Credential Stuffing in Sinatra with Hmac Signatures

Credential stuffing is an automated attack where previously breached username and password pairs are systematically tried against an application to gain unauthorized access. In Sinatra applications that rely on HMAC signatures for request authentication, a common misconfiguration can weaken the protection and enable successful credential stuffing.

HMAC signatures are designed to provide integrity and authenticity by signing a canonical representation of the request using a shared secret. If the server uses only a subset of request properties to build the signature—such as HTTP method, path, and selected headers—but does not include critical mutable or predictable parameters like timestamps, nonces, or account identifiers, an attacker can reuse a captured, valid signature across multiple authentication attempts. Without a per-request nonce or a tightly bound timestamp, the same signature remains valid for different credential pairs, effectively turning the signature into a reusable token that can be replayed across user accounts.

Additionally, if the Sinatra endpoint that validates HMAC signatures does not enforce rate limiting or account-level lockout mechanisms, an attacker can automate thousands of login attempts using the same signature while iterating over many usernames and passwords. Because the signature verification passes, the application may process each request as legitimate until it detects anomalies via other controls, which may not exist. This combination of a reusable HMAC-based authentication scheme and a lack of per-request uniqueness or throttling exposes the authentication path to credential stuffing.

Another subtle risk arises when the HMAC signature is computed over a JSON body that includes user-supplied fields like email or username but the server does not enforce strict canonicalization. Different serialization orders or optional fields can produce different string representations, leading to signature validation inconsistencies. Attackers can exploit these inconsistencies to replay a valid signature with altered credential payloads, especially when the endpoint does not tightly validate that the signed context matches the submitted credentials.

Real-world patterns such as predictable resource IDs or missing binding between the signature and the account being authenticated further enable attackers to test credentials without triggering detection. For example, if the signature does not incorporate the target user identifier, a single intercepted signature might be tried against many user accounts, increasing the likelihood of a successful match. These weaknesses highlight the importance of designing HMAC-based authentication in Sinatra to include non-repeating values and to apply robust, account-aware protections against automated login abuse.

To mitigate credential stuffing risks when using HMAC signatures in Sinatra, you must ensure each signed request is unique, bound to the intended account, and validated with strict canonicalization. Below are concrete code examples that demonstrate a hardened approach.

Include a nonce and timestamp in the signature base string

Ensure the signature covers a combination of method, path, timestamp, nonce, and the account identifier. This prevents signature reuse across requests and across users.

require 'sinatra'
require 'openssl'
require 'base64'
require 'json'
require 'securerandom'

helpers do
  def current_timestamp
    Time.now.utc.strftime('%Y-%m-%dT%H:%M:%S.%LZ')
  end

  def generate_nonce
    SecureRandom.hex(16)
  end

  def build_signature_base(method, path, timestamp, nonce, account_id, body)
    [method.upcase, path, timestamp, nonce, account_id, body].join("\n")
  end

  def compute_hmac(secret, message)
    OpenSSL::HMAC.hexdigest('sha256', secret, message)
  end
end

before do
  request.body.rewind
  @request_body = request.body.read
  # Ensure Content-Type header is preserved for canonicalization
  content_type 'application/json' unless request.content_type.nil?
end

post '/login' do
  account_id = params['account_id'] || JSON.parse(@request_body)['account_id']
  username   = params['username'] || JSON.parse(@request_body)['username']
  password   = params['password'] || JSON.parse(@request_body)['password']

  timestamp = current_timestamp
  nonce     = generate_nonce

  signature_base = build_signature_base(
    request.request_method,
    request.path_info,
    timestamp,
    nonce,
    account_id,
    @request_body
  )

  secret = ENV['HMAC_SECRET']
  signature = compute_hmac(secret, signature_base)

  # Include timestamp, nonce, and account_id in request headers for verification
  env['x-auth-timestamp'] = timestamp
  env['x-auth-nonce']     = nonce
  env['x-auth-account']   = account_id
  env['x-auth-signature'] = signature

  # Proceed with credential verification and authentication logic
  status 200
  body JSON.dump({ status: 'ok', account_id: account_id })
end

Validate signature, timestamp freshness, and nonce uniqueness on the server

On the server side, verify the HMAC using the same canonicalization and reject requests with stale timestamps or repeated nonces to prevent replay and credential stuffing.

post '/login' do
  received_signature = request.env['HTTP_X_AUTH_SIGNATURE']
  timestamp          = request.env['HTTP_X_AUTH_TIMESTAMP']
  nonce              = request.env['HTTP_X_AUTH_NONCE']
  account_id         = request.env['HTTP_X_AUTH_ACCOUNT']

  # Reject if any required header is missing
  halt 400, JSON.dump(error: 'Missing authentication headers') unless [received_signature, timestamp, nonce, account_id].all?

  # Enforce timestamp window (e.g., 5 minutes) to mitigate replay
  request_time = Time.parse(timestamp)
  unless (Time.now - request_time).abs <= 300
    halt 400, JSON.dump(error: 'Stale timestamp')
  end

  # Maintain a short-term store to reject reused nonces (e.g., Redis)
  if nonce_used_recently?(nonce)
    halt 403, JSON.dump(error: 'Duplicate nonce detected')
  end
  mark_nonce_as_used(nonce)

  # Recompute signature on the server using the same canonical base
  request.body.rewind
  body = request.body.read
  signature_base = build_signature_base(
    request.request_method,
    request.path_info,
    timestamp,
    nonce,
    account_id,
    body
  )

  secret = ENV['HMAC_SECRET']
  expected_signature = compute_hmac(secret, signature_base)

  unless secure_compare(expected_signature, received_signature)
    halt 401, JSON.dump(error: 'Invalid signature')
  end

  # Perform account-specific credential validation here
  # Ensure username/password checks are bound to account_id to prevent cross-account attempts
  status 200
  body JSON.dump({ status: 'authenticated', account_id: account_id })
end

# Constant-time comparison to avoid timing attacks
def secure_compare(a, b)
  return false if a.nil? || b.nil?
  return false unless a.bytesize == b.bytesize
  l = a.unpack 'C*'
  r = 0
  b.each_byte { |v| r |= v ^ l.shift }
  r == 0
end

# Placeholder nonce store (use Redis in production)
$used_nonces = {}
def nonce_used_recently?(nonce)
  !!$used_nonces[nonce]
end

def mark_nonce_as_used(nonce)
  $used_nonces[nonce] = true
end