Android's accessibility API was designed to assist users with disabilities, or temporarily preoccupied users unable to interact with a device, e.g., while driving a car. Nowadays, many Android apps rely on the accessibility API for other purposes, including apps like password managers but also malware. From a security perspective, the accessibility API is precarious as it undermines an otherwise strong principle of sandboxing in Android that separates apps. By means of an accessibility service, apps can interact with the UI elements of another app, including reading from its screen and writing to its text fields. As a consequence, design shortcomings in the accessibility API and other UI features such as overlays have grave security implications.
This talk will provide a critical perspective on the current state of Android accessibility and selected UI security features. Starting with an app store centered overview of how accessibility services are used we will continue with currently unpatched flaws in the accessibility design of Android discovered during our assessment. These flaws and vulnerabilities allow information leakages and denial of service attacks up until Android 8.1. With an enabled accessibility service, we are able to sniff sensitive data from apps, including the password of Android's own lock screen.
To evaluate the effectiveness of our attacks against third-party apps, we examined the 1100 most downloaded apps from Google Play and found 99.25% of them to be vulnerable to at least one of the attacks covered in this talk. In the end possible countermeasures are discussed and we shed some light on the reporting process of Android vulnerabilities.