Broken Authentication in Hanami with Hmac Signatures

Broken authentication can occur when Hmac Signatures are implemented inconsistently across Hanami endpoints, enabling attackers to forge or replay requests. Hanami encourages explicit parameter handling, but if the signature is computed over a subset of the request components (e.g., omitting the request path or a critical header), an attacker can manipulate those excluded elements without detection.

Consider a Hanami action that relies on an Hmac-Signature header for integrity. If the signature is generated using only the request body and a shared secret, but does not include the HTTP method, the full URL path, or a timestamp, an attacker can reuse a valid signature across different endpoints or methods. This violates the principle of binding the signature to the full request context. For example, an attacker could take a valid POST /users/1/suspend request with a valid Hmac signature and replay it against /users/1/enable if both endpoints accept the same signature scheme without additional safeguards.

Another common issue is weak secret management. Hanami applications that embed the Hmac shared secret in configuration files checked into version control expose a critical risk. If an attacker gains read access to the repository, they can compute valid signatures for arbitrary requests. Additionally, failing to enforce signature expiration allows replay attacks; an intercepted request with a valid Hmac signature could be resent later to perform unauthorized actions, such as modifying or deleting resources.

Inconsistent verification logic between development and production further exacerbates the problem. A Hanami service might validate the Hmac signature for sensitive endpoints in production but skip verification for certain routes during local testing. This discrepancy can lead to production deployments that inadvertently accept unsigned or tampered requests. The OWASP API Security Top 10 category "2023-A07: Identification and Authentication Failures" aligns with these risks, as broken authentication often stems from incomplete or misconfigured signature validation.

To illustrate a vulnerable pattern, imagine a Hanami action that builds the signature string without canonicalizing the query parameters, leading to different valid signatures for semantically identical requests. An attacker can exploit ordering differences to forge a valid Hmac signature. This is especially dangerous when the application does not enforce a strict parameter sorting policy before hashing. Real-world CVEs in related ecosystems have shown that missing canonicalization and missing coverage of the request target are frequent contributors to authentication bypass via Hmac weaknesses.

Remediation centers on ensuring the Hmac signature covers all aspects of the request that an attacker can control, including the HTTP method, the full path (including normalized query parameters), the request body, and a short-lived timestamp. Below are concrete code examples for a secure Hanami action that validates Hmac Signatures.

First, define a method to build a canonical string for signing. This includes method, path, sorted query parameters, and body, joined with a newline to prevent ambiguity:

# app/services/hmac_verifier.rb
class HmacVerifier
  def self.canonical_string(request)
    query = request.params.to_h.sort.map { |k, v| "#{k}=#{v}" }.join('&')
    [request.request_method, request.path, query, request.body.read].join("\n")
  end

  def self.verify(request, secret)
    received = request.headers['X-Api-Signature']
    return false unless received
    payload = canonical_string(request)
    expected = OpenSSL::HMAC.hexdigest('sha256', secret, payload)
    secure_compare(expected, received)
  end

  def self.secure_compare(a, b)
    return false unless a.bytesize == b.bytesize
    l = a.unpack 'C*'
    res = 0
    b.each_byte.with_index { |byte, i| res |= byte ^ l[i] }
    res == 0
  end
end

In your Hanami action, use the verifier before processing the request:

# app/actions/users/update.rb
class Users::Update
  include Hanami::Action

  def call(params)
    secret = ENV['HMAC_SHARED_SECRET']
    request = self.request

    unless HmacVerifier.verify(request, secret)
      self.status = 401
      self.body = { error: 'invalid_signature' }.to_json
      return
    end

    # Proceed with authorized update logic
    user_id = params[:id]
    # ... update user
  end
end

Ensure the shared secret is stored securely, for example via environment variables or a secrets manager, and never committed to source control. Also include a timestamp (e.g., X-Request-Timestamp) and reject requests older than a short window (such as five minutes) to prevent replay attacks. When combined with Hanami's explicit parameter handling and strict routing, these measures reduce the likelihood of broken authentication related to Hmac Signatures.