Why Webhook Signature Verification Fails and How to Fix It

Webhook signature mismatch errors are among the most frustrating integration bugs, occurring when a provider's signed payload cannot be verified by the receiving endpoint. The root cause is often that developers parse the request body before verification, discarding the original raw bytes that the provider actually signed. Popular platforms like GitHub, Stripe, and Shopify each use HMAC-SHA256 but with slightly different signing formulas, making it critical to follow each provider's exact documentation. The correct approach in frameworks like Express is to capture the raw body buffer before any JSON parsing and use timing-safe comparison functions to validate the signature. Additional pitfalls include mismatched secret encoding, such as treating a hex-stored secret as raw bytes, which can silently break verification.