Android implements a permission system to regulate apps' access to system resources and sensitive user data. One salient feature of this system is its extensibility: apps can define their own custom permissions to expose features and data to other apps. However, little is known about how widespread the usage of custom permissions is, and what is the impact that these permissions can have on users' privacy and security. In this paper, we empirically study the usage of custom permissions at large scale, using a dataset of 2.2M pre-installed and app-store-downloaded apps. We find the usage of custom permissions to be widespread, and seemingly growing over time. Despite this prevalence, we find that custom permissions are virtually invisible to end users, and their purpose mostly undocumented. This lack of transparency can lead to serious security and privacy problems: we show that custom permissions can facilitate access to permission-protected system resources to apps that lack those permissions without user awareness. To detect this practice, we design and implement two static analysis tools, and highlight multiple concerning cases spotted in the wild. We conclude this study with a discussion of potential solutions to mitigate the privacy and security risks of custom permissions.

Android firmware updates are typically managed by the so-called FOTA (Firmware Over-the-Air) apps. Such apps are highly privileged and play a critical role in maintaining devices secured and updated. The Android operating system offers standard mechanisms—available to Original Equipment Manufacturers (OEMs)—to implement their own FOTA apps but such vendor-specific implementations could be a source of security and privacy issues due to poor software engineering practices. This paper performs the first large-scale and systematic analysis of the FOTA ecosystem through a dataset of 2,013 FOTA apps detected with a tool designed for this purpose over 422,121 pre-installed apps. We classify the different stakeholders developing and deploying FOTA apps on the Android update ecosystem, showing that 43% of FOTA apps are developed by third parties. We report that some devices can have as many as 5 apps implementing FOTA capabilities. By means of static analysis of the code of FOTA apps, we show that some apps present behaviors that can be considered privacy intrusive, such as the collection of sensitive user data (e.g., geolocation linked to unique hardware identifiers), and a significant presence of third-party trackers. We also discover implementation issues leading to critical vulnerabilities, such as the use of public AOSP test keys both for signing FOTA apps and for update verification, thus allowing any update signed with the same key to be installed. Finally, we study telemetry data collected from real devices by a commercial security tool. We demonstrate that FOTA apps are responsible for the installation of non-system apps (e.g., entertainment apps and games), including malware and Potentially Unwanted Programs (PUP). Our findings suggest that FOTA development practices are misaligned with Google’s recommendations.