Cache Poisoning in Grape with Basic Auth

Cache poisoning occurs when an attacker causes a cache to store malicious content, which is then served to other users. In Grape, a Ruby API framework, this can happen when responses are cached based on insufficient request validation. When Basic Authentication is used, the Authorization header is typically included in the request. If caching logic (e.g., at a reverse proxy or within application code) uses the request URI as the cache key while ignoring or improperly handling the Authorization header, responses for one user may be incorrectly served to another.

Consider a scenario where an endpoint returns user-specific data, such as /api/profile, and the cache key is derived only from the path and query parameters. If Basic Auth is passed via the Authorization header and not part of the cache key, an authenticated user with a valid token might receive a cached response intended for a different user. This violates confidentiality and can expose one user’s data to another, a classic Insecure Direct Object Reference (IDOR) pattern in the context of caching.

The risk is compounded when the cached response includes sensitive headers or cookies that should not be shared across users. For example, if the server caches a response that includes an Authorization header value or a session-related cookie, downstream requests may inadvertently leak credentials. Cache poisoning in this context does not require malicious input manipulation but relies on misconfigured caching rules that fail to account for authentication context.

Real-world attack patterns mirror issues documented in the OWASP API Top 10, particularly A01: Broken Object Level Authorization. While not directly an IDOR, cache poisoning can enable or amplify such vulnerabilities by allowing unauthorized data access through cached responses. Tools like middleBrick can detect these risks during an unauthenticated scan by analyzing how the API handles authentication and caching headers, identifying missing Vary headers or inconsistent cache key construction that could lead to exposure.

Using middleBrick’s OpenAPI/Swagger spec analysis, the scanner resolves $ref definitions and cross-references them with runtime behavior. If the spec defines security schemes using Basic Auth but does not explicitly exclude cached responses from being shared, middleBrick flags the endpoint with a finding. This helps developers understand how authentication context should influence caching behavior, supporting compliance with frameworks such as OWASP API Top 10 and SOC2 controls related to data segregation.

To prevent cache poisoning when using Basic Authentication in Grape, ensure that the cache key includes elements that differentiate authenticated contexts. The most effective approach is to incorporate the normalized Authorization header or a derived user identifier into the cache key. This prevents responses from being shared across users with different credentials.

Below is a syntactically correct example of Basic Auth usage in Grape, followed by a secure caching pattern.

require 'grape'
require 'base64'

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

  def call(env)
    request = Rack::Request.new(env)
    if request.path.start_with?('/api')
      auth_header = request.env['HTTP_AUTHORIZATION']
      if auth_header&&auth_header.start_with?('Basic ')
        encoded = auth_header.split(' ').last
        decoded = Base64.strict_decode64(encoded)
        username, password = decoded.split(':', 2)
        # Validate credentials against a secure store
        if valid_credentials?(username, password)
          env['api.endpoint'].set_current_user(username)
        else
          return [401, { 'WWW-Authenticate' => 'Basic' }, ['Unauthorized']]
        end
      else
        return [401, { 'WWW-Authenticate' => 'Basic' }, ['Unauthorized']]
      end
    end
    @app.call(env)
  end

  private

  def valid_credentials?(username, password)
    # Replace with secure credential verification
    username == 'admin' && password == 'secure_password'
  end
end

class MyAPI < Grape::API
  use AuthMiddleware

  before do
    # Ensure Vary header is set to differentiate cached responses
    header['Vary'] = 'Authorization'
  end

  get '/profile' do
    # Cache key should include current user context
    user = current_user
    cache_key = "profile:#{user.username}"
    # Assume cache_store is configured elsewhere
    Rails.cache.fetch(cache_key, expires_in: 5.minutes) do
      { username: user.username, email: user.email }
    end
  end
end

In this example, the Vary: Authorization header instructs caches to treat responses with different Authorization headers as distinct. Including the username in the cache key ensures that each user receives their own cached data. middleBrick’s CLI can be used to verify that such headers are present and correctly configured by running middlebrick scan <url> and reviewing the findings related to caching and authentication.

For teams using the middleBrick Pro plan, continuous monitoring can alert when endpoints lack proper Vary headers or when cache keys do not incorporate authentication context. This helps maintain secure caching behavior as APIs evolve, reducing the risk of unintended data exposure through cached responses.