A main aspect of the Android platform is Inter-Application Communication (IAC), which enables reuse of functionality across apps and app components via message passing. While a powerful feature, IAC also constitutes a serious attack surface. A malicious app can embed a payload into an IAC message, thereby driving the recipient app into a potentially vulnerable behavior if the message is processed without its fields first being sanitized or validated. We present what to our knowledge is the first comprehensive testing algorithm for Android IAC vulnerabilities. Toward this end, we first describe a catalog, stemming from our field experience, of 8 concrete vulnerability types that can potentially arise due to unsafe handling of incoming IAC messages. We then explain the main challenges that automated discovery of Android IAC vulnerabilities entails, including in particular path coverage and custom data fields, and present simple yet surprisingly effective solutions to these challenges. We have realized our testing approach as the Intent-Droid system, which is available as a commercial cloud service. IntentDroid utilizes lightweight platform-level in-strumentation, implemented via debug breakpoints (to run atop any Android device without any setup or customization), to recover IAC-relevant app-level behaviors. Evaluation of IntentDroid over a set of 80 top-popular apps has revealed a total 150 IAC vulnerabilities | some already fixed by the developers following our report | with a recall rate of 92% w.r.t. a ground truth established via manual auditing by a security expert.