Security code smells in apps: are we getting better?

Users increasingly rely on mobile apps for everyday tasks, including security- and privacy-sensitive tasks such as online banking, e-health, and e-government. Additionally, a wealth of sensors captures the movements and habits of the users for fitness tracking and convenience. Despite legal regulations imposing requirements and limits on the processing of privacy-sensitive data, users must still trust the app developers to apply suffcient protections. In this paper, we investigate the state of security in Android apps and how security-related code smells have evolved since the introduction of the Android operating system. With an analysis of 300 apps per year over 12 years between 2010 and 2021 from the Google Play Store, we find that the number of code scanner findings per thousand lines of code decreases over time. Still, this development is offset by the increase in code size. Apps have more and more findings, suggesting that the overall security level decreases. This trend is driven by flaws in the use of cryptography, insecure compiler flags, insecure uses of WebView components, and insecure uses of language features such as reflection. Based on our data, we argue for stricter controls on apps before admission to the store.