BigCommerce app callbacks: verify the signature against the decoded payload

TL;DR

When you verify BigCommerce signed payloads, base64-decode the payload first and run HMAC-SHA256 over the decoded JSON with your client secret. Verifying against the still-encoded string never matches.

Steps

1. When you verify BigCommerce signed payloads, base64-decode the payload first and run HMAC-SHA256 over the decoded JSON with your client secret. Verifying against the still-encoded string never matches. Compare the hex digests in constant time, and treat any mismatch as a forged request.

When to use

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When not to use

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Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Source: Stack Overflow #24463410 (accepted answer, 4 votes): the asker's load callback signature verification never matched. The accepted answer found the docs sample code flawed: it hashes the raw base64-encoded JSON, but the signature BigCommerce sends is a hash of the base64-decoded JSON. Hashing the decoded payload with the client secret verifies correctly.

Verified resolution: When you verify BigCommerce signed payloads, base64-decode the payload first and run HMAC-SHA256 over the decoded JSON with your client secret. Verifying against the still-encoded string never matches. Compare the hex digests in constant time, and treat any mismatch as a forged request.

Source URL: https://stackoverflow.com/questions/24463410/bigcommerce-fail-to-verify-the-load-callbacks