Dns Cache Poisoning in Grape with Bearer Tokens

DNS cache poisoning (also known as DNS spoofing) occurs when an attacker injects forged DNS responses into a resolver’s cache, causing the resolver to return an attacker-controlled IP for a legitimate domain. In a Grape-based API, this can intersect with Bearer token handling in subtle ways that do not directly break token cryptography, but change where tokens are resolved or how requests are routed, potentially exposing credentials or causing tokens to be sent to malicious infrastructure.

Consider a Grape API that accepts a hostname or service identifier from the client (for example, a multi-tenant deployment or a request that must forward to an upstream service). If the server resolves that hostname using a system or library resolver and the resolver’s cache is poisoned, the hostname might resolve to an IP under attacker control. Because the API validates only the token format (not the origin server’s authenticity beyond hostname), the request may be processed as intended by the developer, but the Bearer token is inadvertently sent to the attacker. This is especially relevant when tokens are passed via headers such as Authorization: Bearer and the routing or introspection logic depends on the resolved identity of an upstream host.

Another scenario involves token introspection endpoints or OAuth provider discovery. If discovery documents (e.g., .well-known/oauth-authorization-server) are fetched from a domain susceptible to cache poisoning, the API might be directed to an attacker-hosted endpoint that mimics the authorization server. The API then uses Bearer tokens obtained from the client to communicate with this malicious endpoint, potentially leaking tokens or enabling token replay. While Bearer tokens themselves remain cryptographically intact, their confidentiality and integrity depend on the integrity of the network path and resolved endpoints; DNS cache poisoning undermines that integrity by substituting malicious infrastructure without invalidating the token itself.

Grape does not inherently manage DNS resolution, so any reliance on system libraries or HTTP clients that use the OS or shared resolver cache can be affected. The vulnerability therefore resides not in Grape or the token format, but in how the application resolves names and how it ties token usage to network destinations. Attackers need not exploit Grape code directly; they exploit a weak dependency chain where tokens are used after a potentially poisoned resolution step.

Remediation centers on ensuring that token usage does not implicitly trust resolved network endpoints and that sensitive operations validate the intended destination independently of DNS cache state. Below are concrete patterns and code examples for a Grape API.

Pin critical endpoints and avoid dynamic resolution for token validation

When your API validates or introspects Bearer tokens, use pinned or statically configured endpoints rather than dynamically resolving hostnames. For token introspection, prefer direct HTTPS calls to well-known, verified URLs.

require 'net/http'
require 'uri'
require 'json'

module TokenHelpers
  INTROSPECTION_URL = 'https://auth.example.com/introspect'.freeze

  def self.introspect_token(token)
    uri = URI(INTROSPECTION_URL)
    http = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true
    # Pin certificate or verify via a pinned public key in production for stronger assurance
    request = Net::HTTP::Post.new(uri.request_uri, {
      'Content-Type' => 'application/x-www-form-urlencoded',
      'Authorization' => "Basic #{Base64.strict_encode64('client_id:client_secret')}"
    })
    request.body = "token=#{URI.encode_www_form_component(token)}"
    response = http.request(request)
    JSON.parse(response.body)
  rescue StandardError => e
    { active: false, error: e.message }
  end
end

In your Grape endpoint, call the above helper and avoid resolving any hostname from client input when dealing with token validation.

Validate hostname against an allowlist when forwarding or redirecting

If your API must forward requests based on a client-supplied host, resolve the hostname once, verify it against an allowlist or strict pattern, and use an IP or fixed hostname for subsequent connections.

class V1 & Grape::API
  format :json

  ALLOWED_FORWARD_HOSTS = %w[api.partner.com secure.partner.com].freeze

  helpers do
    def safe_forward_host(input_host)
      # Normalize and validate against an allowlist; reject anything not explicitly allowed
      return nil unless input_host.is_a?(String) && !input_host.empty?
      uri = URI.parse("https://#{input_host}")
      return nil unless uri.host && ALLOWED_FORWARD_HOSTS.include?(uri.host)
      # Use a fixed IP or hostname for actual socket connections if desired
      uri.host
    rescue URI::InvalidURIError
      nil
    end
  end

  route_param :service do
    get '/resource' do
      host = safe_forward_host(params[:service])
      unless host
        error!({ error: 'Invalid or unauthorized service host' }, 400)
      end
      # Use a pre-resolved, pinned IP or fixed hostname for outbound requests
      # Do NOT re-resolve the hostname from user input at request time
      target_host = host
      # Perform outbound request to target_host using pinned configuration
      # ... your logic here ...
      { forwarded_to: target_host, status: 'ok' }
    end
  end
end

Enforce HTTPS and certificate verification for all token-related calls

Ensure that any HTTP client used with Bearer tokens enforces TLS and does not skip verification. Avoid passing custom CA stores or disabling verification based on configuration that could be influenced by poisoned discovery.

require 'net/http'
require 'openssl'

ctx = OpenSSL::SSL::SSLContext.new
ctx.verify_mode = OpenSSL::SSL::VERIFY_PEER
ctx.ca_file = '/path/to/pinned-ca-bundle.pem'

http = Net::HTTP.new('auth.example.com', 443)
http.use_ssl = true
http.ssl_context = ctx
request = Net::HTTP::Get.new('/.well-known/oauth-authorization-server')
response = http.request(request)
# Process only if response is valid and served from expected origin

Use environment-controlled configuration for critical endpoints

Store sensitive endpoints and certificate pins in environment variables or secure configuration that is not derived from user input. This reduces the attack surface for manipulation via DNS or runtime configuration poisoning.

# config/environment.rb or platform-specific secure config
ENV['OAUTH_INTROSPECTION_URL'] = 'https://auth.example.com/introspect'
ENV['OAUTH_DISCOVERY_URL'] = 'https://auth.example.com/.well-known/oauth-authorization-server'

Monitoring and anomaly detection

Log resolution failures and unexpected IPs for token-related outbound calls. Correlate with DNS query anomalies where possible, but remember that middleBrick does not fix or block; it detects and reports. Use these logs to identify patterns that may indicate poisoning attempts targeting your token flows.