Zone Transfer in Grape with Bearer Tokens

A Zone Transfer in Grape with Bearer Tokens arises when an API endpoint that supports DNS zone transfers (AXFR/IXFR) is protected only by bearer token authentication, without additional constraints such as IP allowlists or strict scope validation. Grape is a Ruby DSL for building REST-like APIs, and it does not inherently prevent DNS zone transfers; protection depends on how routes and middleware are configured. Bearer tokens are convenient credentials often stored in headers, but if a token is leaked or over-permissioned, it can allow an unauthenticated network attacker to trigger zone transfers that expose internal hostnames, IP addresses, and infrastructure mapping.

In practice, this risk surfaces in APIs that expose administrative endpoints like /dns/zone/:name/transfer and rely solely on bearer token validation. Because the scan tests unauthenticated attack surface where possible, middleBrick can detect endpoints that accept token-like headers but do not enforce network-level restrictions or verify token scope for sensitive operations. The scanner’s Authentication and BOLA/IDOR checks look for cases where a token intended for one purpose is reused to invoke administrative functionality such as zone transfers. When combined with an Inventory Management check, findings may reveal that internal DNS records are exposed without rate limiting or data exposure controls, increasing the risk of reconnaissance by attackers.

Specific attack patterns include enumeration via repeated transfer requests using a single bearer token, or token reuse across environments where staging or debug endpoints mistakenly accept transfers that should be restricted to internal networks. Because middleBrick runs active probe sequences, it can identify whether token-protected zone transfer endpoints respond with detailed error messages or data leakage, which further aids reconnaissance. The scanner also flags missing Transport Layer Security enforcement and insufficient input validation on zone name parameters, which can lead to SSRF or injection alongside data exposure.

Consider a realistic Grape endpoint that lacks network restrictions:

class DNSAPI < Grape::API
  format :json

  before do
    token = env['HTTP_AUTHORIZATION']&.to_s.gsub('Bearer ', '')
    error!('Unauthorized', 401) unless valid_token?(token)
  end

  post '/transfer' do
    zone = params[:zone]
    # No IP restriction, no scope validation
    dns_transfer(zone)
  end
end

def valid_token?(token)
  # simplistic check for example
  token == ENV['BEARER_TOKEN']
end

def dns_transfer(zone)
  # Implementation that may perform an actual zone transfer
  Resolv::DNS.new.query(zone, 'SOA')
end

In this example, if the bearer token is compromised, an attacker can invoke /transfer to request zone data without IP-based controls or token scoping that limits administrative actions. The scanner’s checks for Data Exposure and Encryption will note whether responses contain sensitive records, and the LLM/AI Security probes verify whether verbose error output aids further attacks.

Remediation focuses on reducing the token’s attack surface and adding network and operational constraints so that bearer tokens alone cannot trigger zone transfers. Implement scope-based tokens, enforce IP allowlists for administrative routes, and avoid exposing internal details in error messages. These practices align with the scanner’s findings and reduce risk across Authentication, BOLA/IDOR, and Data Exposure checks.

First, scope bearer tokens to specific actions and validate permissions on each request. Instead of a global token, issue tokens with claims that indicate allowed operations. For zone transfer endpoints, require an additional administrative scope and verify it server-side.

class Auth
  def self.verify_token(token)
    # decode JWT or validate against a store
    # return payload or nil
  end
end

class DNSAPI < Grape::API
  format :json

  helpers do
    def require_scope!(expected)
      token = env['HTTP_AUTHORIZATION']&.to_s.gsub('Bearer ', '')
      payload = Auth.verify_token(token)
      error!('Unauthorized', 401) unless payload
      error!('Insufficient scope', 403) unless payload['scopes']&.include?(expected)
      payload
    end

    def allowed_ip?
      allowed = ['10.0.0.1', '10.0.0.2']
      allowed.include?(env['REMOTE_ADDR'])
    end
  end

  post '/transfer' do
    payload = require_scope!('zone:transfer')
    halt 403, { error: 'IP not allowed' }.to_json unless allowed_ip?
    zone = params[:zone]
    halt 400, { error: 'Invalid zone' }.to_json unless zone =~ /^[a-z0-9.\-]+\.example\.com$/i
    dns_transfer(zone)
  end
end

def dns_transfer(zone)
  Resolv::DNS.new.query(zone, 'SOA')
end

Second, enforce network-level restrictions for administrative endpoints. Even with scoped tokens, combine IP allowlists and, where possible, bind administrative interfaces to internal networks only. This reduces the impact of token leakage and aligns with the scanner’s emphasis on defense in depth.

Third, ensure tokens are transmitted exclusively over TLS and implement short lifetimes with rotation. The scanner’s Encryption check will highlight whether endpoints accept tokens over insecure channels, and the Rate Limiting check can detect abnormal request volumes that suggest abuse. For continuous protection under the Pro plan, configure scanning schedules to detect regressions when endpoints or token handling logic changes.

Finally, validate and sanitize zone name inputs to prevent SSRF or command injection. Use strict regex patterns and avoid direct shell or library calls that interpret input beyond expected DNS names. These measures reduce the attack surface that middleBrick’s Input Validation and SSRF checks monitor, ensuring token usage does not open additional avenues for abuse.