PolyScope: Multi-Policy Access Control Analysis to Triage Android Systems

TL;DR

PolyScope is a tool that analyzes Android access control policies to identify vulnerabilities by computing potential attack operations, revealing previously unknown security issues in OEM Android releases.

Contribution

The paper introduces PolyScope, a novel method for analyzing combined Android access control policies to accurately identify attack vectors and vulnerabilities.

Findings

Permission expansion can increase attack permissions by over 10X.

Approximately 15-20% of permissions are not convertible into attack operations.

PolyScope discovered two previously unknown vulnerabilities.

Abstract

Android filesystem access control provides a foundation for Android system integrity. Android utilizes a combination of mandatory (e.g., SEAndroid) and discretionary (e.g., UNIX permissions) access control, both to protect the Android platform from Android/OEM services and to protect Android/OEM services from third-party apps. However, OEMs often create vulnerabilities when they introduce market-differentiating features because they err when re-configuring this complex combination of Android policies. In this paper, we propose the PolyScope tool to triage the combination of Android filesystem access control policies to vet releases for vulnerabilities. The PolyScope approach leverages two main insights: (1) adversaries may exploit the coarse granularity of mandatory policies and the flexibility of discretionary policies to increase the permissions available to launch attacks, which we call permission expansion, and (2) system configurations may limit the ways adversaries may use their permissions to launch attacks, motivating computation of attack operations. We apply PolyScope to three Google and five OEM Android releases to compute the attack operations accurately to vet these releases for vulnerabilities, finding that permission expansion increases the permissions available to launch attacks, sometimes by more than 10X, but a significant fraction of these permissions (about 15-20%) are not convertible into attack operations. Using PolyScope, we find two previously unknown vulnerabilities, showing how PolyScope helps OEMs triage the complex combination of access control policies down to attack operations worthy of testing.