fastspring-webhooks

FastSpring Webhooks

When to Use This Skill

• Setting up FastSpring webhook handlers
• How do I verify FastSpring webhook signatures?
• Why is my X-FS-Signature verification failing?
• Handling order.completed, subscription.activated, or subscription.charge.completed events
• Iterating the batched events array FastSpring delivers in each POST

Verification (core)

FastSpring signs the exact raw request body with HMAC-SHA256 keyed on your per-webhook HMAC SHA256 Secret, base64-encodes the digest, and sends it in the X-FS-Signature header. Pass the raw body (do not parse/re-serialize first), recompute, and compare timing-safe. Each POST batches multiple events in an events array — verify the signature once against the whole body, then iterate.

Note: Signing is only active when the HMAC secret is set on the webhook. If no secret is configured, no X-FS-Signature header is sent.

Node:

const crypto = require('crypto');

function verify(rawBody, signatureHeader, secret) {
  if (!signatureHeader) return false;
  const expected = crypto.createHmac('sha256', secret).update(rawBody).digest('base64');
  try {
    return crypto.timingSafeEqual(Buffer.from(signatureHeader), Buffer.from(expected));
  } catch {
    return false;
  }
}

Python:

import hmac, hashlib, base64

def verify(raw_body: bytes, signature_header: str, secret: str) -> bool:
    if not signature_header:
        return False
    expected = base64.b64encode(
        hmac.new(secret.encode(), raw_body, hashlib.sha256).digest()
    ).decode()
    return hmac.compare_digest(signature_header, expected)

After verifying, iterate payload.events and dispatch on each event.type. Dedupe on event.id — automatic retries reuse the same id (manual retries get new ids). FastSpring auto-retries over HTTPS until your endpoint returns HTTP 200.

For complete handlers with route wiring, batch iteration, event dispatch, and tests, see:

• examples/express/
• examples/nextjs/
• examples/fastapi/