Dangling Dns in Grape with Mutual Tls

A dangling DNS record in a Grape API backend, combined with Mutual TLS (mTLS) configured for transport-layer identity, can expose authorization and spoofing risks despite mTLS providing client certificate validation. The vulnerability arises when DNS resolution for a hostname used in mTLS certificate verification or in route construction becomes inconsistent with runtime service locations.

Consider a Grape service that validates incoming client certificates against a list of trusted common names (CNs) or subject alternative names (SANs). If the server’s advertised hostname (e.g., api.example.com) has a dangling DNS record that points to a different environment or an unmanaged IP, an attacker could obtain a valid certificate issued for the legitimate CN but connect to a different endpoint. The mTLS handshake will succeed because the certificate matches the CN/SAN, yet the request is routed to a service that may have weaker authorization or data isolation, leading to BOLA/IDOR-like confusion across environments.

Simultaneously, the server-side Grape route may construct internal URLs or perform hostname-based authorization checks using the value from DNS. If DNS resolution returns an unexpected or stale address, the authorization logic that relies on the resolved hostname can be bypassed. For example, an access control that allows "only calls from api.internal.example.com" may evaluate true for a request that resolved to a different IP whose reverse DNS does not match, while mTLS still verified the client identity. This mismatch between identity (mTLS) and address-based controls creates a privilege escalation path.

In practice, this combination is risky when certificate policies embed hostnames and when deployment pipelines promote the same configuration across environments without DNS consistency. A scan using middleBrick’s OpenAPI/Swagger analysis (with full $ref resolution) can surface mismatches between documented hostnames and runtime endpoints, while the LLM/AI Security checks ensure no prompt or configuration leakage compounds the issue. middleBrick’s cross-referencing of spec definitions with runtime findings highlights such inconsistencies before an attacker exploits them.

Concrete risk pattern: an attacker with a valid certificate for api.example.com connects to a dangling-staging endpoint that lacks proper property authorization checks. The mTLS handshake passes, but the request processes with elevated permissions because internal hostname-based guards relied on the dangling record. This maps to OWASP API Top 10 controls around authentication and authorization mismatches and can intersect with compliance frameworks such as PCI-DSS and SOC2.

Remediation centers on ensuring DNS, certificate identity, and runtime authorization are aligned and validated consistently. Below are concrete Grape code examples that enforce strict hostname verification and avoid reliance on DNS-via-resolve for security decisions.

1. Enforce certificate CN/SAN validation explicitly

Configure the SSL context to verify peer certificates and match a specific hostname, rather than trusting the default resolver for access control.

# config/initializers/grape_mtls.rb
require 'openssl'

ENV['RACK_SSL_CERT_STORE'] = '/path/to/ca-bundle.pem'

class MyApi < Grape::API
  ssl_options {
    verify_mode: OpenSSL::SSL::VERIFY_PEER,
    cert_store: OpenSSL::X509::Store.new.tap { |store| store.add_file('/path/to/ca-bundle.pem') },
    verify_hostname: true
  }

  before do
    # Ensure the peer certificate matches the expected hostname(s)
    peer_cert = request.env['ssl.client_cert']
    unless peer_cert
      error!('Client certificate required', 401)
    end

    # Explicit hostname verification (e.g., against SAN or CN)
    allowed_hostnames = ['api.example.com', 'api.internal.example.com']
    cert_hostname = peer_cert.subject.to_a.find { |a| a[0] == 'CN' }&.last
    sans = extract_sans(peer_cert)
    unless allowed_hostnames.include?(cert_hostname) || (sans & allowed_hostnames).any?
      error!('Certificate hostname mismatch', 403)
    end
  end

  helpers do
    def extract_sans(cert)
      ext = cert.extensions.find { |e| e.oid == 'subjectAltName' }
      return [] unless ext
      ext.value.to_a.map(&:value)
    end
  end

  # Example protected endpoint
  get '/profile' do
    { message: 'authorized' }
  end
end

2. Avoid DNS-dependent authorization decisions

Do not use resolved IP or reverse DNS for access control. Instead, authorize based on certificate identity and application-level permissions.

# app/models/authorizer.rb
class Authorizer
  def self.allowed_for?(certificate, request_hostname)
    # Certificate-based allowlist
    allowed_cns = ['api.example.com']
    cert_hostname = certificate.subject.to_a.find { |a| a[0] == 'CN' }&.last
    return false unless cert_hostname
    allowed_cns.include?(cert_hostname)
  end
end

# In your Grape route
class SecureResource < Grape::API
  before do
    peer_cert = request.env['ssl.client_cert']
    halt 401, 'Client certificate required' unless peer_cert
    halt 403, 'Not authorized' unless Authorizer.allowed_for?(peer_cert, nil)
  end

  get '/data' do
    { data: 'confidential' }
  end
end

3. Use consistent DNS and deployment practices

Ensure that the hostname configured in certificates and the DNS records used by clients are synchronized across environments. Automate validation in CI/CD so that any mismatch is caught before promotion. middleBrick’s Pro plan with continuous monitoring can alert on DNS anomalies and certificate-hostname drift.

4. Validate server-side hostname in route constraints

Use Grape route constraints to ensure requests target the intended hostname, reducing reliance on external lookups.

class ConstrainedAPI < Grape::API
  constraints(host: 'api.example.com') do
    before { error!('Invalid host', 400) unless request.host == 'api.example.com' }

    get '/health' do
      { status: 'ok' }
    end
  end
end

By combining explicit hostname verification in the mTLS handshake, avoiding DNS-based authorization, and synchronizing DNS records with certificate CN/SAN, teams mitigate the risk that a dangling DNS record undermines identity assurance. middleBrick’s GitHub Action integration can enforce these checks in CI/CD, failing builds if risk scores exceed your threshold.