Validating webhook deliveries

About validating webhook deliveries

Once your server is configured to receive payloads, it will listen for any delivery that's sent to the endpoint you configured. To ensure that your server only processes webhook deliveries that were sent by Pactima and to ensure that the delivery was not tampered with, you should validate the webhook signature before processing the delivery further. This will help you avoid spending server time to process deliveries that are not from Pactima and will help avoid man-in-the-middle attacks.

To do this, you need to:

1. Create a secret token for a webhook.
2. Store the token securely on your server.
3. Validate incoming webhook payloads against the token, to verify that they are coming from Pactima and were not tampered with.
   

Creating a secret token

In the Developer Hub, you can create a new webhook with a secret token, or you can add a secret token to an existing webhook. When creating a secret token, you should choose a random string of text with high entropy.

Securely storing the secret token

After creating a secret token, you should store it in a secure location that your server can access. Never hardcode a token into an application or push a token to any repository.

Validating webhook deliveries

Pactima will use your secret token to create a hash signature that's sent to you with each payload. The hash signature will appear in each delivery as the value of the X-WEBHOOK-SIGNATURE-256 header.

In your code that handles webhook deliveries, you should calculate a hash using your secret token. Then, compare the hash that Pactima sent with the expected hash that you calculated, and ensure that they match. For examples showing how to validate the hashes in various programming languages, see Examples .

There are a few important things to keep in mind when validating webhook payloads:

• Pactima uses an HMAC hex digest to compute the hash.
• The hash signature always starts with sha256=.
• The hash signature is generated using your webhook's secret token and the payload contents.
• If your language and server implementation specifies a character encoding, ensure that you handle the payload as UTF-8. Webhook payloads can contain unicode characters.
• Never use a plain == operator. Instead consider using a method like secure_compare or crypto.timingSafeEqual , which performs a "constant time" string comparison to help mitigate certain timing attacks against regular equality operators, or regular loops in JIT-optimized languages.
  

Testing the webhook payload validation

You can use the following secret and payload values to verify that your implementation is correct:

• secret: Password123!
• payload: Hello, World!

If your implementation is correct, the signatures that you generate should match the following signature values:

• signature: 459a3b6683149679ad1041b118c67d16e7cb6526e444214e68e7ad9dc17a566c
• X-WEBHOOK-SIGNATURE-256: sha256=459a3b6683149679ad1041b118c67d16e7cb6526e444214e68e7ad9dc17a566c
  

Examples

You can use your programming language of choice to implement HMAC verification in your code. Following are some examples showing how an implementation might look in various programming languages.

Ruby example

For example, you can define the following verify_signature function:

def verify_signature(payload_body)
  signature = 'sha256=' + OpenSSL::HMAC.hexdigest(OpenSSL::Digest.new('sha256'), ENV['SECRET_TOKEN'], payload_body)
  return halt 500, "Signatures didn't match!" unless Rack::Utils.secure_compare(signature, request.env['HTTP_X_HUB_SIGNATURE_256'])
end

Then you can call it when you receive a webhook payload:

post '/payload' do
  request.body.rewind
  payload_body = request.body.read
  verify_signature(payload_body)
  push = JSON.parse(payload_body)
  "I got some JSON: #{push.inspect}"
end

Python example

For example, you can define the following verify_signature function and call it when you receive a webhook payload:

import hashlib
import hmac
def verify_signature(payload_body, secret_token, signature_header):
    """Verify that the payload was sent from GitHub by validating SHA256.

    Raise and return 403 if not authorized.

    Args:
        payload_body: original request body to verify (request.body())
        secret_token: GitHub app webhook token (WEBHOOK_SECRET)
        signature_header: header received from GitHub (X-WEBHOOK-SIGNATURE-256)
    """
    if not signature_header:
        raise HTTPException(status_code=403, detail="X-WEBHOOK-SIGNATURE-256 header is missing!")
    hash_object = hmac.new(secret_token.encode('utf-8'), msg=payload_body, digestmod=hashlib.sha256)
    expected_signature = "sha256=" + hash_object.hexdigest()
    if not hmac.compare_digest(expected_signature, signature_header):
        raise HTTPException(status_code=403, detail="Request signatures didn't match!")

JavaScript example

For example, you can define the following verifySignature function and call it in any JavaScript environment when you receive a webhook payload:

let encoder = new TextEncoder();

async function verifySignature(secret, header, payload) {
    let parts = header.split("=");
    let sigHex = parts[1];

    let algorithm = { name: "HMAC", hash: { name: 'SHA-256' } };

    let keyBytes = encoder.encode(secret);
    let extractable = false;
    let key = await crypto.subtle.importKey(
        "raw",
        keyBytes,
        algorithm,
        extractable,
        [ "sign", "verify" ],
    );

    let sigBytes = hexToBytes(sigHex);
    let dataBytes = encoder.encode(payload);
    let equal = await crypto.subtle.verify(
        algorithm.name,
        key,
        sigBytes,
        dataBytes,
    );

    return equal;
}

function hexToBytes(hex) {
    let len = hex.length / 2;
    let bytes = new Uint8Array(len);

    let index = 0;
    for (let i = 0; i < hex.length; i += 2) {
        let c = hex.slice(i, i + 2);
        let b = parseInt(c, 16);
        bytes[index] = b;
        index += 1;
    }

    return bytes;
}

TypeScript example

For example, you can define the following verify_signature function and call it when you receive a webhook payload:

import { Webhooks } from "@octokit/webhooks";

const webhooks = new Webhooks({
  secret: process.env.WEBHOOK_SECRET,
});

const handleWebhook = async (req, res) => {
  const signature = req.headers["X-WEBHOOK-SIGNATURE-256"];
  const body = await req.text();

  if (!(await webhooks.verify(body, signature))) {
    res.status(401).send("Unauthorized");
    return;
  }

  // The rest of your logic here
};