Pay with Vipps and MobilePay for WooCommerce before 6.4.1 Webhook Signature Forgery

Pay with Vipps and MobilePay for WooCommerce (woo-vipps) before 6.4.1 contains an improper cryptographic signature verification vulnerability in the recurring-payment webhook handler that allows unauthenticated attackers to forge webhook requests when no local webhook secret is stored, because the code fails to return on an empty secret and computes an HMAC keyed with an empty string using a loose == comparison instead of hash_equals. Attackers can craft a recurring.agreement-stopped.v1 event with a known agreementId and a matching empty-key signature to cancel a victim's active WooCommerce subscription.