Simple Membership < 4.7.1 - Unauthenticated Improper Handling of Missing Values

Description

The plugin is vulnerable to Improper Handling of Missing Values via the Stripe webhook handler. This is due to the plugin only validating webhook signatures when the stripe-webhook-signing-secret setting is configured, which is empty by default. This makes it possible for unauthenticated attackers to forge Stripe webhook events to manipulate membership subscriptions, including reactivating expired memberships without payment or canceling legitimate subscriptions, potentially leading to unauthorized access and service disruption.