Explain Webhook Signature Verification.

Webhook signature verification is a security mechanism that ensures incoming webhook payloads are authentic and untampered by validating them against a shared secret.

When to Use

Use it when your application receives third-party webhooks (e.g., Stripe payments, GitHub events, Shopify notifications). It ensures the event really came from the provider and not a malicious actor.

Example

If Stripe sends a payment success webhook, your server computes a signature using the shared secret and compares it with Stripe’s header signature. A match confirms authenticity.

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Why Is It Important

It prevents spoofing attacks and guarantees data integrity. Without signature checks, attackers could inject fake transactions or events.

Interview Tips

Be ready to explain the HMAC verification flow:

1. Read the raw request body.
2. Compute a hash with your secret.
3. Compare with the provider’s signature using constant-time comparison. Mention libraries, replay protection (timestamps, nonces), and secure hash functions.

Trade-offs

Pros: High security and trust. Cons: Slight performance overhead and implementation complexity.

Pitfalls

• Using weak hash algorithms (e.g., MD5).
• Forgetting constant-time comparison (leaks timing info).
• Ignoring replay protection (timestamps, unique IDs).