Webhook signature validation constantly failing (Always getting 401 Unauthorized)

[genetbeneberu825-png]

🏷️ Discussion Type

Bug

💬 Feature/Topic Area

API

Body

Hey everyone,

I’m trying to set up a GitHub Webhook to listen for repository events and trigger an internal sync script on my Node.js/Express backend.

To keep it secure, I added a Webhook Secret in the GitHub repository settings. However, no matter what I try, the signature verification on my server constantly fails. I keep getting a 401 because the hash I calculate locally never matches the x-hub-signature-256 header sent by GitHub.

Here is the logic I'm using to verify it
const crypto = require('crypto');

app.post('/webhook', (req, res) => {
const signature = req.headers['x-hub-signature-256'];
const hmac = crypto.createHmac('sha256', process.env.WEBHOOK_SECRET);

const digest = 'sha256=' + hmac.update(JSON.stringify(req.body)).digest('hex');

if (signature === digest) {
console.log('Verified!');
res.sendStatus(200);
} else {
console.log('Failed verification');
res.sendStatus(401);
}
});

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[biruk-arch]

Ah, welcome to the absolute rite of passage that is webhook signature validation. Almost everyone gets burned by this exact issue the first time.

The culprit is almost certainly this line right here: JSON.stringify(req.body).

The Problem:
If you are using standard middleware like app.use(express.json()) earlier in your code, Express parses the incoming raw string from GitHub and turns it into a JavaScript object (req.body).

When you try to turn it back into a string using JSON.stringify(req.body), the formatting (like spacing, indentation, or property order) changes ever so slightly compared to the exact raw payload GitHub sent. Crypto hashing is incredibly strict—if even a single character or whitespace is off, the signatures won't match. You must compute the hash using the exact raw, unparsed request body text.

The Fix
You need to grab the raw request body buffer before Express parses it into JSON. You can do this elegantly right inside your express.json() middleware config using the verify option.

Update your middleware and route setup to look like this:
const express = require('express');
const crypto = require('crypto');
const app = express();

// 1. Capture the raw body buffer during parsing
app.use(express.json({
verify: (req, res, buf) => {
req.rawBody = buf;
}
}));

app.post('/webhook', (req, res) => {
const signature = req.headers['x-hub-signature-256'];
if (!signature) return res.sendStatus(401);

// 2. Use the raw body buffer to calculate the HMAC
const hmac = crypto.createHmac('sha256', process.env.WEBHOOK_SECRET);
const digest = 'sha256=' + hmac.update(req.rawBody).digest('hex');

// 3. Use timingSafeEqual to prevent timing attacks
const trusted = Buffer.from(digest, 'ascii');
const untrusted = Buffer.from(signature, 'ascii');

if (crypto.timingSafeEqual(trusted, untrusted)) {
console.log('Verified webhook safely!');
// Handle your webhook logic here
res.sendStatus(200);
} else {
console.log('Verification failed.');
res.sendStatus(401);
}
});
Two quick extras:

crypto.timingSafeEqual: I added this to the fix above. Using a standard === string comparison exposes your endpoint to timing attacks. It's a best practice to use a constant-time comparison helper when dealing with security hashes.

Next.js Note: If you ever migrate this endpoint to a Next.js API route or App Router handler, you'll hit the same wall. Instead of req.body, make sure to use await req.text() to get the raw body string directly from the incoming web request object.

Switch over to using the raw body buffer and your webhook test deliveries should instantly go green!