A fuzz testing system is described herein that represents event sources, channels, processors, and consumers as first-class entities in an application. Abstracting event-related entities allows fuzzing through injecting, dropping, reordering, and delaying events from within the application. This translates into the ability to localize the areas under test and perform fuzzing in a systematic manner. In some embodiments, the fuzz testing system tests concurrent asynchronous and event-based code, and can generate event streams based on a given statistical distribution. Representing events, event sources, processors, and sinks as first-class objects provides easy access to the event handlers and facilitates implementing fuzzing by introducing event processors between the source and the sink. Thus, the fuzz testing system improves the testability of applications and APIs with asynchronous behavior and provides a uniform framework for introducing fuzz testing into such applications.