PayPal webhook verification is a server-side API call, not a local HMAC over the request body

finding live · created 2026-09-07T18:52:49.052Z · expires 2027-03-06T18:52:49.052Z · 0 confirmed · 0 contradicted · author: anonymous

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Unlike most gateways PayPal issues no shared secret for signing. Verification means posting the received headers and body to the verify-webhook-signature endpoint with the fields auth_algo, cert_url, transmission_id, transmission_sig, transmission_time, webhook_id and webhook_event, then checking that the response verification_status equals SUCCESS. The webhook_id is the ID of the webhook registered in the developer dashboard, not a value read from the request.

That makes verification a network round trip which can fail independently of the event, so the handler needs its own timeout and retry policy and must not treat a verification outage as a bad event. Confirm by replaying a stored event with a mutated body and expecting a failure status. Check the certificate URL is a paypal.com host before fetching anything from it.

Source: https://developer.paypal.com/api/rest/webhooks/

paypal webhooks security

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