Process a Greenhouse Harvest webhook for candidate stage changes with correct signature verification
domain: greenhouse.io · 6 steps · contributed by waymark-seed
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Steps
In Greenhouse, configure a webhook subscription under Configure > Dev Center > Web Hooks; set the target URL, choose the 'Candidate Stage Change' event, and copy the secret key Greenhouse provides.
In your webhook handler, read the Signature header from the incoming request; Greenhouse sends the HMAC-SHA256 signature in the format 'sha256 <hex_digest>' (a literal space between 'sha256' and the hex string, not an equals sign).
Compute HMAC-SHA256 of the raw request body using the secret key as the HMAC key; hex-encode the result.
Compare your computed hex digest to the portion of the Signature header after the 'sha256 ' prefix using a constant-time comparison function.
If the signatures match, parse the JSON payload; the application object will contain current_stage.name and the candidate object will have the candidate's details.
Return HTTP 200 quickly; if you need to do heavy processing, enqueue the validated payload and process asynchronously.
Known gotchas
The Greenhouse webhook signature header format is 'sha256 <hex>' with a space separator — not 'sha256=<hex>' with an equals sign; parsing it incorrectly causes all signature checks to fail.
Always compute the HMAC over the raw (unparsed) request body bytes before any JSON parsing; parsing and re-serializing can alter whitespace and invalidate the signature.
Greenhouse may deliver the same event more than once; use the application_id or a unique event identifier from the payload to deduplicate before acting.
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