Zone Transfer in Grape with Basic Auth

A Zone Transfer (DNS zone transfer) is the replication of DNS zone data from a primary nameserver to one or more secondary nameservers. When a DNS server allows zone transfers to unauthorized clients, an attacker can obtain the full mapping of hostnames to IP addresses within a domain. This reconnaissance data supports further attacks such as internal service enumeration, targeted phishing, or infrastructure mapping.

In the Ruby web framework Grape, APIs are typically built as namespaced classes that inherit from Grape::API. If such an API is inadvertently configured to support DNS zone transfers and is protected only by HTTP Basic Authentication, the combination can expose internal DNS data in two ways:

• The endpoint logic may directly invoke a DNS library to perform a transfer, and weak or missing authorization checks allow unauthenticated or low-privilege access.
• Basic Auth credentials may be reused across services. If an attacker obtains or guesses these credentials, they can perform a zone transfer that would otherwise be restricted to trusted secondary servers.

Consider a Grape endpoint that performs a zone transfer without properly validating the requester’s authorization to request DNS data:

class DnsApi < Grape::API
  format :json

  helpers do
    def authorized?(token)
      # Weak comparison or hardcoded token
      token == 'secret123'
    end
  end

  before do
    error!('Unauthorized', 401) unless authorized?(env['HTTP_AUTHORIZATION']&.to_s)
  end

  get '/zone' do
    require 'resolv'
    resolver = Resolv::DNS.new
    # Unsafe: performs a full AXFR without IP-based restrictions
    resolver.getresources('example.com', Resolv::DNS::Resource::IN::AXFR)
  end
end

In this example, Basic Auth is used for access control, but if the token is leaked, hardcoded, or weakly compared, an attacker can authenticate and trigger a zone transfer. Even if the token is strong, relying solely on HTTP Basic Auth fails to restrict transfers to authorized secondary servers, because Basic Auth does not enforce network-level trust. Additionally, if the Grape app is exposed in a staging or misconfigured environment, unauthenticated endpoints or misapplied before filters can bypass the intended protection, leading to an unauthenticated or improperly authenticated zone transfer.

Because middleBrick scans the unauthenticated attack surface and tests authenticated routes when credentials are supplied, it can detect whether a zone transfer endpoint is reachable and whether Basic Auth is correctly enforced. The LLM/AI Security checks of middleBrick further probe for system prompt leakage or attempts to bypass controls, ensuring that both the DNS-specific misconfiguration and the authentication mechanism are evaluated.

To mitigate zone transfer risks when using Basic Auth in Grape, apply strict authorization, avoid hardcoded credentials, and enforce network-level restrictions. Use environment variables or a secure vault for credentials, validate tokens rigorously, and limit zone transfers to known IPs.

Below are concrete, secure code examples that implement these practices.

1. Use environment variables for credentials

Never hardcode tokens or passwords. Load them from the environment so they can be managed securely outside the codebase.

class SecureDnsApi < Grape::API
  format :json

  helpers do
    def expected_token
      ENV['DNS_BASIC_AUTH_TOKEN']
    end

    def authorized?(auth_header)
      return false unless auth_header&.start_with?('Basic ')
      encoded = auth_header.split(' ').last
      decoded = Base64.strict_decode64(encoded)
      # Compare securely to avoid timing attacks
      ActiveSupport::SecurityUtils.secure_compare(decoded, expected_token)
    rescue ArgumentError, NoMethodError
      false
    end
  end

  before do
    error!('Unauthorized', 401) unless authorized?(env['HTTP_AUTHORIZATION'])
  end

  get '/zone' do
    require 'resolv'
    resolver = Resolv::DNS.new
    # Optionally restrict by resolver IPs
    allowed_ips = ['192.0.2.10', '198.51.100.20']
    client_ip = request.ip
    error!('Forbidden', 403) unless allowed_ips.include?(client_ip)

    resolver.getresources('example.com', Resolv::DNS::Resource::IN::AXFR)
  end
end

2. Combine Basic Auth with IP allowlisting

Basic Auth protects against casual access, but zone transfers should only be permitted from specific secondary nameservers. Use a whitelist of IPs to enforce this at the application layer.

class RestrictedZoneApi < Grape::API
  format :json

  ALLOWED_ZONE_TRANSFER_IPS = ['203.0.113.5', '198.51.100.15'].to_set

  helpers do
    def client_ip
      # Handle proxies if present; in production, set trusted proxies in config
      env['HTTP_X_FORWARDED_FOR']&.split(',')&.first&.strip || request.ip
    end

    def zone_transfer_allowed?
      ALLOWED_ZONE_TRANSFER_IPS.include?(client_ip)
    end
  end

  before do
    error!('Forbidden: zone transfers restricted by IP', 403) unless zone_transfer_allowed?
  end

  get '/zone' do
    require 'resolv'
    resolver = Resolv::DNS.new
    resolver.getresources('example.com', Resolv::DNS::Resource::IN::AXFR)
  end
end

3. Use strong token validation and secure comparisons

When using token-based Basic Auth, avoid simple string equality. Decode the credentials safely and use constant-time comparison to prevent timing attacks.

middleBrick’s authentication checks validate whether Basic Auth is correctly enforced and whether tokens are handled securely. The CLI command middlebrick scan <url> can be used to test these endpoints from the terminal, while the GitHub Action can enforce security thresholds in CI/CD pipelines.