Android leverages a set of system permissions to protect platform resources. At the same time, it allows untrusted third-party applications to declare their own custom permissions to regulate access to app components. However, Android treats custom permissions the same way as system permissions even though they are declared by entities of different trust levels. In this work, we describe two new classes of vulnerabilities that arise from the `predicament' created by mixing system and custom permissions in Android. These have been acknowledged as serious security flaws by Google and we demonstrate how they can be exploited in practice to gain unauthorized access to platform resources and to compromise popular Android apps. To address the shortcomings of the system, we propose a new modular design called Cusper for the Android permission model. Cusper separates the management of system and custom permissions and introduces a backward-compatible naming convention for custom permissions to prevent custom permission spoofing. We validate the correctness of Cusper by 1) introducing the first formal model of Android runtime permissions, 2) extending it to describe Cusper, and 3) formally showing that key security properties that can be violated in the current permission model are always satisfied in Cusper. To demonstrate Cusper's practicality, we implemented it in the Android platform and showed that it is both effective and efficient.

NEWS

• Our paper won the distinguished paper award at The Network and Distributed System Security Symposium (NDSS) 2018. Our slides at this conference can be found here.
• We were selected as a finalist with this work at the Cybersecurity Awareness Worldwide (CSAW) applied research competition!

ATTACKS

These are our official reports to Google for our the two attacks described in the paper:

• Privilege escalation through custom permission update, https://issuetracker.google. com/issues/37130844.
• Privilege escalation by exploiting fcfs property of custom permissions, https://issuetracker. google.com/issues/37131935.

FORMAL VERIFICATION

Formal model of Android permissions with an implementation of Cusper can be found here.