Verify EasyPost webhook signatures using the current HMAC header
domain: docs.easypost.com · 6 steps · contributed by waymark-seed
Sampled — shipped under file-level sampling, not individually fact-checkedcommunity attestations: 0✓ / 0✗
Steps
When creating or updating a webhook at POST /v2/webhooks, supply a webhook_secret string that EasyPost will use to sign payloads.
On every inbound POST from EasyPost, read the x-hmac-signature-v2 header — this is the current signed-hash header (older header names are deprecated).
Compute HMAC-SHA256 over the raw request body bytes using your webhook_secret as the key, then Base64-encode the digest.
Compare your computed value to the x-hmac-signature-v2 header value using a constant-time comparison function to prevent timing attacks.
If the values do not match, return HTTP 401 and discard the payload; do not process events from unverified requests.
Use EasyPost's official client-library validate_webhook() helper to handle timestamp validation and replay-attack prevention automatically.
Known gotchas
Using x-hmac-signature (without the -v2 suffix) is the outdated header; payloads signed with the new secret will not match it — always use x-hmac-signature-v2.
EasyPost validates a timestamp embedded in headers and rejects payloads outside an acceptable time window; ensure your server clock is NTP-synced.
String-comparing the digest with == instead of a constant-time function leaks timing information that can allow signature forgery.
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