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["WEBHOOK_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:

Use your webhook secret token, not your client secret, for generating the digest for verification.

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.

Local verification

To aid with your development process, here are some example scripts you can use to validate your assumptions.

Ruby

ver.rb

require 'openssl'

webhook_secret_token = ARGV[0]
payload_body = ARGV[1]
expected_signature = ARGV[2]

calculated_signature =
  OpenSSL::HMAC.hexdigest(
    OpenSSL::Digest.new("sha1"),
    webhook_secret_token,
    payload_body,
  )

signature_matches = calculated_signature == expected_signature

puts Hash[
  webhook_secret_token:,
  payload_body:,
  expected_signature:,
  calculated_signature:,
  signature_matches:
].inspect

$ ruby --version
ruby 3.2.2 (2023-03-30 revision e51014f9c0) [x86_64-linux]

$ ruby ver.rb SUP3RS3CR3T my-payload badsig
{:webhook_secret_token=>"SUP3RS3CR3T", :payload_body=>"my-payload", :expected_signature=>"badsig", :calculated_signature=>"6a89633e5f131bfb5f0b5826b33b3bab4bf52068", :signature_matches=>false}

$ ruby ver.rb SUP3RS3CR3T my-payload 6a89633e5f131bfb5f0b5826b33b3bab4bf52068
{:webhook_secret_token=>"SUP3RS3CR3T", :payload_body=>"my-payload", :expected_signature=>"6a89633e5f131bfb5f0b5826b33b3bab4bf52068", :calculated_signature=>"6a89633e5f131bfb5f0b5826b33b3bab4bf52068", :signature_matches=>true}

node.js

ver.js

const { argv } = require('process');
const { createHmac } = require('node:crypto');
argv.splice(0, 2) // Get rid of invocation stuff.

const webhook_secret_token = argv[0]
const payload_body = argv[1]
const expected_signature = argv[2]

const calculated_signature =
  createHmac('sha1', webhook_secret_token)
  .update(payload_body)
  .digest('hex');

signature_matches = calculated_signature == expected_signature

console.log(JSON.stringify({
  webhook_secret_token,
  payload_body,
  expected_signature,
  calculated_signature,
  signature_matches,
}));

$ node --version
v18.15.0

$ node ver.js SUP3RS3CR3T my-payload badsig
{"webhook_secret_token":"SUP3RS3CR3T","payload_body":"my-payload","expected_signature":"badsig","calculated_signature":"6a89633e5f131bfb5f0b5826b33b3bab4bf52068","signature_matches":false}

$ node ver.js SUP3RS3CR3T my-payload 6a89633e5f131bfb5f0b5826b33b3bab4bf52068
{"webhook_secret_token":"SUP3RS3CR3T","payload_body":"my-payload","expected_signature":"6a89633e5f131bfb5f0b5826b33b3bab4bf52068","calculated_signature":"6a89633e5f131bfb5f0b5826b33b3bab4bf52068","signature_matches":true}