The Tamara Checkout WordPress plugin through 1.9.9.20 does not verify the order key, a nonce, or any capability on its public payment cancel/fail return URLs, changing a WooCommerce order's status based solely on an attacker-supplied numeric order id, so an unauthenticated attacker can cancel or fail arbitrary orders store-wide by enumerating ids (triggering downstream stock-release and notification side-effects).