Analyzing information flow is central in assessing the security of applications. However, static and dynamic analyses of information flow are easily challenged by non-available or obscure code. We present a lightweight mutation-based analysis that systematically mutates dynamic values returned by sensitive sources to assess whether the mutation changes the values passed to sensitive sinks. If so, we found a flow between source and sink. In contrast to existing techniques, mutation-based flow analysis does not attempt to identify the specific path of the flow and is thus resilient to obfuscation. In its evaluation, our MUTAFLOW prototype for Android programs showed that mutation-based flow analysis is a lightweight yet effective complement to existing tools. Compared to the popular FlowDroid static analysis tool, MutaFlow requires less than 10% of source code lines but has similar accuracy; on 20 tested real-world apps, it is able to detect 75 flows that FlowDroid misses.

When a user downloads an Android application from a market, she does not know much about its actual behavior. A brief description, a set of screenshots, and the list of permissions, which give a high level intuition of what the application might be doing, are all the user sees before installing and running the application on his device. These elements are not enough to decide whether the application is secure, and for sure they do not indicate whether it might violate the user's privacy by leaking some sensitive data. The goal of my thesis is to employ both static and dynamic taint analyses to gather information on how Android applications use sensitive data. The main hypothesis of this work is that malicious and benign mobile applications differ in how they use sensitive data, and consequently information flow can be used effectively to identify malware.

What is it that makes an app malicious? One important factor is that malicious apps treat sensitive data differently from benign apps. To capture such differences, we mined 2,866 benign Android applications for their data flow from sensitive sources, and compare these flows against those found in malicious apps. We find that (a) for every sensitive source, the data ends up in a small number of typical sinks; (b) these sinks differ considerably between benign and malicious apps; (c) these differences can be used to flag malicious apps due to their abnormal data flow; and (d) malicious apps can be identified by their abnormal data flow alone, without requiring known malware samples. In our evaluation, our MUDFLOW prototype correctly identified 86.4% of all novel malware, and 90.1% of novel malware leaking sensitive data.