Abstract: Live and legitimate user traffic is used with in depth knowledge of the business logic for an API specification to perform security testing on a set of APIs. The present system intercepts and analyzes application program interface (API) traffic, identifies user session data, and identifies traffic suitable to duplicate. The identified traffic is duplicated and modified by addition of malicious code. The modified code is then sent to its intended API destination, where it is processed as normal. The resulting response and other traffic as well as the API system and optionally other systems, such as datastore systems, are analyzed to determine if the malicious code resulted in a valid attack. Results from the modified code attack attempts are reported to a user.