Webhook signature verification in Rest API

Introduction

Webhook signature verification helps you check that messages you get really come from the right sender and not from someone pretending to be them.

When you receive data from a service and want to be sure it is safe and not fake.

When you want to protect your app from bad actors sending false information.

When you need to trust that the webhook data has not been changed during transmission.

When you want to follow security best practices for APIs and webhooks.

Syntax

Rest API

1. Receive webhook request with headers and body.
2. Extract the signature from the request headers.
3. Use your secret key to create a hash of the request body.
4. Compare your hash with the signature from the header.
5. If they match, accept the webhook; otherwise, reject it.

The secret key is shared only between you and the webhook sender.

Hashing algorithms like HMAC with SHA256 are commonly used.

Examples

This example shows how to compare the signature header with your computed hash in Python.

Rest API

signature = request.headers['X-Signature']
computed_hash = hmac.new(secret_key, request.body, hashlib.sha256).hexdigest()
if hmac.compare_digest(signature, computed_hash):
    process_webhook()
else:
    reject_request()

This JavaScript example uses Node.js crypto to verify the webhook signature safely.

Rest API

const signature = req.headers['x-signature'];
const computedHash = crypto.createHmac('sha256', secretKey)
                      .update(req.body)
                      .digest('hex');
if (crypto.timingSafeEqual(Buffer.from(signature, 'utf8'), Buffer.from(computedHash, 'utf8'))) {
  processWebhook();
} else {
  rejectRequest();
}

Sample Program

This program simulates receiving a webhook with a signature and verifies it using HMAC SHA256. It prints a message depending on whether the signature matches.

Rest API

import hmac
import hashlib

# Simulated webhook request data
request_body = b'{"order_id":12345,"status":"paid"}'
secret_key = b'mysecretkey'

# Simulate signature sent by webhook sender
signature = hmac.new(secret_key, request_body, hashlib.sha256).hexdigest()

# Function to verify webhook signature
def verify_webhook(body, signature, secret):
    computed_hash = hmac.new(secret, body, hashlib.sha256).hexdigest()
    return hmac.compare_digest(signature, computed_hash)

# Check the webhook
if verify_webhook(request_body, signature, secret_key):
    print("Webhook verified: process the data")
else:
    print("Webhook verification failed: reject the data")

Important Notes

Always use a timing-safe comparison function like hmac.compare_digest to avoid timing attacks.

Keep your secret key private and never share it publicly.

Check the webhook documentation for the exact header name and hashing method used.

Summary

Webhook signature verification confirms the webhook is from a trusted source.

It uses a secret key and hashing to compare signatures safely.

Always reject webhooks if the signature does not match.

Practice

1. What is the main purpose of webhook signature verification?

easy

A. To confirm the webhook is from a trusted source

B. To speed up the webhook delivery

C. To encrypt the webhook payload

D. To log the webhook request time

Solution

1. Step 1: Understand webhook security

   Webhook signature verification is used to check if the webhook request is really from the expected sender.

2. Step 2: Identify the purpose of verification

   This process confirms the webhook is trusted by comparing the signature with a secret key.

3. Final Answer:

   To confirm the webhook is from a trusted source -> Option A

4. Quick Check:

   Verification = Trusted source confirmation [OK]

Hint: Verification means checking trust, not speed or encryption [OK]

Common Mistakes:

• Confusing verification with encryption
• Thinking it speeds up webhook delivery
• Assuming it logs request time

2. Which of the following is the correct Python code snippet to compute an HMAC SHA256 signature for webhook verification?

easy

A. hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()

B. hmac.new(payload.encode(), secret.encode(), hashlib.md5).hexdigest()

C. hashlib.sha256(secret + payload).hexdigest()

D. hmac.sha256(secret, payload).hexdigest()

Solution

1. Step 1: Understand HMAC function parameters

   The correct order is secret key first, then message (payload), and the hashing algorithm.

2. Step 2: Identify correct syntax and algorithm

   hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest() uses hmac.new with secret encoded, payload encoded, and hashlib.sha256, which is correct.

3. Final Answer:

   hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest() -> Option A

4. Quick Check:

   Secret first, payload second, SHA256 used [OK]

Hint: Secret key is first argument in hmac.new() [OK]

Common Mistakes:

• Swapping secret and payload order
• Using wrong hash function like md5
• Incorrect function call syntax

3. Given the following Python code snippet, what will be the output if the signature matches the computed HMAC?

import hmac
import hashlib

secret = "mysecret"
payload = "testdata"
signature = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()
computed = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()
if computed == signature:
    print("Signature verified")
else:
    print("Signature mismatch")

medium

A. None

B. "Signature verified"

C. RuntimeError

D. "Signature mismatch"

Solution

1. Step 1: Analyze the signature comparison

   The code compares the computed signature with the received signature and prints "Signature verified" if they match.

2. Step 2: Determine output when signatures match

   Since the signatures match, the output will be the success message "Signature verified".

3. Final Answer:

   "Signature verified" -> Option B

4. Quick Check:

   Matching signatures print verification message [OK]

Hint: Match means print verified message [OK]

Common Mistakes:

• Confusing mismatch message as output
• Expecting an error instead of message
• Assuming no output (None)

4. Identify the error in this Python webhook verification snippet:

import hmac
import hashlib

secret = "mysecret"
payload = "data"
signature = "abc123"

computed = hmac.new(secret, payload.encode(), hashlib.sha256).hexdigest()
if computed == signature:
    print("Verified")
else:
    print("Not verified")

medium

A. payload should not be encoded

B. hashlib.md5 should be used instead of sha256

C. secret should be encoded before use in hmac.new

D. signature comparison should use != instead of ==

Solution

1. Step 1: Check data types for hmac.new

   hmac.new requires the secret key as bytes, but secret is a string and not encoded.

2. Step 2: Identify correct fix

   Encoding secret with secret.encode() fixes the error; payload encoding is correct.

3. Final Answer:

   secret should be encoded before use in hmac.new -> Option C

4. Quick Check:

   Secret must be bytes, so encode it [OK]

Hint: Encode secret string before hmac.new call [OK]

Common Mistakes:

• Not encoding secret key
• Changing payload encoding incorrectly
• Using wrong hash function
• Wrong comparison operator

5. You receive webhook requests with a signature header. To securely verify the webhook, which steps should you perform in order? Select the correct sequence.

hard

A. 1. Retrieve secret key
2. Compare signatures
3. Compute HMAC of payload
4. Accept webhook if signatures do not match

B. 1. Accept webhook
2. Compute HMAC of payload
3. Compare signatures
4. Log the webhook

C. 1. Compute HMAC of payload
2. Retrieve secret key
3. Compare signatures
4. Reject webhook if signatures match

D. 1. Retrieve the secret key
2. Compute HMAC of the payload using the secret
3. Compare computed signature with header signature
4. Accept webhook if signatures match

Solution

1. Step 1: Retrieve the secret key

   You need the secret key first to compute the HMAC signature.

2. Step 2: Compute HMAC of the payload using the secret

   Use the secret key and payload to compute the signature.

3. Step 3: Compare computed signature with header signature

   Check if the computed signature matches the one sent in the webhook header.

4. Step 4: Accept webhook if signatures match

   Only accept the webhook if the signatures are the same to ensure trust.

5. Final Answer:

   1. Retrieve the secret key 2. Compute HMAC of the payload using the secret 3. Compare computed signature with header signature 4. Accept webhook if signatures match -> Option D

6. Quick Check:

   Secret -> Compute -> Compare -> Accept [OK]

Hint: Always compute signature before comparing and accepting [OK]

Common Mistakes:

• Accepting webhook before verification
• Comparing before computing signature
• Rejecting on matching signatures