An issue was discovered in OPC cardsystems Webapp Aufwertung 2.1.0. The reference assigned to transactions can be reused. When completing a payment, the first or all transactions with the same reference are completed, depending on timing. This can be used to transfer more money onto employee cards than is paid.
The product does not sufficiently enforce boundaries between the states of different sessions, causing data to be provided to, or used by, the wrong session.
Data can “bleed” from one session to another through member variables of singleton objects, such as Servlets, and objects from a shared pool. In the case of Servlets, developers sometimes do not understand that, unless a Servlet implements the SingleThreadModel interface, the Servlet is a singleton; there is only one instance of the Servlet, and that single instance is used and re-used to handle multiple requests that are processed simultaneously by different threads. A common result is that developers use Servlet member fields in such a way that one user may inadvertently see another user’s data. In other words, storing user data in Servlet member fields introduces a data access race condition.