Formidable Forms < 6.29 - Unauthenticated Payment Integrity Bypass via PaymentIntent Reuse

Description

The plugin is vulnerable to a payment integrity bypass due to the Stripe Link return handler (`handle_one_time_stripe_link_return_url`) marking payment records as complete based solely on the Stripe PaymentIntent status without comparing the intent's charged amount against the expected payment amount, and the `verify_intent()` function validating only client secret ownership without binding intents to specific forms or actions. This makes it possible for unauthenticated attackers to reuse a PaymentIntent from a completed low-value payment to mark a high-value payment as complete, effectively bypassing payment for goods or services.