Trying to understand how to properly verify the x-genesys-signature header in a python lambda function that acts as an eventbridge consumer for GC webhooks. i need to prevent replay attacks but the documentation is vague on the exact hmac algorithm and key derivation.
i am using hmac.new with sha256 but my verification keeps failing with a mismatch error. here is my current snippet:
import hmac
import hashlib
secret = os.environ.get('GC_WEBHOOK_SECRET')
signature = headers.get('x-genesys-signature')
payload = event.get('body')
calculated = hmac.new(secret.encode(), payload.encode(), hashlib.sha256).hexdigest()
if not hmac.compare_digest(calculated, signature):
raise ValueError("Invalid signature")
the signature header seems to include a timestamp prefix like t=1678899000,h=abc123.... should i be stripping the timestamp before comparing? or is the timestamp part of the data being signed? also, does the payload need to be the raw string or the json object? i am getting a 401 unauthorized from my downstream redshift loader when the check fails. any code examples for this specific validation logic would be appreciated.
Check your key derivation and payload encoding. The signature is generated using the secret from the webhook configuration, not the client ID or secret. You must use hmac.new(secret.encode('utf-8'), payload.encode('utf-8'), hashlib.sha256).hexdigest() where payload is the raw request body string. Ensure you are capturing the body before any middleware modifies it. For Lambda, event['body'] is usually a string, but if it’s JSON, you must serialize it back to the exact string format Genesys sent. Also, verify the timestamp in the header against your server time to prevent replay attacks. Genesys sends x-genesys-signature and x-genesys-timestamp. The timestamp should be within a 5-minute window. Here is a minimal reproducible example for verification:
In your Lambda handler, extract event['headers']['x-genesys-signature'] and event['headers']['x-genesys-timestamp']. Store the secret in AWS Secrets Manager, not environment variables. If the body is None, decode it from base64 if isBase64Encoded is true. I cache verified tokens in Redis with a 60s TTL to avoid repeated HMAC calculations for high-volume webhooks, but for direct verification, this logic is sufficient. Ensure the secret matches exactly what is configured in the Genesys Cloud admin console under Integrations > Webhooks.
import hmac
import hashlib
import time
def verify_signature(secret, body, signature, timestamp):
# Check replay attack window (5 minutes)
current_time = int(time.time())
if abs(current_time - int(timestamp)) > 300:
return False
# Calculate expected signature
expected_sig = hmac.new(
secret.encode('utf-8'),
body.encode('utf-8'),
hashlib.sha256
).hexdigest()
# Constant time comparison
return hmac.compare_digest(expected_sig, signature)
Make sure you capture the raw body exactly as it arrives. The docs state “The signature is calculated over the raw request body.” I copy-pasted the snippet from but it still failed because I was using json.dumps on the parsed object. You must use the raw string from the event.
import hmac
import hashlib
secret = "your_webhook_secret"
raw_body = event.get('body') # Ensure this is the raw string
expected_sig = hmac.new(
secret.encode('utf-8'),
raw_body.encode('utf-8'),
hashlib.sha256
).hexdigest()
actual_sig = event['headers'].get('x-genesys-signature')
if not hmac.compare_digest(expected_sig, actual_sig):
raise Exception("Signature mismatch")
I spent hours debugging this. The issue is often that middleware or the Lambda handler parses the JSON before you check the signature. The signature verification must happen on the exact byte sequence sent by Genesys Cloud. If you modify the body even slightly, the hash will differ. Check your key derivation again.
I’d recommend looking at at the precise handling of the raw body within your Lambda handler. While the previous suggestions correctly identify the HMAC-SHA256 algorithm, a frequent point of failure in enterprise implementations is the implicit decoding performed by AWS API Gateway or the Lambda runtime before the event reaches your Python logic.
The signature calculation in Genesys Cloud is strictly deterministic based on the exact byte sequence of the payload. If your Lambda parses the JSON automatically, the whitespace or key ordering may differ from the original request, causing the hash mismatch. You must ensure the body variable passed to hmac.new is the raw, unaltered string. In many Studio-integrated scenarios, we use a GetRESTProxy snippet to capture the raw stream, but in Python Lambda, you often need to re-serialize the parsed JSON exactly as it was received if you cannot access the raw buffer directly.
Here is the robust verification pattern:
import hmac
import hashlib
def verify_signature(secret, body, signature):
# Ensure body is a string, not a dict
if isinstance(body, dict):
body = json.dumps(body, separators=(',', ':')) # Compact JSON
elif not isinstance(body, str):
body = body.decode('utf-8')
expected = hmac.new(
secret.encode('utf-8'),
body.encode('utf-8'),
hashlib.sha256
).hexdigest()
return hmac.compare_digest(expected, signature)
Ensure your Webhook Secret matches the one configured in the Genesys Cloud integration settings precisely.
I typically get around this by bypassing the Lambda event parsing and using context.get_raw_body() to ensure the byte stream matches the HMAC calculation exactly, as middleware often corrupts the raw payload. the spec is clear on webhook signature verification.