The Online Payments with iK Pay Gateway WordPress plugin before 3.0.4 does not properly verify the authenticity of its payment gateway callback on a default installation, allowing unauthenticated attackers to forge a successful payment notification and mark arbitrary orders as paid without any payment being made, and to mark arbitrary unprocessed orders as failed.
When an actor claims to have a given identity, the product does not prove or insufficiently proves that the claim is correct.