Cache Poisoning in Grape with Hmac Signatures

Cache poisoning in the context of a Grape API that uses Hmac Signatures occurs when an attacker causes a cached response to be stored under a URL or cache key that is shared across users, and that response contains user-specific or sensitive data. Because Hmac Signatures are typically used to validate the integrity and origin of requests or query parameters, an implementation that improperly incorporates signature input into the cache key—or fails to include critical differentiating data—can lead to cache collisions.

Consider a Grape endpoint that caches responses based on a combination of path and selected query parameters, and uses an Hmac Signature to authenticate a subset of those parameters. If the signature is computed only over a subset of the request (for example, only the resource identifier) and the cache key does not include the full set of user-specific or tenant-specific inputs (such as an account ID or an accept header), a poisoned cache entry can be reused across different users or roles. Real-world cache poisoning techniques such as parameter pollution or header injection can manipulate the portion of the request that influences caching while keeping the Hmac Signature valid, especially if the signature does not cover all inputs that affect the response.

For instance, an attacker might inject an additional query parameter that the caching layer uses to form the cache key, but which the Hmac verification ignores or treats as non-critical. The signature remains valid because the attacker does not need to forge the secret; they simply rely on the existing, valid signature attached to the request. If the backend uses the cached response for the poisoned key without revalidating user context, other users may receive a response intended for a different user or with modified data. This is particularly relevant when responses contain PII or when the endpoint behavior differs based on authorization attributes that are not part of the signature scope.

In a Grape API, this risk is compounded when OpenAPI/Swagger specifications are used to generate client or server code without carefully mapping which request attributes must be included in both the Hmac Signature and the cache key. If the spec defines query parameters for filtering or sorting but does not explicitly require them to be part of the signature or cache normalization logic, developers might inadvertently create a mismatch between what is signed and what is cached. The scanner checks in middleBrick’s 12 security checks would highlight inconsistencies between authenticated surface, input validation, and data exposure findings, emphasizing that signatures must align with cache boundaries to prevent cross-user leakage.

To remediate cache poisoning when using Hmac Signatures in Grape, ensure that the cache key incorporates all inputs that meaningfully affect the response, and that the Hmac Signature covers those same inputs. Below are concrete code examples demonstrating a secure approach.

First, compute the Hmac Signature over a canonical string that includes all relevant request dimensions—path, query parameters that affect the response, and any user or tenant identifier. Then use the same set of parameters to derive the cache key, ensuring that a valid signature implies a unique cache entry.

# Signature and cache key generation in Grape
require 'openssl'
require 'base64'
require 'cgi'

class SecureEndpoint < Grape::API
  before { authenticate_hmac_signature! }

  helpers do
    def canonical_request(params, headers, request_path)
      # Include parameters that affect the response and a stable header like Accept
      parts = [
        request_path,
        params.sort.map { |k, v| "#{k}=#{CGI.escape(Array(v).join(','))}" },
        headers['Accept']
      ]
      parts.join('\n')
    end

    def compute_hmac(secret, message)
      OpenSSL::HMAC.hexdigest('sha256', secret, message)
    end

    def authenticate_hmac_signature!
      provided = env['HTTP_X_SIGNATURE']
      message  = canonical_request(params, request.headers, request.path)
      expected = compute_hmac(ENV['HMAC_SECRET'], message)
      error!('Unauthorized', 401) unless Rack::secure_compare(provided, expected)
    end

    def cache_key_for(params, headers)
      # Derive cache key from the same canonical input used for Hmac
      parts = [
        request.path,
        params.sort.map { |k, v| "#{k}=#{Array(v).join(',')}" },
        headers['Accept']
      ]
      Digest::SHA256.hexdigest(parts.join('|'))
    end
  end

  get '/reports/:id' do
    cache_key = cache_key_for(params, request.headers)
    # pseudo-cache lookup/store using cache_key
    # response = cache.fetch(cache_key) { generate_report(params) }
    { cached_with_key: cache_key, params: params.to_h }
  end
end

Second, avoid including untrusted or non-essential inputs in the signature scope, and normalize inputs to prevent parameter-pollution variants from producing different canonical strings. For example, always treat multi-valued parameters consistently (e.g., sort and join) and exclude headers that are not part of the security boundary.

Finally, validate that any data used to form the cache key is also validated by input validation checks. middleBrick’s checks for Input Validation, Data Exposure, and Authentication help surface inconsistencies where signature scope and cache scope diverge. In the Pro and Enterprise plans, continuous monitoring can alert you when new parameters appear in requests without corresponding updates to signature or cache logic.