Dns Cache Poisoning in Hanami with Hmac Signatures

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 targeted hostname. In Hanami applications that rely on HMAC signatures to validate the origin or integrity of external data (e.g., webhook signatures, API tokens, or delegated authorization tokens), this attack surface can be compounded if signature verification logic is influenced by or depends on DNS-resolved endpoints.

Consider a Hanami service that verifies an HMAC signature on a webhook request. If the verification process involves fetching a public key or a signing secret from a hostname that is resolved via DNS, a poisoned DNS cache can redirect the application to a malicious server controlled by the attacker. That malicious server can then present a valid public key or shared secret that matches the attacker’s chosen payload, allowing the attacker to forge HMAC-signed messages that the Hanami application will accept as legitimate. Because HMAC signatures rely on shared secrets or public-key material, any substitution at the resolution layer can bypass integrity checks without triggering signature mismatch errors.

The risk is particularly acute when the Hanami application uses runtime DNS resolution for service discovery or configuration endpoints that are baked into HMAC verification workflows. For example, if a microservice calls a DNS-resolved authentication endpoint to validate an HMAC-signed JWT, and the DNS response is poisoned, the service may validate signatures against a key provided by the attacker. Because the signature algorithm and key material are trusted implicitly, the application may treat maliciously crafted requests as authorized, leading to privilege escalation, unauthorized data access, or request smuggling.

In the context of the 12 security checks performed by middleBrick, DNS-related misconfigurations can surface indirectly through signature-validation logic that depends on external hosts. While middleBrick does not perform active DNS poisoning, it can detect whether your API surface includes unauthenticated endpoints or inputs that could be influenced by external resolution paths, and it will highlight findings related to Input Validation and Data Exposure that may intersect with DNS trust assumptions. This is especially relevant when combined with LLM/AI Security checks: an attacker could attempt prompt injection via poisoned inputs that rely on DNS-resolved configuration, testing whether the system treats external data as authoritative without sufficient validation.

Because HMAC signatures require precise key material, any deviation—such as resolving a hostname to an unintended IP—can break the trust chain. Defense requires strict control over how endpoints are resolved, verified, and cached, and it demands that signature validation logic treat DNS resolution as an untrusted input rather than a trusted configuration source.

To mitigate DNS cache poisoning risks in Hanami when using HMAC signatures, you must ensure that key material and endpoints are obtained through trusted, verifiable channels and that runtime DNS resolution does not undermine signature integrity. Below are concrete remediation steps and code examples for Hanami applications.

1. Pin hostnames and avoid runtime DNS for critical keys

Instead of resolving hostnames at runtime for keys or secrets, embed the resolved IP or use a static configuration. If you must use DNS, cache the result securely and validate it against a fingerprint or pinned certificate.

2. Validate public keys using certificate pinning or a trusted registry

When verifying HMAC-signed payloads that include a key identifier, fetch the key from a pinned source rather than a DNS-resolved endpoint.

require "openssl"
require "base64"

module SignatureVerification
  # Pinned public key (e.g., obtained out-of-band or via a trusted registry)
  PUB_KEY_PEM = <<~PEM
    -----BEGIN PUBLIC KEY-----
    MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2exampleverylongkey=
    -----END PUBLIC KEY-----
  PEM

  def self.verify_hmac_signature(payload, signature_b64, key_id)
    # Ensure key_id matches expected pinned key; do not resolve via DNS
    raise "Unexpected key_id" unless key_id == "expected-key-id-2024"

    public_key = OpenSSL::PKey::RSA.new PUB_KEY_PEM
    payload_bytes = payload.encode("ASCII-8BIT")
    signature = Base64.strict_decode64(signature_b64)
    public_key.verify(OpenSSL::Digest::SHA256.new, signature, payload_bytes)
  end
end

3. Use HMAC with a static shared secret stored securely

Avoid deriving the shared secret from external sources that depend on DNS. Store the secret in environment variables or a secrets manager that is not dynamically resolved via DNS at runtime.

require "openssl"
require "base64"

module HmacUtils
  SHARED_SECRET = ENV.fetch("HMAC_SHARED_SECRET") # Set via secure configuration, not DNS

  def self.generate_signature(payload)
    OpenSSL::HMAC.hexdigest("sha256", SHARED_SECRET, payload)
  end

  def self.verify_signature(payload, received_signature)
    expected = generate_signature(payload)
    # Use secure compare to avoid timing attacks
    ActiveSupport::SecurityUtils.secure_compare(expected, received_signature)
  end
end

4. Enforce strict input validation and reject unsigned or mismatched sources

Treat any external input that influences signature verification as untrusted. Validate hostname/IP against an allowlist and ensure that DNS resolution is not used to select keys or secrets.

require "uri"

module EndpointValidator
  ALLOWED_HOSTS = ["keys.example.com", "auth.example.com"]

  def self.allowed_host?(uri)
    ALLOWED_HOSTS.include?(URI.parse(uri).host)
  end
end

# Usage before fetching key material
uri = "https://keys.example.com/public_key.pem"
raise "Blocked: untrusted host" unless EndpointValidator.allowed_host?(uri)

5. Combine with middleBrick checks for ongoing risk visibility

Use the middleBrick CLI to scan your Hanami API endpoints and verify that inputs influencing HMAC validation are properly constrained. The dashboard and GitHub Action integrations can help you enforce that risk scores remain within acceptable thresholds after remediation.