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OSRS Native Android mobile app

This is a project that is still under development

This project aims to reverse engineer the OSRS mobile app to build a working wrapper for tracking the internal game state. It provides a "detailed" learning journey into the world of reverse engineering (RE). It's cool to learn fun shit.

Goals

  • Explore and learn RE techniques.
  • Maintain thorough documentation of the project.
  • Develop a working wrapper for tracking internal game state.

Terminologies

Familiarity with the following terms is essential for understanding this project:

  • ARM ISA
  • Linux kernel
  • Android OS
  • Android NDT, JNI and AGDK
  • Dalvik VM and ART (Android runtime)
  • Android Java bytecode (smali)
  • Dalvik Executable (.dex files)
  • ELF and Shared object (.so)

Tools

Name Description Required
Android studio IDE for android apps
Java 8/11 JDK
apksigner Used to sign the recompiled apk X
emulator The Android emulator that will run the modified apk X
adb Interact with the emulator X
Ghidra Used to decode the binary (Reverse engineering tool)
jadx Used to convert a apk to readable java code
dextools Dalvik executable tools
Hex editor Used to edit the binary (I use Hex Fiend on mac)
apktool Used for de/re-compilation X
frida-tools RE tool for runtime analysis X
LIEF Library to instrument executable formats
QBDI Dyn bin instrumentation

Disassembling and Decompiling

In this section, we delve into RE techniques and best practices. Include header files from NDK, AGDK, and Android from the following locations:

  • Include header files from NDK, AGDK and Android.
    • android-ndk-r25c/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/include
    • android-ndk-r25c/prebuilt/linux-x86_64/include
    • android-ndk-r25c/sources
    • Java/JavaVirtualMachines/jdk-20.jdk/Contents/Home/include

Formats

  • .a files - Archive libraries are statically linked
  • .so files - Shared objects are dynamically linked
  • .oat files - Statically compiled odex
  • .dex - Dalvik executable

Project structure

  • source/*.apk - The apk that will be edited
  • apps/* - The decompiled apk
  • target/*.apk - The recompiled apk
  • frida/* - The frida injection tools

Did you know..

  • Dalvik/ART VM uses a register-based architecture (register machine), while JVM uses stack-based architecture (stack machines).

  • ART (Anroid Runtime) replaced Dalvik VM in Android Lollipop version.

  • ART uses AOT (Ahead-of-time) compilation.

    • It also has a JIT compiler, so the AOT and JIT complement eachother.

  • DVM uses JIT (Just-in-time) compilation.

  • JIT lets Android dynamically compile an app by interpreting a .dex file and then compiling it into executable code during runtime.

  • AOT lets Android statically compile a .dex (into a .oat file), which then can be stored on the device and executed at any time without having to reinterprete and JIT compile it every time the app is launched.

  • Profile-guided compilations is a technique that ART can use to increase app performance.

  • Android apps are first compiled into Java bytecode, then into Dalvik bytcode.

  • The smali format is a human-readable Dex bytecode.

  • JNI (Java Native Interface) bindings are used by the Dalvik/ART VM to call native functions in binaries, and vise versa.

  • NDK (Native Development Kit) is a toolkit that allows developers to write native code to interface with Dalvik/ART VM through JNI.

  • AGDK (Android Game Development Kit) is a toolkit which contains libraries that assists the developer in writing Android games in native code. It can also interface with the Dalvik/ART VM because it also uses NDK and JNI. Note that it does not always need to use them because AGDK contains libraries that can interact independent of JNI.

The story so far

By analysing the .so with Ghidra, and attaching the jni.h file to the data type manager, I've discover a lot more things about the actual binary. For example, it uses the NDK to natively attach it self to the touch event listener and then concurrency polls events from the event queue, without involving the top java wrapper. I'm pretty new at this, but that took me a while to understand. If one inspect the Android Manifest, one could see under <Application> <activity android:configChanges=".." > that it uses touchscreen such that changes to the touchscreen can be handled (or intercepted) by the binary (OBS, this is not the same as regular touch events).

One thing that I still want to explore is the Android NativeActivity class bindings. If I were to inject or override the class, I could potentially intercept all actions being made. This could either be done under compile-time, or during runtime with tools like frida.

TODO subjects Network... Shaders... Protect against .apk tampering... DexGuard... Debugging...

Frida

Using frida with signed apks on jailed android is a bit tricky. We cannot just use the regualar old frida-server and attach it to the process. Instead we need to use the frida-gadget. This requires us to insert a binary (called a gadget) into the apk (next to the other binary), and then call system.loadlibrary in Java. The thing is that our disassembled apk doesn't contain any real Java code, it contains dex decompiled bytecode that has been translated into smali format, human-readable Dalvik bytecode.

When inserting the frida-gadget into the apk, make sure the name of the binary is prefixed with lib and suffixed in .so. Make sure that the loadLibrary call take place before it loads in the next library.

Random notes

The OSRS Android app seems to be developed either as an Android Native c++, Game Activity c++ project, or a port from the RS3 NXT engine - meaning that they might have ported the game engine and then added a android native wrapper on top of it.

The JNI bindings are used in both projects, but it's never automatically added to Game Activity c++ projects (except for the AGDK parts). What I mean is that there's no stringFromJNI native method example present in GA, like there is in the Android Native c++ project.

Loading so files

Injecting into smali.

const-string v0, "abc"
invoke-static {v0}, Ljava/lang/System;->loadLibrary(Ljava/lang/String;)V

or use LIEF to inject the dylib directly into osrs lib binary

Steps to Get Started

  1. Download the OSRS APK: The first step is to download the OSRS apk file. Due to legal reasons, we can't provide a direct link here, but you can easily find it on the official app store or an APK repository.

  2. Decompile the APK: Use JADX or apktool to decompile the APK into readable code.

    java -jar /path/to/apktool_2.7.0.jar d app.apk -o /path/to/folder

  3. Analyze the Decompiled Code: Explore the decompiled APK to understand its structure and operation. Look for any interesting methods or classes that could be used or modified for your purposes.

  4. Inject Frida Gadget: Inject the Frida Gadget into the APK. This will allow for runtime analysis and manipulation of the app. Ensure that the gadget is correctly named (prefixed with lib and suffixed with .so) and that the loadLibrary call is made before loading any other libraries.

  5. Recompile the APK: Once you've made your modifications, recompile the APK with apktool:

    java -jar /path/to/apktool_2.7.0.jar b /path/to/folder -o app-changed.apk

  6. Sign the APK: Use apksigner to sign your recompiled APK. This is necessary because Android requires all apps to be digitally signed with a certificate before they can be installed or updated.

    apksigner sign --verbose --ks-pass pass:android --ks debug.keystore "app.apk"

  7. Install and Run the APK: Install the signed, recompiled APK on your Android device or emulator. Monitor its operation with adb logcat, and perform any necessary testing or analysis.

  8. ????

  9. Profit

Command examples

# For decoding the dex files and making the apk file readable
$ jadx -d /path/to/output -e /path/to/app.apk

# For decompiling and recompiling an apk
$ java -jar /path/to/apktool_2.7.0.jar d app.apk -o /path/to/folder
$ java -jar /path/to/apktool_2.7.0.jar b /path/to/folder -o app-changed.apk

# Find header files quickly on mac
$ find /Library -name jni.h

# Terminal adb loggin
$ adb logcat | logcat-colorize

# Frida (-U is the default usb device)
$ frida-trace -U -i "Java_*" 8425
$ frida-ps -U # ps but for android processes

# Creating keystore
$ keytool -genkey -v -keystore custom.keystore -alias mykeyaliasname -keyalg RSA -keysize 2048 -validity 10000

# Sign recompiled apks
$ apksigner sign --verbose --ks-pass pass:android --ks debug.keystore "app.apk"
$ apksigner verify --verbose "app.apk"

$ java -jar ~/Downloads/apktool_2.7.0.jar d ./source/Old\ School\ RuneScape_213.2_Apkpure_original.apk -o ./apps/osrs-app
$ java -jar ~/Downloads/apktool_2.7.0.jar b ./apps/osrs-app -o ./target/osrs.apk

$ adb shell netstat -ln | grep 27042

Resources

This project leverages several resources for understanding Android runtime, Frida, LIEF, and QBDI. Android runtime

Frida

LIEF

QBDI