Dns Cache Poisoning in Grape with Mutual Tls

DNS cache poisoning (also known as DNS spoofing) occurs when an attacker injects a malicious DNS response into a resolver’s cache, causing the resolver to return an attacker-controlled IP for a target hostname. In a Grape API service that relies on outbound HTTP calls to downstream services, poisoned DNS records can redirect those calls to a rogue endpoint. When mutual TLS (mTLS) is used, the client presents a client certificate during the TLS handshake, but the validation performed by the server (or by the client when verifying the server) may still be vulnerable if name checks are incomplete.

With Grape, DNS resolution typically happens at the TCP connection layer when the HTTP client (for example, using Net::HTTP or a wrapper such as Faraday) establishes a connection to the target host. If an attacker on the network path can persuade a DNS resolver used by the service to cache a spoofed record, subsequent connections to that hostname may reach the attacker. Even with mTLS, the client may still complete a TLS handshake with the rogue server if the server’s certificate is accepted. The key exposure arises when hostname verification is not strictly enforced: the client may validate the certificate’s presence and chain, but fail to ensure the certificate’s subject alternative names (SANs) or common name (CN) match the expected hostname. This mismatch allows a poisoned DNS entry to be leveraged in a man-in-the-middle scenario where the attacker presents a valid certificate for a different name, and the client incorrectly accepts it.

Mutual TLS does not prevent DNS cache poisoning; it changes the risk profile. An attacker who can poison DNS might attempt to serve a certificate that matches the poisoned hostname or rely on weak client-side hostname validation. In Grape applications, if the HTTP client does not enforce strict hostname verification, an attacker who successfully poisons DNS could intercept traffic even when mTLS is in use. Moreover, if the client caches DNS entries and does not respect DNS TTLs or uses a resolver with a long cache window, the poisoned entry may persist across requests. The combination of mTLS and DNS cache poisoning therefore centers on whether the client validates server identity (and, in bidirectional scenarios, whether the server validates client identity) alongside DNS correctness.

To summarize the specific interaction: mTLS protects against unauthorized TLS endpoints, but only when hostname verification is rigorous. DNS cache poisoning bypasses name-based assurances by steering connections to an attacker-controlled host. If the Grape client does not enforce strict hostname-to-certificate matching, the mTLS channel can be established with the rogue host, leading to potential data interception or manipulation. Secure configuration requires both proper DNS hygiene and strict certificate policy checks.

Remediation focuses on ensuring strict hostname verification and robust mTLS configuration in Grape HTTP clients and servers. Below are concrete, working examples that you can adapt.

1. Grape API client with mTLS and strict hostname verification

Use a reliable HTTP client such as Faraday with an adapter like Net::HTTP, and enforce hostname verification by providing a custom SSL verification callback. This ensures that even if DNS is poisoned, the client will reject connections where the server certificate does not match the expected hostname.

require 'faraday'
require 'openssl'

conn = Faraday.new(url: 'https://api.example.com') do |faraday|
  faraday.request :url_encoded
  faraday.response :logger
  faraday.adapter Faraday.default_adapter

  # Configure TLS with client certificate and key
  faraday.ssl = {
    client_cert: OpenSSL::X509::Certificate.new(File.read('client.crt')),
    client_key: OpenSSL::PKey::RSA.new(File.read('client.key')),
    verify: true,
    verify_mode: OpenSSL::SSL::VERIFY_PEER,
    # Custom verification to enforce hostname match
    ssl_verify_callback: proc do |preverify_ok, store_ctx|
      # Default OpenSSL verification
      next false unless preverify_ok

      # Extract peer certificate
      cert = store_ctx.current_cert
      # Expected hostname (could be derived from request)
      expected_host = 'api.example.com'

      # Check SANs first, fall back to CN
      san = cert.extensions.find { |e| e.oid == 'subjectAltName' }
      if san
        names = san.value.split(/,\s*/).map { |part| part.strip.sub('DNS:', '') }
        return true if names.any? { |name| name == expected_host }
      end

      # Fallback to CN
      cn = cert.subject.to_s.match(/CN=([^,\s]+)/)&.[](1)
      return cn == expected_host
    end
  }
end

# Example request
response = conn.get('/v1/resource')
puts response.body

2. Grape API server with mTLS and client certificate validation

Configure the server to require client certificates and validate them appropriately. This prevents unauthorized clients from connecting even if DNS is poisoned on the client side (though server-side DNS poisoning is also a concern for inbound connections).

require 'grape'
require 'openssl'

class MyAPI < Grape::API
  format :json

  # In a real deployment, you would terminate TLS at the web server (e.g., Puma with SSL)
  # and forward headers; this example shows how to enforce mTLS within a Rackup setup.
  # For production, prefer a reverse proxy (NGINX, HAProxy) that handles client cert verification.
end

# Puma/SSL configuration example (config/puma.rb or boot file)
ssl_options = {
  cert: OpenSSL::X509::Certificate.new(File.read('server.crt')),
  key: OpenSSL::PKey::RSA.new(File.read('server.key')),
  ca_file: 'ca.pem',
  verify_mode: OpenSSL::SSL::VERIFY_PEER,
  # Optional: reject if client cert is missing or invalid
  verify_callback: proc do |preverify_ok, store_ctx|
    next false unless preverify_ok
    # Additional checks: revocation, policy, etc.
    true
  end
}

Rack::Handler.get(:puma).run MyAPI, ssl_options

3. General recommendations

• Ensure HTTP clients validate server certificates and enforce strict hostname checks (do not disable verification).
• Keep CA bundles up to date and pin certificates where appropriate.
• Use short DNS TTLs for critical endpoints to reduce the window of poisoned cache impact.
• Monitor for unexpected certificate changes and anomalies in TLS handshakes.