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If a webhook’s JSON looks right but signature verification fails, the most likely issue is that your server is verifying a different byte sequence from the one the provider signed. Preserve the original request body, verify it using that provider’s exact rules, and parse the JSON only after verification succeeds.
Why identical-looking JSON can produce a different signature
Webhook signatures authenticate a provider-defined input, not the JSON object your application eventually sees. Whitespace, key order, text encoding, and other byte-level differences can change that input even when two payloads appear equivalent when printed.
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A common cause is middleware that parses the request body and then application code serializes the resulting object again. The re-created JSON may look the same but differ from the original bytes. Keep the incoming body intact rather than reconstructing it from parsed data. GitHub documents signing the payload contents, while Slack specifically requires the raw request body before deserialization (GitHub’s webhook validation guidance; Slack’s request verification guidance).
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Debug the failure in this order
- Confirm the provider and endpoint. Identify the exact webhook endpoint that sent the failing request, then check that you are using its configured secret. Secrets may differ by endpoint or environment.
- Preserve the original body early. Capture the body bytes—or the exact raw representation required by the provider’s official SDK—at the earliest point in request handling. Do this before JSON-parsing middleware,
request.json(), or an equivalent body transformation. - Verify with the provider’s own rules. Use its official SDK or documented formula, signature header, algorithm, and encoding. Do not assume another provider’s signing scheme applies.
- Parse only after verification succeeds. Once the signature passes, decode and parse the body for application logic.
- If it still fails, check configuration and transformations. Verify the secret, header name, algorithm, and encoding; check whether earlier code consumed the body; and investigate proxies or load balancers that might alter the body or headers.
- Keep diagnostics safe. Record which verification stage failed without logging signing secrets or exposing sensitive payload contents.
Provider rules differ: GitHub and Slack
The raw-body requirement is shared, but the signature formula and headers are provider-specific. These examples illustrate why you should follow the documentation for the service that sent the request.
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| Provider | Documented verification details | What to check |
|---|---|---|
| GitHub | Uses the payload contents as the HMAC-SHA256 input and sends a hexadecimal digest with the sha256= prefix in X-Hub-Signature-256. |
Use HMAC-SHA256 and the configured webhook secret. GitHub’s troubleshooting guidance also calls out the legacy X-Hub-Signature/HMAC-SHA1 header, UTF-8 handling where applicable, and possible proxy or load-balancer changes. |
| Slack | Uses a signing secret, X-Slack-Signature, a timestamp header, and the raw request body before deserialization. |
Follow Slack’s timestamp-based procedure, including its recency check to help prevent replay. Do not apply Slack’s timestamp procedure to a different provider unless that provider documents it. |
See GitHub’s validation documentation and Slack’s request verification documentation for their respective requirements.
Use a constant-time comparison
If you implement verification yourself, compare the expected signature and the received signature with a constant-time comparison function rather than ordinary string equality. GitHub gives crypto.timingSafeEqual and Python’s hmac.compare_digest as examples, and its guidance warns against using a plain == operator. Follow the function’s requirements for equal-length inputs and handle malformed or missing signatures safely before comparing.
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What to inspect when raw-body handling is correct
- Secret: Make sure it belongs to the failing endpoint and environment, and that the application is using the current configured value.
- Header and algorithm: Confirm you read the provider’s current signature header and use its specified algorithm and output format.
- Body lifecycle: Check whether middleware or another handler parsed, decoded, consumed, or re-encoded the body before verification.
- Encoding and intermediaries: Check the runtime’s encoding behavior and whether a proxy or load balancer changed the payload or headers.
GitHub lists secret configuration, the correct signature header and algorithm, secret-value errors, UTF-8 handling, and intermediary modifications among its troubleshooting checks (GitHub Docs). The exact checks for other providers should come from their own documentation.
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