Dns Cache Poisoning in Grape with Dynamodb

DNS cache poisoning can occur in a Grape API when service discovery or routing depends on DNS records that an attacker can manipulate. When the backend stores configuration or endpoint metadata in DynamoDB, poisoned DNS can redirect the application to malicious or unexpected network locations, undermining the integrity of service communication. In this scenario, the Grape service retrieves an external service hostname or IP from DynamoDB, then performs a DNS lookup to connect. If a previous cache entry has been poisoned, the lookup may return a rogue IP, causing the application to route sensitive traffic to an attacker-controlled host.

DynamoDB itself does not perform DNS resolution; it stores records such as service endpoints, CNAMEs, or IPs used by the Grape application. If these records reference hostnames rather than direct IPs, and the application relies on system or language-level DNS caching, an attacker who compromises or misconfigures DNS can alter the resolution path between the time the record is read and the connection is made. This timing window is especially relevant in microservice environments where services frequently read endpoint configurations from DynamoDB and then initiate outbound HTTP or gRPC calls through Grape.

The combination increases risk when the Grape app caches DNS results at the runtime level (for example, via a DNS client cache in Ruby or the underlying OS) and uses those results across multiple requests. An attacker does not need to modify DynamoDB content; they only need to affect DNS responses seen by the application or its host. Because DynamoDB is often used as a central configuration store, changes there are typically well-protected, so defenders may overlook the DNS layer, assuming the stored hostname is authoritative and immutable. However, if the hostname resolves to a mutable public DNS zone, the effective endpoint used by Grape can be hijacked without any DynamoDB write being detected.

Common contributing patterns include: using CNAME records in DynamoDB that point to external services with shared hosting, long DNS TTL values that prolong the impact of a poisoned entry, and outbound HTTP clients in Grape that do not re-resolve DNS per request or lack mechanisms to validate endpoint identity. Without active validation of the resolved address against an expected baseline, the poisoned cache can persist across deployments, making the issue difficult to detect via routine log review. MiddleBrick’s checks for SSRF and unsafe consumption are particularly relevant here, as they can surface unexpected network destinations and missing hostname verification in DynamoDB-driven configurations.

Remediation focuses on reducing reliance on mutable DNS-based resolution and ensuring that connections initiated by Grape are as predictable and verifiable as possible. Prefer storing and using direct IP addresses or fixed internal hostnames in DynamoDB where feasible, and avoid long DNS TTLs for externally facing service endpoints that the Grape app consumes. Implement runtime hostname verification and, where possible, pin connections to known certificate identities. The following examples show how to integrate safer patterns into a Grape service that reads endpoint configuration from DynamoDB using the AWS SDK for Ruby.

First, store and retrieve endpoint records that favor explicit IPs or controlled hostnames. When using hostnames, validate them against an allowlist before use. The example below reads a service entry from DynamoDB and ensures the hostname matches an expected pattern before constructing a request.

require 'aws-sdk-dynamodb'
require 'net/http'
require 'uri'

class SafeGrapeService
  ALLOWED_HOST_SUFFIX = 'internal.example.com'

  def initialize
    @ddb = Aws::DynamoDB::Client.new(region: 'us-east-1')
  end

  def call(env)
    # Fetch endpoint record by a known key
    resp = @ddb.get_item(
      table_name: 'service_endpoints',
      key: { service_id: { s: 'payment-processor' } }
    )
    item = resp.item
    raise 'Endpoint not found' unless item && item['hostname']

    hostname = item['hostname']['s']
    # Validate hostname to reduce DNS poisoning impact
    unless hostname.end_with?(ALLOWED_HOST_SUFFIX)
      raise "Disallowed hostname: #{hostname}"
    end

    uri = URI("https://#{hostname}/v1/charge")
    request = Net::HTTP::Post.new(uri)
    request['Content-Type'] = 'application/json'
    request.body = { amount: env['rack.request.form_hash'] }.to_json

    # Use an HTTP client that does not blindly cache DNS; ensure connect_timeout and open_timeout are set
    http = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true
    http.open_timeout = 5
    http.read_timeout = 10

    response = http.request(request)
    # Handle response securely
    { status: response.code, body: response.body }
  rescue => e
    # Log and return a safe error; avoid exposing internal details
    { status: 500, body: { error: 'service_unavailable' }.to_json }
  end
end

Second, if you must rely on DNS, re-resolve for each critical request or use a library that disables persistent DNS caching for the client. The example below disables the default DNS cache for the HTTP client and forces a fresh lookup on each call, which reduces the window during which a poisoned cache can be used. Combine this with short timeouts and explicit IP binding when possible.

require 'aws-sdk-dynamodb'
require 'net/http'
require 'uri'

class FreshDnsGrapeService
  def initialize
    @ddb = Aws::DynamoDB::Client.new(region: 'us-east-1')
  end

  def call(env)
    item = @ddb.get_item(
      table_name: 'service_endpoints',
      key: { service_id: { s: 'reporting-api' } }
    ).item
    raise 'Endpoint missing' unless item&.dig('hostname')

    hostname = item['hostname']['s']
    uri = URI("https://#{hostname}/v1/report")

    # Force a new DNS lookup per request by using IPAddress or low-level socket options
    # Ruby's Net::HTTP does not expose cache control directly; use a custom resolver or
    # a lower-level client (e.g., Excon with `resolver: :noop` or system DNS disabled)
    # Here we demonstrate explicit IP binding as a safer alternative:
    # resolved_ip = resolve_ip_fresh(hostname) # implement a short-lived DNS query
    # ip = resolved_ip or raise 'Unable to resolve'
    # http = Net::HTTP.new(ip, uri.port)
    # http.address = hostname # for SNI if needed

    # Fallback: ensure timeouts are short and verify server certificate
    http = Net::HTTP.new(uri.host, uri.port)
    http.use_ssl = true
    http.open_timeout = 3
    http.read_timeout = 5
    http.verify_mode = OpenSSL::SSL::VERIFY_PEER

    request = Net::HTTP::Get.new(uri)
    response = http.request(request)
    { status: response.code, body: response.body }
  rescue => e
    { status: 500, body: { error: 'request_failed' }.to_json }
  end
end

Finally, integrate MiddleBrick’s scans to detect SSRF, unsafe consumption, and unexpected external destinations that could be influenced by DNS or configuration stored in DynamoDB. Use the GitHub Action to fail builds if risk scores exceed your threshold, and leverage the CLI for on-demand checks during development. The MCP Server can also help validate endpoints directly from your IDE when you modify DynamoDB-backed routing logic. These practices reduce the impact of DNS cache poisoning by ensuring that your Grape service validates, restricts, and monitors the destinations it contacts based on DynamoDB configuration.