poisoning when using Hmac Signatures in Sinatra, ensure that the cache key and the signed inputs are aligned. The signature must cover all parameters that influence the response and are used to construct the cache key. Below is a concrete, working example that demonstrates a secure approach.

First, define a helper to compute the Hmac signature over a canonical set of request components. This example uses the openssl library and includes the HTTP method, the full path with sorted query parameters, and a server-side secret. All components that affect the cached response are included in the signature.

require 'sinatra'
require 'openssl'
require 'uri'
require 'cgi'

SECRET = ENV['HMAC_SECRET'] || 'replace-with-strong-secret'

def compute_hmac(request)
  uri = URI(request.url)
  params = CGI.parse(uri.query || '')
  # Normalize: sort keys, include only values relevant to the response
  canonical_params = params.sort.map { |k, vs| vs.map { |v| "#{k}=#{v}" } }.flatten.join('&')
  message = "#{request.request_method}:#{uri.path}?#{canonical_params}"
  OpenSSL::HMAC.hexdigest('sha256', SECRET, message)
end

def valid_signature?(request)
  provided = request.get_header('HTTP_X_API_SIGNATURE') || ''
  expected = compute_hmac(request)
  # Constant-time comparison to avoid timing attacks
  ActiveSupport::SecurityUtils.secure_compare(expected, provided)
end

Next, use the signature in your route and tie the cache key to the same canonical representation. This ensures that any variation that changes the signed data also changes the cache key, preventing mismatches.

helpers do
  def cache_key(request)
    uri = URI(request.url)
    params = CGI.parse(uri.query || {})
    sorted = params.sort.map { |k, vs| vs.map { |v| "#{k}=#{v}" } }.flatten.join('&')
    "v1:#{request.request_method}:#{uri.path}:#{sorted}"
  end
end

get '/api/items' do
  halt 401, { error: 'invalid signature' }.to_json unless valid_signature?(request)
  key = cache_key(request)
  cache_store.fetch(key) do
    # Expensive operation that depends on all query parameters
    items = fetch_items_from_source(request.params)
    { items: items }.to_json
  end
end

In this setup, cache_store represents your caching layer (e.g., Redis or a memory store). The key incorporates the method, path, and normalized query parameters, so responses are cached separately when any of those inputs differ. The signature covers the same set, ensuring that a cached response cannot be reused for a different set of parameters unless the signature also matches. Additionally, always use a strong secret stored securely, prefer constant-time comparison to avoid timing leaks, and avoid including user-controlled data that should not affect the response in either the signature or the cache key.