Clickjacking in Grape with Hmac Signatures

Clickjacking is a client-side attack that tricks a user into interacting with a hidden or disguised UI element inside an embedded frame. When a Grape API serves HTML or embeds endpoints used by web pages, missing frame-protection headers can allow an attacker to embed the application’s actions inside an invisible iframe. If the API also relies on Hmac Signatures for request authentication but does not protect the signing process or the rendering context, the combination can amplify risk.

Consider a Grape endpoint that performs a sensitive action (for example, changing an email or initiating a transfer) and validates requests using an Hmac signature in a header. The signature is typically computed over selected parts of the request (method, path, timestamp, and body). If the API response is served with an absent or permissive Content-Security-Policy frame-ancestors directive, an attacker can embed the endpoint in a malicious page. A victim who is authenticated to the API (e.g., via session cookie or a predictable Hmac scheme that relies only on headers) may unknowingly load this page, triggering forged signed requests from the victim’s browser.

Crucially, Hmac Signatures that are computed solely from headers and static parameters can be predictable if the attacker can control or guess part of the data (e.g., a known timestamp window or a nonced body). If the API does not enforce strict referrer or origin checks alongside the Hmac verification, and does not bind the signature to the request’s execution context, a clickjacked request can be successfully validated. The API sees a legitimate Hmac and a valid user session, but the intent of the request is hijacked.

Additionally, if the API returns JSON or form data that is rendered in a page under weak CSP, an attacker might also exfiltrate sensitive information via the forged request flow. While Hmac Signatures help ensure data integrity and origin authenticity, they do not inherently prevent the UI from being embedded. Without explicit frame-ancestors protections and careful binding of the signature to the execution context, the API remains vulnerable to clickjacking despite strong cryptographic integrity checks.

Remediation focuses on preventing the API response from being embedded and ensuring Hmac verification accounts for execution context and anti-forgery tokens. Use strict frame-protection headers, bind nonces or per-request tokens to the Hmac computation, and validate origins rigorously.

1. Prevent embedding with CSP and X-Frame-Options

Ensure every response includes headers that prohibit embedding in iframes:

class < Grape::API
  before do
    header['X-Frame-Options'] = 'DENY'
    header['Content-Security-Policy'] = "frame-ancestors 'none'"
  end

  # your endpoints
end

2. Include a nonce or request token in Hmac

Instead of signing only static headers, include a server-generated nonce or a per-request token. The client must obtain the nonce from a safe endpoint (protected by CSP and SameSite cookies) and include it in the signature base string.

require 'openssl' require 'base64'

def generate_hmac(params, secret)
  data = [params['method'], params['path'], params['nonce'], params['timestamp'], params['body']].join("|")
  Base64.strict_encode64(OpenSSL::HMAC.digest('sha256', secret, data))
end

# Server-side verification example
def verify_hmac(headers, body, secret)
  received = headers['HTTP_X_API_SIGNATURE']
  expected = generate_hmac({
    'method' => env['REQUEST_METHOD'],
    'path'   => env['PATH_INFO'],
    'nonce'  => request.env['HTTP_X_API_NONCE'],
    'timestamp' => request.env['HTTP_X_API_TIMESTAMP'],
    'body'   => body
  }, secret)
  secure_compare(received, expected)
end

def secure_compare(a, b)
  return false unless a && b
  return false unless a.bytesize == b.bytesize
  l = a.unpack "H*"[0]
  r = b.unpack "H*"[0]
  return false unless l.bytesize == r.bytesize
  (l ^ r).unpack('H*').first.to_i(16) == 0
end

3. Enforce strict Origin and Referer validation

Validate the Origin and Referer headers on sensitive endpoints to mitigate cross-origin forged requests:

before do
  origin = request.env['HTTP_ORIGIN']
  referer = request.env['HTTP_REFERER']
  allowed_origin = 'https://your-trusted-domain.com'
  unless origin == allowed_origin || referer&.start_with?(allowed_origin)
    error!('Forbidden', 403)
  end
end

4. Use SameSite and Secure cookies for session binding

Ensure session cookies used in the request flow include SameSite and Secure attributes to reduce the likelihood of cross-origin cookie inclusion:

# In your app configuration or middleware
Rack::Session::Cookie.new(app, {
  key: '_middlebrick_session',
  httponly: true,
  secure: Rails.env.production?,
  same_site: :strict
})

5. Combine Hmac with per-request anti-CSRF tokens

For highly sensitive actions, pair Hmac verification with a synchronizer token pattern. The server provides a short-lived token that must be included both in the Hmac base string and in a request header or body field.

# Server generates token and returns it alongside the signed request
before { env['X_API_CSRF_TOKEN'] = SecureRandom.uuid }

# Client includes this token in the signature and sends it in a header
# Server recomputes Hmac including the token and validates it

By integrating these patterns, you protect both the integrity of Hmac Signatures and the rendering context of your API, effectively mitigating clickjacking risks while preserving strong request authentication.