Verifying webhook signature headers to prevent replay attacks in Data Actions

Is it possible to validate the X-Genesys-Signature header inside a Genesys Cloud Data Action before processing the payload?

Background

We route outbound event notifications to a custom endpoint via EventBridge. The payload contains sensitive PII, so replay protection is mandatory. I wrote a Node.js middleware that computes HMAC-SHA256 over the raw body using a shared secret.

Issue

Verification works locally. When deployed, the Data Action consistently drops the request with a 401 Unauthorized. The incoming header format looks like t=1716234567,v1=a8f5f167f44f4964e6c998dee827110c. I strip the timestamp and compare the hash, but it never matches. The retry pattern in the orchestrator fires three times before dead-lettering.

Troubleshooting

I logged the raw buffer and the computed digest. They diverge immediately. Could the platform be URL-encoding the body before signing? Here is the verification snippet:

const hmac = crypto.createHmac('sha256', process.env.GC_SECRET);
hmac.update(`t=${req.headers['x-request-timestamp']}&${rawBody}`);
const digest = hmac.digest('hex');

Any idea why the hash mismatch occurs in production?

Is it possible to validate the X-Genesys-Signature header inside a Genesys Cloud Data Action before processing the payload?

You might want to look at implementing the verification in your Node.js middleware before the data hits the action.

const crypto = require('crypto');
const isValid = (req, secret) => {
 const sig = req.headers['x-genesys-signature'];
 const expected = crypto.createHmac('sha256', secret).update(req.body).digest('hex');
 return crypto.timingSafeEqual(Buffer.from(sig), Buffer.from(expected));
};

This prevents replay attacks by ensuring the request originated from Genesys Cloud.