Successful HMAC Verification Shows What Webhook Forwarding Preserves — and What It Does Not

A payment webhook routed through an intermediary delivery service — from a payment provider through a server and CLI tool to a final handler — successfully passed HMAC signature verification at the destination. The verification confirmed that all values included in the provider's signed message, typically the raw request body and timestamp, were not altered during transit. However, a passing HMAC check does not guarantee that every part of the HTTP request arrived unchanged, since headers like Host or User-Agent may differ across forwarded connections without affecting the signature. This distinction matters because intermediaries that parse and reserialize a JSON body can silently break HMAC verification even when the data appears semantically identical. Developers using webhook relay or forwarding layers must ensure raw payload bytes are preserved, not just the logical content of the message.