Auth Bypass in Grape with Hmac Signatures

Grape is a REST-like API micro-framework for Ruby that lets you define endpoints and attach authentication logic via helpers. When HMAC signatures are used, the client typically sends a signature header derived from a shared secret, the request method, path, timestamp, and payload. The server recomputes the signature and compares it to the value provided. Auth bypass can occur when the comparison is implemented incorrectly, for example by using a non-constant-time string comparison, which enables timing attacks that let an attacker forge valid signatures. Another common pattern is accepting timestamps that are too far in the past or future without strict bounds, allowing replay of intercepted requests within the allowed window. If the server-side logic skips signature validation for certain routes or for specific HTTP methods (such as GET) under the assumption they are safe, an attacker can exploit those routes to access protected resources. Additionally, if the shared secret is exposed in client-side code, logs, or error messages, an attacker can compute valid HMACs and bypass authentication entirely. These issues are not inherent to HMAC or Grape, but arise from implementation mistakes that expand the unauthenticated attack surface that middleBrick scans for as part of its Authentication and BOLA/IDOR checks.

To securely use HMAC signatures in Grape, enforce constant-time comparison, strict timestamp validation, and consistent verification on all relevant routes. Below is a complete, realistic example that you can adapt to your service.

# Gemfile
gem 'grape'
gem 'openssl' # for secure HMAC

# app/api/base_api.rb
require 'openssl'
require 'json'

class BaseApi < Grape::API
  format :json

  helpers do
    def hmac_secret
      # Use environment-managed secrets; avoid hardcoding
      ENV['HMAC_SECRET_KEY']
    end

    def verify_hmac_signature(request)
      return false unless request.env['HTTP_X_API_SIGNATURE']
      return false unless request.env['HTTP_X_API_TIMESTAMP']

      timestamp = request.env['HTTP_X_API_TIMESTAMP']
      signature_header = request.env['HTTP_X_API_SIGNATURE']

      # Reject requests older than 5 minutes to prevent replay
      now = Time.now.to_i
      request_time = Integer(timestamp)
      return false if (now - request_time).abs > 300

      payload_body = request.body.read
      # Reset body for downstream use
      request.body.rewind

      data_to_sign = "#{request.request_method}#{request.path}#{timestamp}#{payload_body}"
      expected_signature = OpenSSL::HMAC.hexdigest('sha256', hmac_secret, data_to_sign)

      # Constant-time comparison to prevent timing attacks
      secure_compare(expected_signature, signature_header)
    end

    def secure_compare(a, b)
      return false unless a.bytesize == b.bytesize

      l = a.unpack 'C*'
      res = 0
      b.each_byte { |byte| res |= byte ^ l.shift }
      res == 0
    end
  end

  before { verified if verified != false }

  route_setting(:auth_required, value: true)

  before do
    verified unless settings.auth_required == false
  end

  helpers do
    def verified
      error!('Unauthorized', 401) unless verify_hmac_signature(request)
    end
  end

  resource :items do
    desc 'List items, requires HMAC auth'
    get do
      { items: [] }
    end

    desc 'Create item, requires HMAC auth'
    post do
      { status: 'created' }
    end
  end
end

Key practices to prevent bypass:

• Always use a constant-time comparison to avoid timing side-channels.
• Include a timestamp or nonce and enforce a tight window (for example, 300 seconds) to prevent replay attacks.
• Verify the signature for all endpoints that access protected data, regardless of HTTP method.
• Never log the shared secret or expose it in error messages; return a generic 401 on verification failure.
• Store the secret in environment variables or a secrets manager, and rotate it periodically.

These steps reduce the risk of authentication bypass and align with findings you may see in a middleBrick scan, which checks for weak authentication mechanisms and authorization issues across the unauthenticated attack surface.