2. Android Security

Android Architecture

Android is based on the Linux O.S.

• Linux file permission model (e.g. drwxr-xr-x) - Chmod Calculator

Virtual Machine

• Dalvik - original runtime VM, not used anymore

• Android Runtime (ART) - modern runtime for executing app's bytecode into the device, within the app's own sandbox and isolated file system with a unique user

Android Identity and Access Management

User - owner of the application directory

• UID 10000 to 999999 - username e.g. u0_a129 (with UID 10129)

• Various app's directories

  • /data/app/com.example.basic - generic app data

  • /data/data/com.example.basic - runtime storage of data

  • /mnt/sdcard/Android/data/com.example.basic - external stored data for runtime

• root (system level) user is necessary to access every application directory

Apps can communicate with each other either through granted permissions or, more commonly, by utilizing an exposed Content Provider / Broadcast Receiver (e.g. Open with option).

• Profiles (work BYOD, personal, etc) - access system level functions, but can have isolated app's data, clipboard, contacts, etc

• Primary User - first created user, removed only by factory reset

• Secondary User - additional users

• Guest User - guest access to the phone

• Kid Mode - specific account for kids

Android Architecture

The Android software stack - https://developer.android.com/guide/platform

Kernel

Android relies on a modified Linux Kernel with additional features, with key components such as the managed runtime (ART/Dalvik) for applications and Bionic, Android’s equivalent of glibc (GNU C library)

• Support for various CPU types

• Apps run on specific Android Runtime/API version (minSDKVersion) - Check for Android API Levels - https://apilevels.com/

  • Lower SDK/Android level can have more security vulnerabilities

  • The higher the SDK/API level, the better/more secure

• Physical components are controlled by drivers

Hardware Abstraction Layer (HAL)

HAL is an interface between the operating system and hardware components, providing a standardized way for the OS to communicate with various manufacturer and type of hardware components.

• HAL types: Automotive, iOT, Gaming peripherals, etc

Native C vs Android Runtime

C and C++ is the device's native language.

• no VM required

• Webkit, Libc, OpenMAX AL / OpenGL ES / Media (UI APIs)

Java / Kotlin are preferred over Native C, due to their portability and compatibility across various devices.

Java API Framework

A collection of predefined classes and methods provided by Java for building applications, offering a standardized set of functionalities.

• Content Providers - for data sharing to other apps via a specific directory

• View System - App's UI

• Managers - Notifications, Package management, Telephony, Location, Activity, etc

System Application Layer

The layer of software that includes essential system applications and services, Google and vendor specific pre-installed applications.

Signing Process

Android apps can be decompiled (reverse-engineered), rebuilt, re-signed and re-run.

DEX Bytecode ➡️ SMALI ➡️ Decompiled JAVA

• Tools for reverse engineering Android apps - apktool, jadx / jadx-gui

Android build process

Android application signing involves the use of cryptographic signatures, including additional features like Google Play Signing, to verify the authenticity and integrity of an Android app.

• Signing Tools - keytool (Generate a private key), jarsigner (apk signing), zipalign (zip & optimize apk).

📌 Android Applications need to be signed to run on Android devices.

APK validation process v4 - source.android.com

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