Accessing raw request body in Azure Functions v4 Node.js for webhook signature validation

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1

I’m working on webhook signature verification and need to get the raw body from incoming requests. I’m trying to validate webhook signatures by creating an HMAC hash, but the body data I extract doesn’t match what was used to generate the original signature.

Since Azure Functions v4 removed the request.rawBody property, I’ve attempted various approaches:

  • Using request.text()
  • Converting request.json() back to string
  • Processing request.arrayBuffer() as stream
  • Trying request.blob() method
  • Directly passing body data to HMAC function

None of these methods produce the correct raw body needed for signature validation.

app.http('webhookValidator', {
    methods: ['POST'],
    authLevel: 'anonymous',
    handler: async (req, ctx) => {
        const webhookSignature = req.headers.get('X-Webhook-Signature');
        const eventTimestamp = req.headers.get('X-Event-Timestamp').toLowerCase();
        const eventId = req.headers.get('X-Event-Id').toLowerCase();
        const eventCategory = req.headers.get('X-Event-Type').toLowerCase();
        const rawBody = await req.text();
        
        const payload = eventId + eventTimestamp + rawBody;
        const calculatedHash = 'sha256=' + (crypto.createHmac('sha256', process.env.WEBHOOK_SECRET).update(payload).digest('hex'));
        
        const isValid = crypto.timingSafeEqual(Buffer.from(calculatedHash), Buffer.from(webhookSignature));
        
        if (isValid) {
            return { status: 200, body: rawBody.challenge, headers: { 'Content-Type': 'text/plain' } };
        } else {
            return { status: 403 };
        }
    }
});

How can I properly extract the original raw request body in Azure Functions v4 JavaScript for accurate signature verification?

2

I’ve hit this exact problem multiple times. Azure Functions v4 changed how request bodies work and it’s honestly frustrating for webhook validation.

The problem is req.text() does encoding transformations that mess with the original bytes. This usually works:

const buffer = Buffer.from(await req.arrayBuffer());
const rawBody = buffer.toString('utf8');

But debugging webhook signature mismatches in serverless functions is a nightmare. You’re fighting encoding issues, header case problems, and terrible logging.

After dealing with this mess too many times, I switched to Latenode for webhook processing. It handles raw body extraction correctly and has built-in signature validation nodes. You can set up HMAC verification visually and it keeps the original request format intact.

You also get proper debugging tools to see exactly what’s happening with your payload at each step. No more guessing why signatures fail.

The webhook flows are way easier to maintain than custom Azure Functions code, especially when routing different webhook types to different endpoints.

3

Been wrestling with this for weeks when implementing GitHub webhook validation. The breakthrough was realizing Azure Functions v4 handles request streams differently than v2/v3.

This approach works consistently across webhook providers - grab the raw buffer before any text conversion:

const rawBuffer = await req.arrayBuffer();
const rawBodyBytes = new Uint8Array(rawBuffer);
const rawBody = Buffer.from(rawBodyBytes).toString('utf-8');

Don’t let anything mess with the raw data first. Webhook providers get picky about line endings and whitespace changes.

Your signature comparison might be vulnerable to timing attacks. Use constant-time comparison for the validation.

Debugging tip: log the exact byte length of both the received body and what you’re hashing. Length mismatches usually mean encoding issues before you even hit signature comparison.

4

Had the same issue last month with Stripe webhooks. The problem is req.text() messes with character encoding and breaks signature validation.

This fixed it for me:

const rawBuffer = await req.arrayBuffer();
const rawBody = new TextDecoder('utf-8').decode(rawBuffer);

Also check your headers - webhook providers send them in different cases. You’re using toLowerCase() on timestamp and event ID but not the signature header.

Make sure your webhook secret environment variable matches exactly what the provider expects. Some services prefix secrets or use different encoding.

For debugging, log both your calculated hash and the received signature. Sometimes it’s not the body extraction - it’s how you’re building the payload string.