(A)AssetHook

(A)AssetHook is an LD_PRELOAD-based hooking library that allows for replacing APK asset files dynamically without modifying an APK. It's also written in rust. It redirects asset file loads to a filepath on an Android device under /data/local/tmp/assethook/com.pkg.name. Needless to say, due to this (really SEAndroid's ridiculous DRM scheme that now blocks app access to /data/local/tmp), AssetHook currently requires that the SELinux mode be set to permissive. If the raw asset path (typically assets/path/to/file.name) exists under the respective package's AssetHook path, the replacement will be loaded instead. AssetHook consists of two separate LD_PRELOAD hook implementations, one for the "public" C API, and one for the internal C++ API. In general, use the C++ hook, but if it breaks on a new version of Android, try the C one (just note the limitations of it described below).

Why?

Because things like React Native use the native asset manager API, which can't be hooked via things like Xposed.

C++ API (cppapi_assethook) Hook

A "relatively stable" LD_PRELOAD-based hook that hooks the underlying C++ asset implementation called by the public C API and the Java APIs among other things. As this lower level API is occasionally used by other internal Android code to load/check other static resources from APKs, it can also be used to replace those files (if read via this API) by placing replacements under non-assets/ APK paths.

Note: While files such as res/layout/activity_main.xml are at least opened via this API, they are not read with it and cannot be replaced at runtime with AssetHook.

Legacy C API (capi_aassethook) Hook

(Note the two 'a's in "aasset")

A "conservative" LD_PRELOAD-based hook that won't hook the Java-level android.content.res.AssetManager. As the C API it hooks (AAsset*) is public, this is a stable hook to be used if new changes in the internal C++ implementation break the C++ API hook. Using this hook, only files under /data/local/tmp/assethook/com.example.name/assets/ can be hooked.

Usage

1. Install AAssetHook and enable it for a particular app package

2. Push (as shell to make sure the file is world-readable) replacement files:

   adb push index.android.bundle.mod /data/local/tmp/assethook/com.pkg.name/assets/index.android.bundle
   

3. Restart the hooked app

Building

Note: Shared objects for 32-bit and 64-bit arm are embedded in the repository.

Note: The C API hooking AAssetHook uses some rust syntax extensions to cut down on a tremendous amount of boilerplate, and therefore needs to be compile using rust nightly. It is known to build using rust at commit daf8c1dfc from 2016-12-05 (in case internal changes in the rust compiler break the build).

1. Install the Android NDK

2. Install rust via rustup (https://github.com/rust-lang-nursery/rustup.rs)

3. Run the following commands:

   rustup install nightly # C API hook only
   rustup default nightly # C API hook only
   rustup target add aarch64-linux-android
   rustup target add arm-linux-androideabi
   rustup target add i686-linux-android # AVD/HAXM
   mkdir ~/.cargo/toolchains
   cd ~/.cargo/toolchains
   /path/to/ndk/build/tools/make-standalone-toolchain.sh \
     --platform=android-22 --toolchain=arm-linux-android-4.9 \
     --install-dir=android-22-arm-toolchain`
   /path/to/ndk/build/tools/make-standalone-toolchain.sh \
     --platform=android-22 --toolchain=aarch64-linux-android-4.9 \
     --install-dir=android-22-aarch64-toolchain`
   /path/to/ndk/build/tools/make-standalone-toolchain.sh \
     --platform=android-22 --toolchain=x86-linux-android-4.9 \
     --install-dir=android-22-x86-toolchain`
   

4. Add the following to ~/.cargo/config

   [target.arm-linux-androideabi]
   linker = "/path/to/home/.cargo/toolchains/android-22-arm-toolchain/bin/clang"
   
   [target.aarch64-linux-android]
   linker = "/path/to/home/.cargo/toolchains/android-22-aarch64-toolchain/bin/clang"
   
   [target.i686-linux-android]
   linker = "/path/to/home/.cargo/toolchains/android-22-x86-toolchain/bin/clang"
   

5. cd /path/to/aassethook/cppapi

6. cargo build --target=arm-linux-androideabi --release

7. cargo build --target=aarch64-linux-android --release

8. cargo build --target=i686-linux-android --release

Note: The build.sh script in the cppapi directory performs the last three commands.

Installing

Note: If falling back to the capi implementation, swap all instances of cppapi for capi.

First install SELinuxModeChanger from F-Droid or MrBIMC/SELinuxModeChanger, and set it to permissive mode.

Note: You can also just run /system/bin/setenforce 0 after each boot.

cd cppapi
./install.sh

Enabling

Note: If falling back to the capi implementation, swap all instances of cppapi for capi.

cd cppapi
./hook.sh <pkg.name> <32|64|unhook>

Future Work

• Migrate hook file storage to an application's local file directory to bypass the need for SEAndroid disabling
• Resource hooking