Webhook-Signature

Yookassa Webhook Flaw Allowed Forging Payment Confirmations via Single Curl Request

A technical audit of a car rental service uncovered a critical flaw in the Yookassa webhook handler where payment.succeeded events were trusted without verification. The endpoint accepted the event type and gateway_payment_id directly from the client-supplied payload, enabling attackers to mark pending bookings as paid using a simple unauthenticated POST request. The vulnerability existed because the integration did not use Yookassa's optional Webhook-Signature mechanism and avoided IP allowlisting due to proxy infrastructure concerns. Developers addressed the issue by implementing a reverse API call to fetch authoritative payment status from Yookassa before updating records, ensuring fail-closed behavior on errors. New tests cover forgery attempts, amount mismatches, and idempotency, while several additional hardening measures were deliberately deferred to keep the patch minimal and reviewable. The original unit tests had masked the problem by simulating trusted input rather than validating source authenticity.