Cassandra: A Security-Certifying App Store for Android

Apps running on Android smartphones have access to various private information of their users, e.g., contacts, appointments, and the GPS location. To protect such information, the Android operating system provides a mechanism for restricting access to it: the Android permission system. An app may only access a protected source of private information if it was granted the appropriate permission at the time of the app's installation. However, the user has no control on how private information is propagated by an app after it has been accessed legitimately. In fact, it has been observed that many apps abuse entrusted information by leaking it, e.g., to the Internet.

Cassandra aims at increasing the transparency of how apps use private information and, thus, supporting users in protecting their privacy. The primary goal of Cassandra is that no private data or other secret information is leaked by running an app.

Overview of the Tool

From a user's perspective, Cassandra resembles conventional app stores with respect to browsing and installing apps. In addition, Cassandra offers an interface for specifying security policies and for checking that apps adhere to these policies prior to installation.

Cassandra's architecture follows the client-server paradigm. The client of Cassandra is an Android app that provides an interface for browsing apps, for specifying security policies, and for viewing analysis results. The server is implemented in Java and PHP and provides its clients with apps to install, templates for specifying security policies, and a service for conducting the information-flow analysis.

The screenshots show a part of the user interface of Cassandra's client app. From left to right, they show the app browser, the app viewer, the policy editor, and the reporter, respectively. To conduct a security analysis with Cassandra, the user first selects an app in the app browser. Second, he clicks the button "Analyze" below the app's description. Third, the user chooses sources of private information, and untrusted information sinks in the policy editor and confirms the selection by clicking "Analyze" again. After a few seconds, the user either gets a notification that the app respects his security requirements or he gets an overview of found information leaks in the app. Finally, the user can take an informed decision on whether to install the analyzed app or not. The video below shows the application of Cassandra at the example of the app "Minute Man", which reveals a leak of private information in the app.

Further details about the Architecture of Cassandra and its implementation are provided in [LMSBSW14].

 

Security Analysis

The security analysis of Cassandra automatically checks that no information obtained from a selected private source of information is leaked to a selected untrusted sink by running the analyzed app. The analysis implements a type-based information-flow analysis for Dalvik bytecode. This information-flow analysis has been proven to soundly enforce a noninterference-like, declarative security property. The definition of the security type system, the security property, and the soundness proof are given in [LMSW2014].

At this point in time, the sound security analysis of Cassandra supports 211 out of 218 Dalvik bytecode instructions. This is already sufficient for analyzing apps with meaningful functionality as those available in the Cassandra Demo below. The work to further improve the practical applicability of Cassandra's security analysis is in progress.

Demo

A demo of the Cassandra app store is available. It comprises an app store with selected apps, and a client app to access this store, which can be downloaded here. If you have trouble with running the demo or encounter bugs or other problems, please contact David Schneider.

The app store offers three self-written apps that can be analyzed and installed: Distance Tracker, Minute Man, and Private Notes. A short description of the apps as well as an overview of their potential information leaks is provided in the following.

Distance Tracker Minute Man Private Notes
Functionality: When started, the Distance Tracker measures the traveled distance as well as the speed of the mobile device. Besides this information, the app displays a link to the website of the app provider. Functionality: When activated, Minute Man runs as a background service and offers the user to terminate outgoing phone calls within 59 seconds to save the money for the second minute. Functionality: Private Notes allows its user to take and view notes. The notes are stored in the private persistent memory of the app. The app displays an advertisement banner that it loads from the Internet.
Leaks: The app leaks the recorded location information to the Internet when the link to the website is clicked. The location information is encoded into the URL that is opened with the web-browser. Leaks: The app leaks the called phone number to a URL that is opened in the web- browser immediately after the app has terminated a call. Leaks: The app does not leak sensitive information to any potentially untrusted information sink.

Source Code

The source code of Cassandra is available under the MIT license and can be downloaded here.

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