HMAC-SHA256 · hex · timestamped

Slack

Its basestring format and the separate timestamp header are what everyone gets wrong.

Try it — live, in your browser

import { verify } from "hooksig";

const result = await verify("slack", {
  payload: rawBody,            // the RAW body, not parsed JSON
  headers: request.headers,
  secret: env.WEBHOOK_SECRET,
});

if (!result.verified) {
  return new Response(result.reason, { status: 400 });
}

The scheme

• HeaderX-Slack-Signature
• AlgorithmHMAC-SHA256
• Encodinghex
• Timestampyes, so it has replay protection
• SignsHMAC-SHA256 over `v0:{timestamp}:{body}`, hex, prefixed `v0=`. The timestamp arrives in its own header.

Gotchas

• The signed string is `v0:{timestamp}:{body}` with colons, and the timestamp comes from X-Slack-Request-Timestamp, a separate header.
• Prefix your computed digest with v0= before comparing.
• Reject requests whose timestamp is more than five minutes old to stop replays.
• Sign the raw body, before any URL-decoding of the form payload.

Official Slack docs →