Securing webhooks

Callback URL
Fractal ID accepts only secure sites (HTTPS) as callback URLs.
Validating payloads from Fractal ID
When your secret token is set, Fractal ID uses it to create a hash signature with each payload. The hash signature is passed along with each request in the headers as X-Fractal-Signature.
Fractal ID generates signatures using a hash-based message authentication code (HMAC) with SHA-1.
Your endpoint should verify the signature to make sure it came from Fractal ID. Example implementation in Ruby:
def verify_signature
  payload_body = request.body.read
  signature = "sha1=" + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new("sha1"), ENV["SECRET_TOKEN"], payload_body)
​
  if Rack::Utils.secure_compare(signature, request.headers["X-Fractal-Signature"])
    render json: {}, status: 200
  else
    render json: { error: "signature_mismatch" }, status: 400
  end
end
Your language and server implementations may differ than this code. There are a couple of very important things to point out, however:
No matter which implementation you use, the hash signature starts with sha1=, using the key of your secret token and your payload body.
Using a plain == operator is not advised. A method like secure_compare performs a "constant time" string comparison, which renders it safe from certain timing attacks against regular equality operators.
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Last updated 2 years ago