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Securing webhooks
Fractal ID accepts only secure sites (HTTPS) as callback URLs.
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.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:
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.To aid with your development process, here are some example scripts you can use to validate your assumptions.
Ruby
node.js
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 sig
{: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}
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 sig
{"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}
Last modified 4mo ago