Sifalo Pay 2.0 - 2.5.2 - Unauthenticated Order Payment Metadata Modification and Forced Order Failure

The Sifalo Pay WordPress plugin before 2.5.3 does not verify ownership of, or authorisation for, the order being updated when it handles a payment return, allowing unauthenticated attackers to write arbitrary payment session metadata onto any order in the store and to force any order awaiting payment into a failed state.