The Stripe Payment Forms by WP Full Pay WordPress plugin before 8.5.5 does not verify that a subscription belongs to the customer bound to the requesting customer-portal session before cancelling it, allowing a user with a confirmed portal session to cancel subscriptions belonging to other customers.
Exploitation requires the attacker to know the target subscription's identifier, which is high-entropy and not enumerable through the Stripe Payment Forms by WP Full Pay WordPress plugin before 8.5.5.