Webhooks are how a lot of third-party services, payment processors, email tools, notification systems, tell your app that something happened elsewhere: a payment succeeded, an email bounced, a subscription renewed. Your app listens for these on a specific URL, and it's a specific, common gap when that URL trusts anything sent to it without checking where it actually came from.
Why this matters more than it sounds like it should
A webhook endpoint URL, once it exists, is just a normal address on the internet. Anyone who finds or guesses it can send it a request shaped like a real message from the service it's expecting. If your app processes "payment succeeded" messages by, say, unlocking premium features, and it doesn't verify the message genuinely came from your payment processor, anyone who can guess the shape of that message can send a fake one and get the same result.
How legitimate services actually protect against this
Most reputable services that send webhooks include a cryptographic signature with each message, generated using a secret only your app and that service know. Your endpoint is supposed to verify that signature before trusting anything in the message. If the signature doesn't match what it should be given your shared secret, the message didn't actually come from the real service, and should be rejected outright, regardless of how convincing its contents look.
How to test your own webhook endpoints
- Find every webhook endpoint your app exposes for any connected third-party service.
- Check your code for whether it actually verifies the provided signature against your shared secret, rather than just reading and acting on the message's contents directly.
- If there's no verification step at all, try sending a fake, correctly-shaped request to that endpoint yourself and see whether your app processes it as if it were real.
- If it does, that's the gap, confirmed directly, and worth fixing before anyone else finds the same endpoint.
The fix
Every webhook endpoint needs to verify the sender's signature before doing anything with the message's contents, using the verification method the service itself documents, and reject anything that doesn't check out. This is usually a small, well-documented step most third-party services explain clearly, and it's exactly the kind of step that's easy to skip when a webhook feature is built quickly and tested only with real, legitimate messages that never needed to prove where they came from.