The Syliud PayPal Plugin is the Sylius Core Team’s plugin for the PayPal Commerce Platform. A vulnerability in versions prior to 1.6.1, 1.7.1, and 2.0.1 allows users to manipulate the final payment amount processed by PayPal. If a user modifies the item quantity in their shopping cart after initiating the PayPal Express Checkout process, PayPal will not receive the updated total amount. As a result, PayPal captures only the initially transmitted amount, while Sylius incorrectly considers the order fully paid based on the modified total. This flaw can be exploited both accidentally and intentionally, potentially enabling fraud by allowing customers to pay less than the actual order value. Attackers can intentionally pay less than the actual total order amount, business owners may suffer financial losses due to underpaid orders, and integrity of payment processing is compromised. The issue is fixed in versions 1.6.1, 1.7.1, 2.0.1, and above. To resolve the problem in the end application without updating to the newest patches, there is a need to overwrite `ProcessPayPalOrderAction`, `CompletePayPalOrderFromPaymentPageAction`, and `CaptureAction` with modified logic.
The web application does not sufficiently verify inputs that are assumed to be immutable but are actually externally controllable, such as hidden form fields.