Why Webhook Signature Checks Break After Deploy and How to Fix Them

Webhook signature verification failures after deployment are commonly caused by JSON middleware parsing the request body before the signature check runs, meaning the server compares a different byte sequence than what the sender originally signed. Frameworks like Express replace the raw request stream with a JavaScript object, and re-serializing that object does not reliably reproduce the original bytes due to differences in whitespace, escaping, or number formatting. The fix is to preserve the raw request buffer and run signature verification before any JSON middleware processes the body, since middleware order effectively functions as executable security policy. A robust implementation should also assign each signing secret a key ID, accept both old and new secrets only during a time-bounded rotation window, and log which key validated each event. For sensitive operations such as updating a payment ledger, an unverified signature should result in no action taken and no response that leaks details about which key nearly matched.