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Why Verify Webhooks

Because of the way webhooks work, attackers can impersonate services by simply sending a fake webhook to an endpoint. Think about it: it’s just an HTTP POST from an unknown source. This is a potential security hole for many applications, or at the very least, a source of problems. In order to prevent it, SafetyKit signs every webhook and its metadata with a unique key for each endpoint. This signature can then be used to verify the webhook indeed comes from SafetyKit, and only process it if it is. Another potential security hole is what’s called replay attacks. A replay attack is when an attacker intercepts a valid payload (including the signature), and re-transmits it to your endpoint. This payload will pass signature validation, and will therefore be acted upon. To mitigate this attack, SafetyKit includes a timestamp for when the webhook attempt occurred. Our libraries automatically reject webhooks with a timestamp that are more than five minutes away (past or future) from the current time. This requires your server’s clock to be synchronised and accurate, and it’s recommended that you use NTP to achieve this.

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Verifying Webhooks with a Library

The simplest way to verify SafetyKit webhooks is with an open source library.

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Verifying Webhooks Manually

Each webhook call includes three headers with additional information that are used for verification:
  • webhook-id: the unique message identifier for the webhook message. This identifier is unique across all messages, but will be the same when the same webhook is being resent (e.g. due to a previous failure).
  • webhook-timestamp: timestamp in seconds since epoch.
  • webhook-signature: the Base64 encoded list of signatures (space delimited).

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Constructing the signed content

The content to sign is composed by concatenating the id, timestamp and payload, separated by the full-stop character (.). In code, it will look something like: 
signedContent = "${webhook_id}.${webhook_timestamp}.${body}"
Where body is the raw body of the request. The signature is sensitive to any changes, so even a small change in the body will cause the signature to be completely different. This means that you should not change the body in any way before verifying.

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Determining the expected signature

SafetyKit uses an HMAC with SHA-256 to sign its webhooks. So to calculate the expected signature, you should HMAC the signed_content from above using the base64 portion of your signing secret (this is the part after the whsec_ prefix) as the key. For example, given the secret whsec_MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw you will want to use MfKQ9r8GKYqrTwjUPD8ILPZIo2LaLaSw. For example, this is how you can calculate the signature in Node.js: 
const crypto = require('crypto');

signedContent = `${webhook_id}.${webhook_timestamp}.${body}`
const secret = "whsec_5WbX5kEWLlfzsGNjH64I8lOOqUB6e8FH";

// Need to base64 decode the secret
const secretBytes = new Buffer(secret.split('\_')[1], "base64");
const signature = crypto
.createHmac('sha256', secretBytes)
.update(signedContent)
.digest('base64');
console.log(signature);
This generated signature should match one of the ones sent in the webhook-signature header. The webhook-signature header is composed of a list of space delimited signatures and their corresponding version identifiers. The signature list is most commonly of length one. Though there could be any number of signatures. For example: 
v1,g0hM9SsE+OTPJTGt/tmIKtSyZlE3uFJELVlNIOLJ1OE= v1,bm9ldHUjKzFob2VudXRob2VodWUzMjRvdWVvdW9ldQo= v2,MzJsNDk4MzI0K2VvdSMjMTEjQEBAQDEyMzMzMzEyMwo=
Make sure to remove the version prefix and delimiter (e.g. v1,) before verifying the signature. Please note that to compare the signatures it’s recommended to use a constant-time string comparison method in order to prevent timing attacks.

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Verify timestamp

As mentioned above, SafetyKit also sends the timestamp of the attempt in the webhook-timestamp header. You should compare this timestamp against your system timestamp and make sure it’s within your tolerance in order to prevent timestamp attacks.