Need some help troubleshooting the signature verification logic for incoming GC webhooks in my Lambda consumer. I am trying to validate the X-Genesys-Signature header against the request body to ensure integrity and prevent replay attacks, but the computed HMAC never matches the header value.
- Environment: Node.js 20.x, AWS Lambda, Genesys Cloud EventBridge
- Issue:
crypto.createHmac output differs from header despite using the correct secret and raw body
Here is the verification snippet:
const crypto = require('crypto');
const secret = process.env.GENESYS_WEBHOOK_SECRET;
const signature = crypto.createHmac('sha256', secret).update(event.body).digest('hex');
// signature !== event.headers['x-genesys-signature']
Am I missing a specific encoding step or is the secret format different?
It depends, but generally… The signature mismatch usually stems from how the payload is serialized before hashing, not the secret itself. Genesys Cloud signs the raw request body as a UTF-8 string. If you are logging or parsing the body into a JSON object before hashing, you have altered the input string, causing the HMAC to diverge.
In an async Python context, I handle this by capturing the raw bytes before any parsing. Here is how I structure the verification logic using httpx and hmac:
import hmac
import hashlib
from fastapi import Request, HTTPException
async def verify_gc_signature(request: Request):
# 1. Get raw body bytes FIRST
body_bytes = await request.body()
signature_header = request.headers.get("x-genesys-signature")
if not signature_header:
raise HTTPException(status_code=401, detail="Missing signature")
# 2. Reconstruct the signed string: method + path + body
# Note: GC signs "POST\n/path\nbody" format in some older docs,
# but modern EventBridge often signs just the raw body or specific fields.
# For standard webhooks, it's typically just the raw body.
secret = "your_webhook_secret" # Retrieve from env/Redis
# Compute HMAC SHA256
computed_hmac = hmac.new(
secret.encode('utf-8'),
body_bytes,
hashlib.sha256
).hexdigest()
# 3. Constant-time comparison to prevent timing attacks
if not hmac.compare_digest(computed_hmac, signature_header):
raise HTTPException(status_code=403, detail="Invalid signature")
return body_bytes # Safe to parse now
Lambda cold starts are the primary bottleneck here. The standard PureCloudPlatformClientV2 SDK initialization is synchronous and heavy, blocking the event loop while it loads. Do not initialize the SDK inside the handler. Instead, keep the secret in memory or fetch it once at cold start. Also, ensure you are not stripping whitespace from the body during logging, as that changes the hash. Check if your Lambda environment variable for the secret has trailing newlines; that is a common gotcha.
You need to ensure the raw request body is consumed exactly as received before any JSON parsing occurs, as this is the most common cause of HMAC mismatch in serverless environments. The suggestion above regarding serialization is correct, but there is a critical timing issue in Lambda handlers that often gets overlooked.
- Capture Raw Bytes Early: In your Lambda handler, do not use
event.body directly if it has already been parsed by API Gateway or an integration layer. You must access the raw binary payload or ensure the body is treated as a string without normalization.
- Verify Secret Encoding: Ensure your webhook secret from the Genesys Cloud Admin portal is being passed to
crypto.createHmac as a UTF-8 encoded buffer. A common mistake is using a base64-decoded secret when the platform expects the raw secret string, or vice versa.
- Compare Constant-Time: Use
crypto.timingSafeEqual to compare the computed HMAC digest against the X-Genesys-Signature header value to prevent timing attacks.
Here is the robust implementation pattern for Node.js:
const crypto = require('crypto');
exports.handler = async (event) => {
const signature = event.headers['x-genesys-signature'];
const body = event.body; // Ensure this is the raw string, not parsed JSON
// 1. Compute HMAC-SHA256
const hmac = crypto.createHmac('sha256', process.env.GC_WEBHOOK_SECRET);
hmac.update(body);
const digest = hmac.digest('hex');
// 2. Constant-time comparison
if (!crypto.timingSafeEqual(Buffer.from(digest), Buffer.from(signature))) {
return { statusCode: 401, body: 'Invalid signature' };
}
// 3. Proceed with processing
return { statusCode: 200, body: 'Verified' };
};
In my Android SDK work, I see similar issues when OkHttp interceptors modify headers before the body is read. Double-check that your API Gateway integration is set to pass the body as application/json without transformation.
It depends, but generally… you need to ensure the raw buffer is passed to crypto.createHmac before any JSON.parse calls mutate the string encoding. I use the exact bytes from the EventBridge payload for my DogStatsD ingestion traces to avoid this exact drift.
Make sure you capture the raw body before any middleware processes it. The documentation states “the signature is calculated over the raw request body.” In my .NET integrations using Azure Functions, I often see this fail because the function runtime parses JSON before the verification step.
- Use the
HttpRequest.Body stream directly instead of req.Body.AsStringAsync().
- Do not convert the stream to a string first. The HMAC-SHA256 calculation requires the exact byte sequence.
- Verify the encoding. The documentation states “the payload must be UTF-8 encoded.”
Here is the C# pattern I use in my IAsyncFunction implementation:
var bodyStream = req.Body;
bodyStream.Seek(0, SeekOrigin.Begin);
var bodyBytes = new byte[bodyStream.Length];
await bodyStream.ReadAsync(bodyBytes, 0, bodyBytes.Length);
var hmac = new HMACSHA256(Encoding.UTF8.GetBytes(_webhookSecret));
var hash = hmac.ComputeHash(bodyBytes);
var computedSignature = Convert.ToBase64String(hash);
If you parse the JSON first, the spacing or order might change, breaking the hash. Keep the bytes raw.