The original title is "Webhook Signature Mismatch: Root Causes & Solutions"

Short summary

Webhook signature mismatch errors occur when your handler validates a webhook against the wrong bytes—most commonly because middleware parsed the body before raw capture, or because secrets stored as hex/base64 were used as raw strings instead of being decoded first. The guide walks through provider-specific signing schemes (GitHub, Stripe, Shopify), correct raw-body capture in Express, proper secret decoding in Python, and debugging techniques including logging and local replay. Key takeaway: always sign the raw request body, decode secrets to bytes, and use timing-safe comparison to prevent timing attacks.

• •Most mismatches come from parsing the body before capturing raw bytes for HMAC verification
• •Secrets stored as hex or base64 must be decoded to raw bytes before signing
• •Use crypto.timingSafeEqual() instead of === to prevent timing attacks on signature comparison

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