A case study was developed to illustrate how a logic flaw
could cause security vulnerabilities. The case is based on the
research article How to Shop for Free Online: Security
Analysis of Cashier-as-a-Service Based Web Stores [10]. It is
about the eCommerce merchant software Bigcommerce,
previously known as Interspire, using PayPal Express as a
third-party payment collector. A logic flaw allowed attackers to
complete an expensive order using the payment intended for a
cheaper order. A PowerPoint animation was created to trace
the API calls and back-end code representing the steps of the
exploit from this case, and explain the manual testing method
used to discover the exploit. A set of discussion questions has
students apply this method to similar code in order to find
potential vulnerabilities and then fix them.